View Full Version : A question about "opposite and equal reactions"
Newtype
Feb12-09, 07:20 PM
Newton's third law of motion says "For every action there is an opposite and equal reaction". So when I throw a rock at a brick wall (action), that rock bounces off that wall (opposite and equal reaction). But if I jump up and while in the air and my shoes are not touching the ground when I throw a rock at a glass window (action) that rock smashes through that window (reaction but not opposite since that rock goes in the same direction I threw it). It appears Newton's third law of motion is not accurate. Can someone clarify this third law of motion? Did I disprove it or is Newton somehow correct? Please explain.
Momentum was conserved, the window acclerated in the direction of the rock, and the rock decelerated. The accelerations correspond to the equal and opposing forces, divided by the mass of the rock and the affected part of the window.
Newtype
Feb12-09, 07:49 PM
But what is the opposite and equal reaction when that rock smashed through that window? That rock didn't deaccelerate very much; that window didn't have enough resilency to totally deaccelerate that rock to force it backwards. So that rock still carried with it most of its forward thrown momentum knocking glass fragments forward. It doesn't look like there was an "opposite reaction" and/or an "equal reaction".
Newton's third law of motion says "For every action there is an opposite and equal reaction". So when I throw a rock at a brick wall (action), that rock bounces off that wall (opposite and equal reaction).
You throwing a rock and the rock hitting a wall are not "action/reaction" pairs.
Newton's third law describes how bodies exert forces on each other when they interact. When you are throwing the rock, your hand exerts a force on the rock; Newton's third law says that the rock will exert an equal and opposite force on your hand.
When the rock hits something, such as a wall or window, that's a totally different interaction. The rock exerts a force on the wall, and thus the wall exerts an equal and opposite force on the rock.
Newtype
Feb12-09, 08:03 PM
But that window didn't exert an equal and opposite force onto that rock like the force that wall exerted onto that rock. Please provide an example to explain what you're saying. Here's an example to explain what I'm saying: A 10kg rock thrown at 1 meter per second northwise will bounce off that wall (wall supposedly exerts 10kg of 1m/s southwise force), but a 10kg rock thrown at 1 meter per second northwise will smash through that window; that window will not exert an opposite and equal 10kg at 1 m/s southwise force to repel that rock since that window doesn't have the resilency to do so, that window will be smashed to pieces northwise instead.
But that window didn't exert an equal and opposite force onto that rock like the force that wall exerted onto that rock. Please provide an example to explain what you're saying. Here's an example to explain what I'm saying: A 10kg rock thrown at 1 meter per second northwise will bounce off that wall (wall supposedly exerts 10kg of 1m/s southwise force), but a 10kg rock thrown at 1 meter per second northwise will smash through that window; that window will not exert an opposite and equal 10kg at 1 m/s southwise force to repel that rock since that window doesn't have the resilency to do so, that window will be smashed to pieces northwise instead.
When the rock hits the wall, the wall and rock exert equal and opposite forces on each other. When the rock hits the window, the window and rock exert equal and opposite forces on each other. And if the rock hit a butterfly, the butterfly and rock would exert equal and opposite forces on each other. Of course, since these are all different interactions the forces created will be different. But in every case, Newton's third law will apply.
Clearly the force exerted by the window was not enough to slow the rock down much, nevermind send it bouncing backwards. But it was clearly enough force to break the window.
Note that momentum and force are different concepts. A 10 kg rock moving at 1 m/s has a certain amount of momentum, not force.
pallidin
Feb12-09, 08:28 PM
Newtype, the window DOES exert an equal and opposite force, but ONLY to it's own limit to withstand the impacting force of the rock.
Thus, to balance things off, the impacting force continues through the window, yet exiting with a reduced force EXACTLY as less as the window inhibited it.
Hopes that makes any sense...
Newtype
Feb12-09, 08:45 PM
Please provide an example like I did. Northwise moving glass fragments are not opposite a northwise moving rock. Force = mass multiplied by acceleration. Acceleration is the change of velocity, no change of velocity, then no acceleration (same velocity). Momentum = mass multiplied by velocity. So force and momentum are pretty much the same.
zoobyshoe
Feb12-09, 09:07 PM
Newtype, the window DOES exert an equal and opposite force, but ONLY to it's own limit to withstand the impacting force of the rock.
Thus, to balance things off, the impacting force continues through the window, yet exiting with a reduced force EXACTLY as less as the window inhibited it.
Hopes that makes any sense...
Yes, from the rock's frame the window represents a certain amount of resistance to its forward motion. That resistance constitutes a force exerted on the rock: it is acting to change the rock's inertia. The rock is inert at this point, discounting gravity, and the window, therefore, represents a force capable of accelerating it: changing its state of motion or rest.
From the window's frame the rock is a force which is capable of changing the window's state of motion or rest. The window is inert, in its own frame, and the rock will exert a force to change the state of motion or rest of the window.
The rock will experience an impulse in its encounter with the window: I = ft, or I = mat, and the value of that impulse will be exactly the same as the impulse experienced by the window. In this case the magnitude of the impulse will be capped by the window's material strength. In the case of an immovable wall, the impulse received by both will have a higher value, but will still be the same for both masses.
Newton III does not mean that both masses which are in relative motion can automatically match the full possible force of the other. It simply means that, whichever wins, informally speaking, they will both have experienced the same amount of acceleration from the other.
Please provide an example like I did. Northwise moving glass fragments are not opposite a northwise moving rock. Force = mass multiplied by acceleration. Acceleration is the change of velocity, no change of velocity, then no acceleration (same velocity). Momentum = mass multiplied by velocity. So force and momentum are pretty much the same.
Force is the rate of change of momentum. The change in momentum of the glass fragments is equal and opposite to the change in momentum of the rock.
Here's an example (with made up numbers) that might help. A 10 kg rock moves north at 1 m/s (let's ignore gravity) and thus has a momentum of 10 kg-m/s north. It encounters a window pane that is able to exert a force of 10 N on it for 0.1 seconds toward the south. (In the course of exerting that force, the window pane shatters, but so what?) According to Newton's 3rd law, both window and rock exert equal and opposite forces on each other, thus the rock exerts a force of 10 N on the window for 0.1 seconds toward the north.
What happens to the rock? Since the window has exerted an impulse (FΔt) on the rock equal to 10 N * 0.1 sec = 1 N-s south, the rock's new momentum is 10 - 1 = 9 kg-m/s north. So the rock is still moving north, only now its speed is only 0.9 m/s.
What happens to the window? If the window is attached to a window pane, etc, more forces act on it than just the rock. For simplicity, let's ignore all that and just pretend that the window pane was suspended in mid-air when the rock hit it. The window shatters from the 10 N force in the north direction, and glass bits go flying. Since the rock exerted an impulse of 1 N-s north on the glass, we know that the new momentum of all the glass pieces (you have to add them up) will total 1 kg-m/s north.
Note that the total momentum of "rock + window" hasn't changed. It started out as 10 kg-m/s north (all rock), and it ended up as 9 (rock) + 1 (window fragments) = 10 kg-m/s north.
Make sense?
