A question about opposite and equal reactions

[mordechai9]

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]

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]

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?

 

[Doc Al]

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]

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?

 

[Doc Al]

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".

 

[Newtype]

So how does the glass window create enough force to be opposite and equal the rock's thrown kinetic force?

 

[Dale]

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.

 

[Newtype]

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.

 

[Doc Al]

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"?

 

[Newtype]

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).

 

[Doc Al]

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]

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 let's 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]

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 let's 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."

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.

 

[Newtype]

AIR&SPACE, And how exactly would that equal and opposite reaction be on my fist?

 

[AIR&SPACE]

Because your fist is the object imparting the force on the nose. Where else would it show up?

 

[AIR&SPACE]

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.

 

[Newtype]

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]

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.

 

[Newtype]

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?

 

[Dale]

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.

 

[Doc Al]

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.

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]

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]

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.

 

[vin300]

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]

That's all just plain wrong. Every bit of it. You're making the same basic mistakes as Newtype.

 

[ideasrule]

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.

 

[vin300]

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]

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.

 

[vin300]

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.