A question about opposite and equal reactions

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Newton's third law of motion states that for every action, there is an equal and opposite reaction, which applies to interactions between objects. When a rock is thrown at a wall, it bounces back due to the equal and opposite force exerted by the wall. However, when the rock hits a window, the window may not exert enough force to reverse the rock's momentum, resulting in the rock breaking through instead. The forces are still equal and opposite, but the material properties of the window limit its ability to respond like a wall. Understanding these interactions clarifies that Newton's law remains valid across different scenarios, despite variations in material resilience.
  • #51


Newtype said:
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

AIR&SPACE said:
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.
 
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  • #52


Newtype said:
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.
 
  • #53


We should really slow this down because certain important points have been ignored:
Doc Al said:
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.
 
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  • #54


DaleSpam said:
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.
 
  • #55


pallidin said:
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.
 
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  • #56


That's all just plain wrong. Every bit of it. You're making the same basic mistakes as Newtype.
 
  • #57


vin300 said:
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.
 
  • #58


russ_watters said:
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?
 
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  • #59


vin300 said:
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.
 
  • #60


ideasrule said:
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.
 
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  • #61


vin300 said:
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.
 
  • #62


Doc Al said:
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.
 
  • #63


vin300 said:
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.
 
  • #64


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.
 
  • #65


Doc Al said:
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.
 
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  • #66


vin300 said:
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.
 
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  • #67


Doc Al said:
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.
 
  • #68


vin300 said:
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.
 
  • #69


Doc Al said:
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?
 
  • #70


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).
 
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  • #71


Newtype said:
I still don't understand. Momentum is basically force.
No, they have different units.


Newtype said:
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.
 
  • #72


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.
 
  • #73


Newtype said:
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.
 
  • #74


Newtype said:
It doesn't contradict my other statement
It does contradct it. If two things have different units they cannot be the same.
 
  • #75


Newtype said:
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?
 
  • #76


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 don't know if this will help but I but i tried.
 
  • #77


vin300 said:
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.
 
  • #78


Gokul43201 said:
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.
 
  • #79


Newtype said:
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.
 
  • #80


Newtype said:
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?

:smile: 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.
 
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  • #82


Jasongreat said:
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.
 
  • #83


Newtype said:
And where's the opposite and equal reactions during Yogic Flying?


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.
 
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  • #84


Newtype said:
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
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.
 
  • #85


DaleSpam said:
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.
 
  • #86


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.
 
  • #87


Newtype said:
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.
 
  • #88


Newtype said:
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.
 
  • #89


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?
 
  • #90


Newtype said:
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.
 
  • #91


Newtype said:
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).
 
  • #92


Newtype said:
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. :smile:)

Nonetheless, as palladin points out, it's easy to fake such things.
 
  • #93


Newtype said:
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.
 
  • #94


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.
 
  • #95


Newtype said:
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.
 
  • #96


Yes yes yes
 
  • #97


sophiecentaur said:
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.
 
  • #98


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.
 
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  • #99


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?
 
  • #100


Newtype said:
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?
 

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