I'm guessing the issue is the difference between the wall and window case. The forces in the window case are much less than in the wall case. Still, the smaller forces between rock and window are equal and opposing, and the larger forces between rock and wall are equal and opposing.
zoobyshoe
Feb13-09, 08:25 PM
I'm guessing the issue is the difference between the wall and window case. The forces in the window case are much less than in the wall case. Still, the smaller forces between rock and window are equal and opposing, and the larger forces between rock and wall are equal and opposing.
I think the problem is the wording of Newton III, which is tricky and requires explication:
To every action there is always opposed an equal reaction: or, the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.
It's quite possible to read that and suppose he's making the extravagant claim that when, say, the rock hits a butterfly the rock should, by this law, always bounce off the butterfly in the opposite direction from, but at the same velocity with which it hit the butterfly.
To avoid that misreading, explication and context are required, which Newton provided:
"Whatever draws or presses another is as much drawn or pressed by that other. If you press a stone with your finger the finger is also pressed by the stone. If a horse draws a stone tied to a rope, the horse (if I may say so) will be equally drawn back toward the stone; for the distended rope, by the same endeavor to unbend or relax itself, will draw the horse as much toward the stone as it does the stone toward the horse, and will obstruct the progress of the one as much as it advances that of the other. If a body impinge upon another, and by its force change the motion of the other, that body also (because of the equality of the mutual pressure) will undergo an equal change, in its own motion, toward the contrary part. The changes made by these actions are equal, not in the velocities but in the motions of bodies; that is to say, if the bodies are not hindered by any other impediments. For, because the motions are equally changed, the changes of the velocities made toward contrary parts are inversely proportional to the bodies. This law also takes place in attractions and will be proved in the next scholium."
-Law III
Mathematical Principles of Natural Philosophy
I have a tendancy to resort to frames fo reference to resolve these kinds of ideas. Look at it from the rock's point fo view; it is moving forward with a certain amount of momentum (the "force" to which you refer). From the rock's point of view, it is not moving at all. The rock hits the wall, and experiences a tremendous jolt. The wall experiences exactly the same jolt. But the wall, being much more massive, doesn't move much, so the rock moves alot.
When hitting a window, the rock experiences a much lesser jolt. This slight jolt is not enough to impart much acceleration to the rock, but for the tiny shards of glass it is sufficient force to accelerate them quite a bit.
In each case, the same amount of force is applied to the rock and the obsticle, and both accelerate in the direction opposite one another. And the difference in acceleration is determined by the difference in the masses of the two objects involved in the colision.
Does that help any?
Archosaur
Feb17-09, 02:21 AM
I find that, as a general rule of thumb, if you ever think one of Newtons laws is wrong, don't tell anyone.
redargon
Feb17-09, 02:48 AM
Ok, here goes, with an example (your example in fact):
The rock hits the wall and bounces off (here we're assuming an infinitely hard rock and wall). The energy is conserved. The force on the wall from the rock is the same as the force on the rock from the wall and in oppsite directions at the point and exact moment of impact
Now you throw your infinitely hard rock through a window. At the point and exact moment of impact the force on the rock is equal and opposite to the force on the glass. Because this force is not large enough to stop the rock, the window breaks and momentum (motion) is given to all the glass shards (they will all move in different directions and at different velocities depending on their mass) and the same amount of momentum (motion) will be removed from the rock, so it will slow down slightly and may change direction or begin rotating. The rock will eventually hit something else or more things and come to a complete stop when it has transferred all its momentum to those things. At every moment and exact point of impact there will be a force on the rock from the object and a force from the object on the rock. This force will be equal and will act in opposite directions. The question if the rock will stop or not is only related to how much of its momentum it will give to the objects that it hits.
If this is not clear for now, don't worry, it'll become clear as you advance in your studies. Unlike archosaur (no offence archosaur and you're right in 99.9% of the cases (rule of thumb, like you say)) I think that it's ok to question everything, even Newton's laws, but keep an open mind and allow people with experience to guide you to a better understanding of these and other laws and accept them when you find out how to use them better.
Archosaur
Feb17-09, 06:02 AM
no offence archosaur
None taken! I'm just having fun.
Great example, redargon!
HallsofIvy
Feb17-09, 07:07 AM
I find that, as a general rule of thumb, if you ever think one of Newtons laws is wrong, don't tell anyone.
Yes, they are liable to knock you UP!
(Our British friends may not get that joke.)
Redargon, How is there a "force on the rock is equal and opposite to the force on the glass"? Where does this force come from?
maverick_starstrider
Jul1-09, 08:47 PM
The glass.
maverick_starstrider
Jul1-09, 08:52 PM
You seem to have it in your head that just because something breaks that that in any way negates reactive forces. Take a wooden board and punch through it martial arts style. Yes you broke the board, but I bet your fist really hurts.
Cantab Morgan
Jul1-09, 09:01 PM
I find that, as a general rule of thumb, if you ever think one of Newtons laws is wrong, don't tell anyone.
LOL!
I hope I'm not derailing the thread, but Newton probably didn't mean for his third law simply to mean that F_{12} = - F_{21} as we commonly summarize it today. Elsewhere in Principia he applies it a bit more powerfully, anticipating D'Alembert's principle.
russ_watters
Jul1-09, 09:39 PM
How is there a "force on the rock is equal and opposite to the force on the glass"? Where does this force come from?
The glass. No: the rock hitting the glass. The force is due to the interaction of (collision between) the pair of objects. If the rock is thrown from the left and the glass is on the right, the rock exerts a force on the glass from left to right and the glass exerts a force on the rock from right to left. Equal and opposite.
Archosaur
Jul2-09, 12:25 PM
If rock hits glass -> glass hits rock.
Yes, the glass is made up of particles and since the glass is stationary it will put forth a resistive physical force. But how do you determine the opposite and equal force that the glass is putting forth against the rock's physical force? To say that the glass is putting forth an equal and opposite force against the rock's force is like saying that an eye of yours can put forth an equal and opposite force against a spear thrown at it. Are you going to tell me that it's impossible for a spear that's thrown to destroy a human eye because that eye will put forth a force equal and opposite to the spear's thrown force? Are you going to tell me that on September 11, 2001 airplanes could not have crashed into the World Trade Center buildings because the buildings' "opposite and equal" reaction forces cancelled the airplanes' impact forces and that the airplanes "simply bounced off the World Trade Center buildings"? Really, why are you so adamant to continue putting forth that "for every action there is an opposite and equal reaction"? What's really going on?
To say that the glass is putting forth and equal and opposite force against the rock's force is like saying that an eye of yours can put forth an equal and opposite force against a spear thrown at it.
Well, that's true! When the spear exerts a force on your eye, your eye exerts an equal and opposite force on the spear. So what?
Are you going to tell me that it's impossible for a spear that's thrown to destroy a human eye because that eye will put forth a force equal and opposite to the spear's thrown force?
:confused: What does exerting equal and opposite forces on each other--which always happens when two objects interact--have to do with the effects of such forces. If I smack an egg over my head, the forces they exert on each other are equal--but the egg is destroyed while my head is just fine, albeit messy. So what? Same thing with the spear and a human eye. The eye will not survive the experience unscathed, but the spear will.
DaleSpam
Jul2-09, 04:37 PM
To say that the glass is putting forth and equal and opposite force against the rock's force is like saying that an eye of yours can put forth an equal and opposite force against a spear thrown at it. The eye does in fact put forth an equal and opposite force against a spear thrown at it.Are you going to tell me that it's impossible for a spear that's thrown to destroy a human eye because that eye will put forth a force equal and opposite to the spear's thrown force?A given force will deform an eye much more than a spear. So the fact that the eye is destroyed and the spear is not does not in any way contradict the fact that the forces on each are equal.
EDIT: Doc Al beat me to it!
DaleSpam, how are those forces "equal"?
DaleSpam, how are those forces "equal"?
Again, you are mistaking the effect of a force on a particular body with the fact that the forces exerted are equal. The same force acting on different objects can easily have different effects.
Archosaur
Jul2-09, 06:57 PM
In your wonderfully morbid spear-eye example, the forces felt by both objects are equal, but an eye cannot handle as much force as the metal tip of a spear can, so it breaks.
maverick_starstrider
Jul3-09, 03:41 AM
Alright, Object X is thrown at material Y. If material Y experiences Z amount of force it will shatter. So if object X is chucked with a force GREATER THAN Z when it comes in contact with a material Y, X will exert a force Z (since it is all Y can muster) on Y (the result of this on Y will be that it shatters), Y will also exert an equal and opposite force Z on X (which, if the total collision/shattering took t seconds then this will result in object X loosing Z*t momentum), this may also dent X or what have you. However, if the force with which is much greater than Z it will still have a non zero momentum after its shattering collision with Y and will keep on going. It's like static friction, the force of static friction equals f_s \leq \mu_s N . Notice the less than equal to.
mordechai9
Jul4-09, 12:51 PM
It looks like everybody did a pretty good job of getting at this, but I thought I would just chip in my two cents with an alternative sort of answer.
First let's consider the throwing of the rock. Maybe you are familiar with the notion of a "free body diagram". Basically it just consists of a picture of the object in question, and arrows representing all the forces acting on the body. Now, if you draw a FBD of the rock when it's being thrown, this picture consists in the rock, and only the rock. In the picture, you will see a force arrow which represents the hand pushing on the rock. (You will also see an arrow pointing downwards representing gravity. Let's just ignore that for now.) Now at the same instant, if you draw a FBD of the hand (and only the hand), then you see an arrow of the same length, pointing in the opposite direction. This represents the rock pushing back on the hand. That's all that Newton's third law means.
If you do the same thing for the window and the rock at the initial point of impact, again, you will have a similar situation if you draw instantaneous pictures of the window or the rock. The window breaks because it is unable to sustain this force, which it feels in terms of a sudden, nonuniform increase in load. After the window shatters, the rock is no longer exerting a force on the window at all, so it is correct to say that there is not an equal and opposite reaction for a long time. The rock does not stop because the work done by the window is insufficient to fully dissipate the rock's kinetic energy.
Question: since the window is trying to push backwards against the flight of the rock, do you see bits of the window flying outwards, in the opposite direction? The answer is yes, I believe. However, the distribution of forces in the shattered glass is much more complicated, and it is not straightforward to predict the trajectories of the shards. Elastic and nonelastic deformation are complicated problems.
JanClaesen
Jul5-09, 07:16 AM
So the more momentum the rock has, the more force it will take to make that momentum change direction, because if the rock doesn't change direction, it will break the glass apart. The amount of force the glass can exert on the rock is an intrinsic property of the glass-rock interaction, so a piece of wood with the same momentum won't feel the same change in momentum, or force, because it's a different interaction.
Am I right?
JanClaesen
Jul7-09, 11:33 AM
So the more momentum the rock has, the more force it will take to make that momentum change direction, because if the rock doesn't change direction, it will break the glass apart. The amount of force the glass can exert on the rock is an intrinsic property of the glass-rock interaction, so a piece of wood with the same momentum won't feel the same change in momentum, or force, because it's a different interaction.
Am I right?
No one?
Is the law of Archimedes also a reaction force?
If it is: (m is the mass of the object I drop in a cup of water)
mg = pVg (p is mass density), so the mass of the object I drop in the water (pV) must be the same as the mass of the water, what's wrong here?
And what about Hooke's law?
So the more momentum the rock has, the more force it will take to make that momentum change direction, because if the rock doesn't change direction, it will break the glass apart. The amount of force the glass can exert on the rock is an intrinsic property of the glass-rock interaction, so a piece of wood with the same momentum won't feel the same change in momentum, or force, because it's a different interaction.
Am I right?
Not bad. I'll nitpick a bit: Force is not just change in momentum, it's the rate of change of momentum--change in momentum per unit time. I can throw a rock and a pillow against a wall with the same momentum. Assuming they come to rest, they both experience the same change in momentum, but the pillow-wall interaction will take longer and produce less force.
Is the law of Archimedes also a reaction force?
Not sure what you mean by that. (Certainly if the water pushes on the object, the object pushes back on the water with an equal and opposite force.)
If it is: (m is the mass of the object I drop in a cup of water)
mg = pVg (p is mass density), so the mass of the object I drop in the water (pV) must be the same as the mass of the water, what's wrong here?
Archimedes's principle says that the buoyant force (exerted by the water on the object) will equal the weight of the displaced water. Only if the object has the same density as water will its mass equal the mass of the displaced water.
And what about Hooke's law?
What about it?
JanClaesen
Jul7-09, 02:06 PM
On a second thought, those questions about the Archimedes force were rather stupid. :)
So about Hooke's law ;-), is that 'Hooke force' a reaction force on the force I pull with?
So about Hooke's law ;-), is that 'Hooke force' a reaction force on the force I pull with?
Any time you have something pulling (or pushing) something else, there will be a "reaction" force. Forces always come in pairs.
So if the spring pulls on you (with a force given by Hooke's law), you pull back on the spring with an equal and opposite force. Those two forces are "third law pairs".
So how does the glass window create enough force to be opposite and equal the rock's thrown kinetic force?
DaleSpam
Jul7-09, 03:42 PM
So how does the glass window create enough force to be opposite and equal the rock's thrown kinetic force?Fundamentally it is by the symmetry of the laws of physics. The known laws exhibit symmetry under spatial translations, this gives rise to the conservation of momentum, which is described by Newton's third law.
Could you clarify as to how the glass creates such a force? Like say give an example. So far I haven't seen any credible explanations and/or examples that VERIFY WITH EVIDENCE Newton's Third Law.
So how does the glass window create enough force to be opposite and equal the rock's thrown kinetic force?
What do you mean "enough force"? What do you mean by the rock's "kinetic force"?
Enough force of the glass as in an amount of force that can be equal in Newtons and opposite in direction to that of the rock's kinetic force (moving force, as in the force that is imposed on that rock that causes it to be thrown in a movement towards the glass that causes that glass to be broken).
Enough force of the glass as in an amount of force that can be equal in Newtons and opposite in direction to that of the rock's kinetic force (moving force, as in the force that is imposed on that rock that causes it to be thrown in a movement towards the glass that causes that glass to be broken).
The force used to create the rock's original momentum (before colliding with glass or anything else) is irrelevant. There's no such a thing as "moving force". You seem to think that something moving has a force attached to it that must be "overcome".
What the rock has is momentum. If it interacts with any object with any force, its momentum will be changed. If it interacts with any object with any force, the force that the object exerts on the rock will be equal and opposite to the force that the rock exerts on the object. That's how forces work.
AIR&SPACE
Jul7-09, 04:20 PM
So how does the glass window create enough force to be opposite and equal the rock's thrown kinetic force?
Unless it's bullet-proof glass or the like, it doesn't.
Your problem is that you assume that ANY interaction with the brick requires all of it's energy to be dissipated. This is not necessarily the case.
Let's look at a baseball being hit by a bat. A 100mph pitch when hit at the center of mass by a bat swung by Albert Pujols will hit out of the park. Now lets say that you don't really like Albert. So you decide to put a crack in the handle. You make sure that the crack is not easily detected -- the bat is exactly the same (mass, c.o.m etc) except for a very small crack. Now when Albert hits a pitch that just happens to be exactly the same, in exactly the same spot on the bat... he hits a ground ball to the shortstop and his bat breaks.
The reason is that the bat broke when the internal stresses, caused by the force being acted upon it, caused it to break.
This is exactly what's happening in your window case. The window shatters and then can't impart any more force on the brick.
AIR&SPACE
Jul7-09, 04:38 PM
Also, the earlier explanation of a karate chop WAS a credible explanation.
Let's say, for kicks and giggles, that if ANY BONE (for you or anyone else) experiences a force of 100 N you will break that bones.
NOW lets say that you have a friend hold up a piece of paper. You punch it and your hand easily goes through. Unfortunately, the paper doesn't stop your hand which goes on to hit your friend in the nose with 256 Newtons of Force. YOU have a broke hand, and HE has a broken nose. THIS is the force pair. It's not, "YOUR hand only has 20 N of force on it while HIS nose has 492 N."
http://i28.tinypic.com/2ij19hj.jpg
That is what a force-pair is. Because I STATE there is a 256 N force on his nose directly caused by your fist, then we know there MUST BE an equal and opposite reaction on your fist.
AIR&SPACE, And how exactly would that equal and opposite reaction be on my fist?
AIR&SPACE
Jul7-09, 04:53 PM
Because your fist is the object imparting the force on the nose. Where else would it show up?
AIR&SPACE
Jul7-09, 04:58 PM
Another example is this.
Have a friend hold a mechanical scale like one uses to measure themselves on. Not a digital scale... mechanical.
Now, you pick one up as well. Make sure you can both read the weight. Remember weight is a force. Then try all sorts of different combos of pushing them together. They should always read, within reasonable error, the same value.
No, my fist would not be broken because I'd be wearing an iron gauntlet with padding on the inside to protect my fist. Therefore, no equal reaction force on my fist to get it broken. That means that Newton's Third Law is invalid. Also, if there was really an opposite and equal reaction for every action, then why is it that we all look differently? Seems like if Newton's Third Law was really valid, then since the beginning of time a chain reaction would occur in which there should be only two colors -one the action color and the other the reaction color.
AIR&SPACE
Jul7-09, 05:11 PM
Well, I would say the reason for padding is to distribute the force (this is how bullet proof vests work)
But I think it is obvious that you have no real intention of using this thread/forum for any mature purpose.
I broke nose with force (action), my fist encountered very little force (reaction). I'd call that unequal. And what exactly did you mean by mature?
DaleSpam
Jul7-09, 05:29 PM
So far I haven't seen any credible explanations and/or examples that VERIFY WITH EVIDENCE Newton's Third Law. ... That means that Newton's Third Law is invalid.Wow! Usually this kind of comment comes from the Anti-Relativity Crackpots, this is the first time that I have ever encountered a Anti-Newton Crackpot. Here is a brief and very incomplete list of evidence:
Rockets
Billiards
Car crashes
Newton's cradles
Recoil
Freshman physics labs
Have a friend hold a mechanical scale like one uses to measure themselves on. Not a digital scale... mechanical.
Now, you pick one up as well. Make sure you can both read the weight. Remember weight is a force. Then try all sorts of different combos of pushing them together. They should always read, within reasonable error, the same value.Very good example. He could also use a force gauge under the same principle.
Newtype, why don't you perform the experiment suggested by AIR&SPACE. There is no need to trust us, you can verify the validity of Newton's 3rd law yourself. If you do not like his experiment then you can look up lots of Freshman physics lab experiments on the internet.
No, my fist would not be broken because I'd be wearing an iron gauntlet with padding on the inside to protect my fist.
So? If the gauntlet hits the nose with a force of X, the nose hits the gauntlet with a force of X.
Therefore, no equal reaction force on my fist to get it broken.
You fist exerts a force on the inside of the gauntlet--and the gauntlet exerts an equal and opposite force on your hand. So?
That means that Newton's Third Law is invalid.
:rolleyes: Just because you don't understand it, doesn't make it invalid.
Also, if there was really an opposite and equal reaction for every action, then why is it that we all look differently?
Huh?
Seems like if Newton's Third Law was really valid, then since the beginning of time a chain reaction would occur in which there should be only two colors -one the action color and the other the reaction color.
What may be confusing you is the rather old-fashioned terminology of "action" and "reaction". In everyday English, those terms imply that the action comes first and causes the reaction, but that's not how it works in physics. Better to use the term "third law pair" in describing the relationship between the two forces in Newton's third law.
russ_watters
Jul7-09, 06:17 PM
We should really slow this down because certain important points have been ignored: The force used to create the rock's original momentum (before colliding with glass or anything else) is irrelevant. Let that sink in for a moment, Newtype.....
Next: There's no such a thing as "moving force". You seem to think that something moving has a force attached to it that must be "overcome". Expanding, an object in motion needs no force to maintain that motion. There is no force associated with constant speed motion.
These two basic ideas are really what the entire misunderstanding you have is about and you gave no response to either, you simply ignored them. You are claiming that the glass must do something that it really doesn't have to do. Going further and explaining what "momentum" is won't help until you accept the reality of those two points first.
russ_watters
Jul7-09, 06:23 PM
Wow! Usually this kind of comment comes from the Anti-Relativity Crackpots, this is the first time that I have ever encountered a Anti-Newton Crackpot. Here is a brief and very incomplete list of evidence:
Rockets
Billiards
Car crashes
Newton's cradles
Recoil
Freshman physics labs As an engineer, my favorite and perhaps the most relevant to the thread is the Charpy impact tester: http://en.wikipedia.org/wiki/Charpy_impact_test
It measures the amount of energy absorbed by an object when it breaks upon being struck by another object. It is a breathtakingly simple and elegant device: it is basically an axe on a fulcrum, turning it into a pendulum. You start it from a known height, it hits and breaks an object, then swings up to a new height. Measuring the difference in height and applying e=mgh gives you the energy.
Note that in most cases for objects in a charpy test, energy is more relevant that momentum: the objects being struck don't move, they just break. In the case of glass (the original example), the fracture energy is so low it is probably irrelevant, so the final momentum of the glass shards is more relevant.
Newtype, the window DOES exert an equal and opposite force, but ONLY to it's own limit to withstand the impacting force of the rock.
Thus, to balance things off, the impacting force continues through the window, yet exiting with a reduced force EXACTLY as less as the window inhibited it.
Hopes that makes any sense...Newtype was right, it's simple. Looks like Newton's third law needs a modification. Every action has an equal and opposite reaction unless the acting and reacting bodies do not deform. when the mass is thrown at the window, the mass has to do extra work in breaking the binding forces,so the conservation of energy may be obeyed by estimates.Moreover, the mass is going through air molecules and if they always exerted equal and opposite reaction the mass would not move.
russ_watters
Jul8-09, 12:51 AM
That's all just plain wrong. Every bit of it. You're making the same basic mistakes as Newtype.
ideasrule
Jul8-09, 01:02 AM
Newtype was right, it's simple. Looks like Newton's third law needs a modification. Every action has an equal and opposite reaction unless the acting and reacting bodies do not deform. when the mass is thrown at the window, the mass has to do extra work in breaking the binding forces,so the conservation of energy may be obeyed by estimates.Moreover, the mass is going through air molecules and if they always exerted equal and opposite reaction the mass would not move.
Umm, but the air molecules do exert equal and opposite reaction forces. That's where air resistance comes from.
That's all just plain wrong. Every bit of it. You're making the same basic mistakes as Newtype.
Then could you explain, if according to Newton's third law, everything in all cases exerts equal and opposite forces to any magnitude of force how would the air molecules move to make way for the mass, or why should the window in your example break?
ideasrule
Jul8-09, 03:08 AM
Then could you explain, if according to Newton's third law, everything in all cases exerts equal and opposite forces to any magnitude of force how would the air molecules move to make way for the mass, or why should the window in your example break?
The mass exerts the same force on the air as the air exerts on the mass; it's just that this force is enough to push tiny air particles away but not enough to deflect the large mass. The mass exerts the same force on the window as the window does on it, but glass is more fragile, so this force is enough to shatter glass but not the mass.
The mass exerts the same force on the air as the air exerts on the mass; it's just that this force is enough to push tiny air particles away but not enough to deflect the large mass. The mass exerts the same force on the window as the window does on it, but glass is more fragile, so this force is enough to shatter glass but not the mass.
Let's think. The moment the mass is thrown, it encounters gas molecules all over its front hemisphere(if it is spherical) and each of it exerts a force equivalent to what it experiences according to its distance from the centre of the hemispherical surface, in exactly the opposite direction so the forces cancel out and there is no net force so no motion possible anywhere in the atmosphere and all gravity has no result except the experience of weight the motion of anything that comes in contact with anything else is retarded light does not propogate nor does sound.No electricity.Even inertia means absolute opposite force by the molecules in the system itself.
Does this new theory based on Newton's third law look good? It violates his other laws too, because there is no force.
Completely futile work,definitely.
The actuality is this:
Some of the energy is imparted to the air molecules so they are set in motion and the energy of the mass decreases by a negligible amount.If there was exactly equal and opposite reaction, the scenario doesn't quite fit to observe the conventional action-reaction.
Let's think. The moment the mass is thrown, it encounters gas molecules all over its front hemisphere(if it is spherical) and each of it exerts a force equivalent to what it experiences according to its distance from the centre of the hemispherical surface, in exactly the opposite direction so the forces cancel out and there is no net force so no motion possible anywhere in the atmosphere and all gravity has no result except the experience of weight the motion of anything that comes in contact with anything else is retarded light does not propogate neither does sound.No electricity.Even inertia means absolute opposite force by the molecules in the system itself.
Comically incorrect. :rolleyes: The equal and opposite forces involved in Newton's third law never cancel out--they act on different bodies.
Does this new theory based on Newton's third law look good? It violates his other laws too, because there is no force.
Completely futile work,definitely.
Futile? Yes. Based on Newton's third law? Not even close.
The actuality is this:
Some of the energy is imparted to the air molecules so they are set in motion and the energy of the mass decreases by a negligible amount.If there was exactly equal and opposite reaction, the scenario doesn't quite fit to observe the conventional action-reaction.
It's true that energy is transferred from the mass to the surrounding air, but that says nothing about how Newton's third law operates.
Comically incorrect. :rolleyes: The equal and opposite forces involved in Newton's third law never cancel out--they act on different bodies.
Futile? Yes. Based on Newton's third law? Not even close.
It's true that energy is transferred from the mass to the surrounding air, but that says nothing about how Newton's third law operates.That still means the mass exerts force on an envelope of air ahead of it, and the latter exerts an equal amount of force on the mass, which can be lesser not to nullify the motion of the body only if the force by the body was less, which is contradictory.
That still means the mass exerts force on an envelope of air ahead of it, and the latter exerts an equal amount of force on the mass,
That's true.
which can be lesser not to nullify the motion of the body only if the force by the body was less, which is contradictory.
That's gibberish.
redargon
Jul8-09, 06:40 AM
what is being discussed by vin300 and Newtype has been moving further and further away from force reactions and resultants and closer and closer to energy and momentum. I think both of you need to do some reading up on conservation of momentum (mass multiplied by velocity) and conservation of energy.
That's true.
That's gibberish.If I say the body exerts 5N force on the envelope of air ahead of it you agree that the envelope of air exerts an equal 5N force on the body exactly in the opposite direction. Be it the envelope of air or a denser wall, the force exerted by the body on any of them and the reaction force remain the same, still the body is reflected by the wall and not by air simply because the air is free to move and the wall isn't.
If newton's third law is to be obeyed, where is the result of the 5N force exerted by the air on the body?
If you say that energy is lost due to air resistance that is not the satisfactory explanation of the law.
If I say the body exerts 5N force on the envelope of air ahead of it you agree that the envelope of air exerts an equal 5N force on the body exactly in the opposite direction.
True.
Be it the envelope of air or a denser wall, the force exerted by the body on any of them and the reaction force remain the same, still the body is reflected by the wall and not by air simply because the air is free to move and the wall isn't.
No. The body is reflected by the wall because the wall exerts a greater force on the body than does the air.
If newton's third law is to be obeyed, where is the result of the 5N force exerted by the air on the body?
The result of the 5N force exerted by the air on the body is given by Newton's 2nd law. That force accelerates the body, slowing it down.
If you say that energy is lost due to air resistance that is not the satisfactory explanation of the law.
That has nothing to do with Newton's 3rd law.
True.
No. The body is reflected by the wall because the wall exerts a greater force on the body than does the air.So the wall exerts a greater force than the air while the action forces are the same. Isn't that a violation of newton's third law?
The result of the 5N force exerted by the air on the body is given by Newton's 2nd law.
That's kiddish.
So the wall exerts a greater force than the air while the action forces are the same.
What do you mean by "the action forces are the same"? When the body hits the wall, the body and wall exert equal and opposite forces on each other. Same deal when the body hits the air. Of course, the forces involved are different in each case.
Isn't that a violation of newton's third law?
No.
As russ_watters emphasized in post #53, you really need to go back to the points I raised in post #42. You have some basic misconceptions about motion and forces.
What do you mean by "the action forces are the same"? When the body hits the wall, the body and wall exert equal and opposite forces on each other. Same deal when the body hits the air. Of course, the forces involved are different in each case.
No.
As russ_watters emphasized in post #53, you really need to go back to the points I raised in post #42. You have some basic misconceptions about motion and forces.
I got it.when the body hits the wall it loses momentum in very less impact time to the wall so the force is too much, and the wall reacts with equal force. When it hits a gas molecule only little of the momentum is imparted to it and considering the impact time, the force to the molecule is much lesser and it reacts only with this lesser force.
I had worked on this several times before, but every next time I make mistakes and I need to do it again. That's ridiculous!
Could you get some help for my "stationary shifts"problem?
Newtype
Aug12-10, 04:11 PM
I still don't understand. Momentum is basically force. Momentum equals mass multiplied by velocity, and force equals mass multiplied by acceleration (acceleration is the rate of change of velocity over time).
DaleSpam
Aug12-10, 04:27 PM
I still don't understand. Momentum is basically force. No, they have different units.
Momentum equals mass multiplied by velocity, and force equals mass multiplied by acceleration (acceleration is the rate of change of velocity over time).Yes, this is correct and contradicts your other statement.
Newtype
Aug12-10, 04:30 PM
It doesn't contradict my other statement because I've been saying throughout this topic that momentum (force) of a glass window is insufficient to stop a brick thrown at it.
Here's another way to look at what I'm saying: get with somebody else and have him/her poke out one of you eyes using a nail. You'll then become half blind and terrified. You poke a hole in his/her arm using a nail. I'm sure afterwards you wouldn't feel like you had experienced an opposite and equal reaction.
It doesn't contradict my other statement because I've been saying throughout this topic that momentum (force) of a glass window is insufficient to stop a brick thrown at it.
As already pointed out, you are confusing momentum and force. Nonetheless, perhaps you mean something like: "If you throw a brick through a window, the force that the window exerts on the brick was insufficient to stop the brick." Well, that's true. So what? It's still true that the force that the window exerted on the brick was "equal and opposite" to the force that the brick exerted on the window.
Here's another way to look at what I'm saying: get with somebody else and have him/her poke out one of you eyes using a nail. You'll then become half blind and terrified. You poke a hole in his/her arm using a nail. I'm sure afterwards you wouldn't feel like you had experienced an opposite and equal reaction.
I think part of your confusion is your use of the term 'reaction'--it doesn't mean what you think it means. "Equal and opposite reaction" is an old-fashioned term that means in physics "equal and opposite force". If I jab you in the arm (or eye!) with a nail, the nail and your arm (or eye!) exert equal and opposite forces on each other. Of course the consequences of that equal force are vastly different; that same force destroys your eye while the nail is unperturbed. Same thing with the brick and the window. The window smashes while the brick is unscathed--nonetheless they exerted equal and opposite forces on each other.
DaleSpam
Aug12-10, 05:40 PM
It doesn't contradict my other statementIt does contradct it. If two things have different units they cannot be the same.
Gokul43201
Aug12-10, 06:31 PM
Here's another way to look at what I'm saying: get with somebody else and have him/her poke out one of you eyes using a nail. You'll then become half blind and terrified. You poke a hole in his/her arm using a nail. I'm sure afterwards you wouldn't feel like you had experienced an opposite and equal reaction.Are you being serious with this example?
vlado_skopsko
Aug13-10, 04:08 AM
The wall and the glass are made from different materials and I guess have different thickness so they can exert some maximal pressure before they brake. If that force is not enough to stop the brick, to bring the bricks momentum to zero ( in perfect inelastic case ), then the stone will continue to travel with the rest of his momentum whatever it is.
So we know from the material the obstacle is made, what maximum force it can oppose to an external pressure ( because very important is the area on which its acted but if you use the same brick we will suppose that is the same area and it will cancel. ).
For example the maximum force the glass can exert is 10 N for that area of the brick. If you throw the brick with some force, you give some momentum on that brick, the glass does not care how much, when there is contact between them, the glass can oppose ( it can feel ) only that much (10 N ) so the stone will feel 10 N, so the momentum of the stone will change. If its enough to stop the brick or it will continue to travel with that smaller momentum, depends on the previous momentum of the brick.
Same for the wall.
The guys before me did excellent job trying to explain this, so i dont know if this will help but I but i tried.
So the wall exerts a greater force than the air while the action forces are the same. Isn't that a violation of newton's third law?
This is the problem. The action forces are not the same. The force the ball exerts on the air is much, much smaller than the force the ball exerts on the wall. The force is not an intrinsic property of the ball's motion - it is a property of each interaction, and the force will be different depending on what the ball is hitting.
If you throw a ball, and it hits a pillow, the force on the ball will be lower than the force on the ball if it hits a brick wall. If you throw the ball and it hits a piece of glass, the glass will push back on the ball just as hard as the ball pushes on the glass. The glass may break, because the glass is more fragile, but that doesn't negate newton's 3rd law. It just means that the glass took less force to break than would be required to stop the ball.
Are you being serious with this example?
Yes. Is there any way to physically measure a glass window exerting 10 N on a brick thrown at it and exerting 10 N on it? If so, then how?
And here's another example: a boulder attached to a scale (scale1) hits another scale (scale2). According to you scale2 will push up against that boulder and scale1 with an equal force that can be measured by scale1, but I'm guessing scale2 can't because that boulder smashes scale2 to pieces.
And here's another example: a boulder attached to a scale (scale1) hits another scale (scale2). According to you scale2 will push up against that boulder and scale1 with an equal force that can be measured by scale1, but I'm guessing scale2 can't because that boulder smashes scale2 to pieces.
And what does that have to do with anything? Just because you've managed to smash the scale to pieces says nothing about the fact that whatever hit the scale and scale itself exerted equal and opposite forces on each other.
You're still confusing the exertion of a force with the effect of the force.
Then clarify the distinction between an exertion of a force and effect of a force.
You seem puzzled by the fact that a brick smashing through a plate of glass can exert equal and opposite forces on each other. I can only imagine it's because the same force can have very different effects on different objects. A force X might smash the glass to bits, but the same force X might just barely slow the brick down.
And where's the opposite and equal reactions during Yogic Flying?
http://www.youtube.com/watch?v=NHwhGUo90jw
:rofl: OK, now we know you're joking. At least I hope so! All forces involved in "Yogic Flying", a.k.a. bouncing on the ground, comply with Newton's 3rd law. That video is complete crackpottery.
MagnetDave
Nov9-10, 02:31 PM
I hereby invoke the Moon Principle:
http://www.physicsforums.com/showthread.php?t=182413&page=9
It is no longer possible to explain this any more clearly.
A pane of glass can only exert so much force on a brick before it breaks, but the brick does not have to exert its full force on the glass to break it. If a brick has 10N of force, and the glass can only withstand 2N of force, before breaking, the brick can only exert 2N of the 10N of force it carries on the glass, the floor exerts the other 8N of force on the brick to stop it when it lands. Is this correct? I am trying to understand this myself. Thanks.
No, not really correct. Don't think of a brick as 'having force'. A moving brick doesn't 'have force', but it does have mass and momentum. Only when the brick interacts with something else are forces involved. And depending on what it interacts with will determine the size of those forces. For example, if the brick hits a wad of cotton, the forces produced will be small. But if it the brick hits a solid wall, then the forces will be much greater.
Archosaur
Nov9-10, 04:06 PM
And where's the opposite and equal reactions during Yogic Flying?
http://www.youtube.com/watch?v=NHwhGUo90jw
The action of their knees on the ground is opposed by the action of the ground on their knees. If it weren't, their knees would go through the ground.
Also, this is ridiculous. It's upsetting that National Geographic even entertains this. Show me that they're accelerating at anything less than g, and then you've got something, but I don't pay my cable bill just so I can watch people in their pajamas jump around on their knees.
If you want to learn about force pairs, fine, but bringing stuff like this into the thread is a great way to get it locked.
DaleSpam
Nov9-10, 05:03 PM
a boulder attached to a scale (scale1) hits another scale (scale2). According to you scale2 will push up against that boulder and scale1 with an equal force that can be measured by scale1Exactly correct. The two scales will measure the same force (obviously assuming that the forces are in the range that the scales can measure). This is what the 3rd law means.
Newtype
Nov12-10, 10:18 AM
Exactly correct. The two scales will measure the same force (obviously assuming that the forces are in the range that the scales can measure). This is what the 3rd law means.
You should test that over water (having scale2 on water, not ground). I have a theory that scale2 will be smashed to pieces or forced underwater and scale1 will measure nothing.
Assuming the scales are identical, they'll either both get crunched, or both not get crunched. What makes you think the scale attached to the rock wouldn't get crunched? It has the rock on top of it just as much as the scale attached to the ground does.
You should test that over water (having scale2 on water, not ground). I have a theory that scale2 will be smashed to pieces or forced underwater and scale1 will measure nothing.
What happens to each scale depends on the net force on it, among other things. Nonetheless, the forces two objects exert on each other are equal and opposite.
DaleSpam
Nov12-10, 10:43 AM
You should test that over water (having scale2 on water, not ground). I have a theory that scale2 will be smashed to pieces or forced underwater and scale1 will measure nothing.Many variations of this test have been performed. Your theory is wrong. Scale 1 will measure the same as scale 2.
Newtype
Nov23-10, 08:54 PM
So if a scale was attached to a brick and that brick was thrown through a glass window, the scale would have displayed the force of the glass as equal to the force of that thrown brick? I'd call that impossible.
And getting back to that Yogic Flying video, what's the opposite and equal reaction of that guy who is simply hovering in midair with his legs crossed?
pallidin
Nov23-10, 09:12 PM
And getting back to that Yogic Flying video, what's the opposite and equal reaction of that guy who is simply hovering in midair with his legs crossed?
It's called an iron-type rod poised secretly under the garments.
No magic at all, just deception.
So if a scale was attached to a brick and that brick was thrown through a glass window, the scale would have displayed the force of the glass as equal to the force of that thrown brick? I'd call that impossible.
Yep - the force of the glass on the brick would be the same as the force of the brick on the glass (and both would be equal to the breaking strength of the glass, since the glass broke).
So if a scale was attached to a brick and that brick was thrown through a glass window, the scale would have displayed the force of the glass as equal to the force of that thrown brick?
Yes.
I'd call that impossible.
Why?
And getting back to that Yogic Flying video, what's the opposite and equal reaction of that guy who is simply hovering in midair with his legs crossed?
There was only bouncing, no hovering. All rather silly. (There was a still photograph of someone in mid-air, though. :rofl:)
Nonetheless, as palladin points out, it's easy to fake such things.
sophiecentaur
Nov24-10, 06:00 AM
So if a scale was attached to a brick and that brick was thrown through a glass window, the scale would have displayed the force of the glass as equal to the force of that thrown brick? I'd call that impossible.
We are confusing Force with Impulse (change of Momentum).
There is no reason at all why the two forces should be the same. Your force on the brick can last much longer than the force of the glass on the brick. What could be the same would be the force times the time for which the force was acting. In any case, the brick through window scenario is not a good one to discuss because it does not specify at what speed the brick emerges on the other side. It will probably still have some momentum left so there's no answer yet.
sophiecentaur
Nov24-10, 06:02 AM
That Yogic thing. The equal and opposite forces when the guy is in the air are 1. the force pulling HIM down and 2. the force pulling THE EARTH up. Gravity is supplying the 'piece of string' between them.
So if a scale was attached to a brick and that brick was thrown through a glass window, the scale would have displayed the force of the glass as equal to the force of that thrown brick? I'd call that impossible.
Perhaps there's some confusion as to what forces you are talking about. Are you, as sophiecentaur suggests, comparing these two forces:
(1) the force that your hand exerted on the brick when you threw it with
(2) the force that the brick exerts on the glass when they collide
Those forces are not connected by Newton's 3rd law (they are are not 'equal and opposite reactions') and have no simple relationship to each other.
The Newton's 3rd law pairs would be:
For (1): The force that your hand exerts on the brick is equal and opposite to the force that the brick exerts on your hand.
For (2): The force that the brick exerts on the glass is equal and opposite to the force that the glass exerts on the brick.
sophiecentaur
Nov24-10, 06:39 AM
Yes yes yes
Your force on the brick can last much longer than the force of the glass on the brick. What could be the same would be the force times the time for which the force was acting. .
The first part isn't right, the force on the glass can exist only as long as that on the brick, because only an action can cause a reaction.From what follows,the second statement is true.
sophiecentaur
Nov25-10, 03:17 AM
I wasn't, perhaps, clear enough. By "your force", I was referring to the force you exerted on the brick and not 'yer force', in the Irish sense. If I had meant the force from the glass on the brick, then I would have said it was equal to the force of the brick on the glass.
But my intro emphasised the difference between force and impulse - so you could have inferred what I really meant.
(Edit) When you throw a brick, the brick isn't making contact with the glass at same time so the forces can be different.
And here's another question: what was the opposite and equal reaction when Earth stopped rotating during the Chilean Earthquake and then started rotating again?
Archosaur
Dec1-10, 07:59 PM
And here's another question: what was the opposite and equal reaction when Earth stopped rotating during the Chilean Earthquake and then started rotating again?
A force pair formed at the point of contact between "What the f%$k" and "are you talking about?!"
Yogic flying?!
The Earth stopping?!
When will this thread die?
Also, what newspaper do you get?
And here's another question: what was the opposite and equal reaction when Earth stopped rotating during the Chilean Earthquake and then started rotating again?
Don't believe everything you read in the tabloids.
CNN covered the story of how scientists detected how Earth stopped spinning during the recent Chilean earthquake (it was for 1.26 microseconds).
http://www.google.com/search?hl=en&source=hp&q=Chilean+earthquake+stopped+rotating+Earth&aq=f&aqi=&aql=&oq=&gs_rfai=
Archosaur
Dec1-10, 09:00 PM
That is not at all what that article says.
CNN covered the story of how scientists detected how Earth stopped spinning during the recent Chilean earthquake (it was for 1.26 microseconds).
http://www.google.com/search?hl=en&source=hp&q=Chilean+earthquake+stopped+rotating+Earth&aq=f&aqi=&aql=&oq=&gs_rfai=
The earthquake didn't stop the earth for 1.26 microseconds and then start it again. That would be impossible, and it would also cause incredible devastation if it were possible. What it did is it changed the rotation rate of earth, so days are (permanently) 1.26 microseconds shorter than they were before. This didn't require any force, since the earth's angular momentum didn't change. What did change was the earth's mass distribution, which changed the moment of inertia.
Oh, and linking a google search on a topic does NOT count as evidence for it, especially when none of the top search results actually support what you are saying. Link directly to the page making the claims please.
And here's another question:
Let's stick to your 'brick through the window' example, which has the benefit of being real. Please respond to my questions in post #95.
DaS Energy
Dec2-10, 05:39 AM
Hello Newtype,
A point of rock throwing the equal opposite effect was in play.
At point of rock touching window equal opposite forces came into play.
The fact the rock did not bounce back but passed through did not take away the equal opposite force as the rock first contacted the glass
Cheers
Peter
DaleSpam
Dec2-10, 07:02 AM
So if a scale was attached to a brick and that brick was thrown through a glass window, the scale would have displayed the force of the glass as equal to the force of that thrown brick? I'd call that impossible. Have you any experimental evidence to support that assertion?
Newtype
Feb26-11, 12:47 PM
Perhaps there's some confusion as to what forces you are talking about. Are you, as sophiecentaur suggests, comparing these two forces:
(1) the force that your hand exerted on the brick when you threw it with
(2) the force that the brick exerts on the glass when they collide
Yes.
Those forces are not connected by Newton's 3rd law (they are are not 'equal and opposite reactions') and have no simple relationship to each other.
The Newton's 3rd law pairs would be:
For (1): The force that your hand exerts on the brick is equal and opposite to the force that the brick exerts on your hand.
For (2): The force that the brick exerts on the glass is equal and opposite to the force that the glass exerts on the brick.
So why is it that in the movie Sidekicks, Jonathan Brandis was able to smash stacks of bricks with his hand and that the bricks didn't obviously exert equal and opposite reactions onto his hand enough to break it?
DaleSpam
Feb26-11, 12:49 PM
Don't be stupid. It's movie special effects, not experimental data.
Newtype
Feb26-11, 12:52 PM
I'm not being stupid. I simply responded and then asked a question. Watch that movie Sidekicks if you don't believe me. In that movie, Jonathan Brandis uses his hand to karate break stacks of bricks.
DaleSpam
Feb26-11, 12:54 PM
You are being completely stupid if you think that everything which is depicted in a movie is real and scientifically plausible. Have you never watched "Mythbusters"?
Get some real data, not a movie reference.
sophiecentaur
Feb26-11, 12:58 PM
Clearly the force that broke the bricks just wasn't enough to break his hand. There are no special dispensations for karate exponents to violate N3. You must look elsewhere for your explanation.
DaleSpam
Feb26-11, 01:00 PM
Here is a series of questions for you Newtype.
A 1 kg toy boat is floating in a still bucket of water.
1) How much force does the toy hull exert on the water?
2) How much force does the water exert on the hull?
3) How much did the water deform?
4) How much did the hull deform?
Newtype
Feb26-11, 01:05 PM
Brandis's hand wasn't broken in that movie.
AlephZero
Feb26-11, 01:07 PM
So why is it that in the movie Sidekicks, Jonathan Brandis was able to smash stacks of bricks with his hand and that the bricks didn't obviously exert equal and
opposite reactions onto his hand enough to break it?
The reason is (and I'm not just being facetious) that bricks are hard lumps of baked clay with lots of tiny internal cracks in them, and hands are fairly soft squidgy thngs.
A large force applied for a very short amount of time is enough to join up the cracks inside a brick and split it into two. If you hadn't done any training beforehand, the same force applied to your hand for the same length of time would hurt and bruise you, but it wouldn't "break" anything.
It hurts a lot more if you pull the punch and don't break the brick, because then the force between your hand and the brick continues for a much longer time than if the brick "instantly" breaks.
The only "trick" involved in this is to convince your brain that you can actualy do it. That may take years of training, of course.
sophiecentaur
Feb26-11, 01:44 PM
@newtype
You may find it more fruitful not to look for flaws in accepted Science before you have actually understood what it tells you. When you are a 'big boy' like Albert I or Isaac N you may be in a position to shake the foundations but Not Yet, I think.
JaredJames
Feb26-11, 02:31 PM
Yes.
Then you are looking at and comparing the wrong forces. They are completely independent. This is where your misunderstanding stems from.
Your links so far are "Yogic Flying", "Earth stopping" and "movie science". Do you want people to take you seriously?
So why is it that in the movie Sidekicks, Jonathan Brandis was able to smash stacks of bricks with his hand and that the bricks didn't obviously exert equal and opposite reactions onto his hand enough to break it?
Two points (for real life, not just movie effects):
(1) The force exerted by the bricks on the hand is exactly equal and opposite to the force the hand exerts on the bricks.
(2) Just because the force is equal doesn't mean the effect of the force is equal. (I've broken bricks and wood with my hands--my hands didn't break.)
JaredJames
Feb26-11, 03:27 PM
I don't think there is any way to put this differently for the OP, but here goes:
You throw a brick with force X, the brick exerts is force back on your hand (this is one equal and opposite force pair).
The brick hits a window with force Y and the window exerts force Y back on the brick (this is another separate equal and opposite force pair). Because the window breaks, force Y = the maximum force the window could take.
Force X that the brick was thrown with does not have to equal force Y. It can equal it, such as when it hits a wall, but in the case of a window smashing X doesn't equal Y.
This is a key point that you are missing.
The force the brick imparts on the window does not have to be equal to the force it was thrown with. They are separate events and not connected with Newton's third law.
I find that, as a general rule of thumb, if you ever think one of Newtons laws is wrong, don't tell anyone.
Is it the 3rd law
What the laws which we declared as universal are not applicable at all the portions of the universe. With out knowing at least 1% about the universe, how we can declare a law as universal.
How many laws are applicable at the center of the black hole?
Archosaur
May4-11, 10:17 AM
When will this thread die?
Good question, me-from-5-months-ago.
In fact, this question is even more pertinent today, considering that this mediocre thread has been sporadically active for... roughly 9% of my life.
*Sigh*... see you all in another 5 months.
DaveC426913
May4-11, 10:28 AM
Is it the 3rd law
What the laws which we declared as universal are not applicable at all the portions of the universe. With out knowing at least 1% about the universe, how we can declare a law as universal.
How many laws are applicable at the center of the black hole?
You are being far too literal, and you are assuming the laws are applied literally too.
We know the context in which the laws hold, and we know that they do not hold everywhere. You don't need to go to the centre of a BH to find that.
Two objects moving toward each other do not simply add their velocities, they use relativistic velocity addition, even at human speeds. However, the effect is so vanishingly small that, unless we are dealing with objects moving at near relativistic velocities (or time spans where it's relevant, such as GPS navigation), we do not have to factor it in.
It does not mean we just throw the laws out the window.
Bloodthunder
May4-11, 11:55 AM
A Scientific Law is sort of defined to be an observation that will always occur when under specific conditions. So... we don't throw laws out of the window, we have a new set of them to use under different conditions.
Libohove90
May30-11, 01:36 AM
I still don't understand. Momentum is basically force. Momentum equals mass multiplied by velocity, and force equals mass multiplied by acceleration (acceleration is the rate of change of velocity over time).
No, momentum is not force. The rate of change of momentum is force.
Suppose you had an object that is 10 kg and moving at 10 m/s.
It's momentum is (10 kg)(10 m/s) = 100 kg m/s.
Force is defined as the rate of change of momentum. This moving object is not accelerating, it's just moving at a constant velocity in a straight line. So what's its force?
F = ma = (10 kg)(0 m/s^2) = 0 N.
This object exerts no force, but has momentum of 100 kg m/s. Momentum and force are completely different.
sophiecentaur
May31-11, 04:07 PM
Some of you guys really crack me up. You seem to argue against some of the most fundamental ideas in basic Physics when you just haven't done your homework. Go back to basics and learn them. Only then should you feel qualified to 'question' stuff. This isn't a subjective subject. It's hard and as objective as it can possibly be.
vBulletin® v3.8.7, Copyright ©2000-2012, vBulletin Solutions, Inc.