Is Newton's third law always correct?

Question

Newton's third law states that every force has an equal and opposite reaction. But this doesn't seem like the case in the following scenario:

For example, a person punches a wall and the wall breaks. The wall wasn't able to withstand the force, nor provide equal force in opposite direction to stop the punch.

If the force was indeed equal, wouldn't the punch not break the wall? I.e., like punching concrete, you'll just hurt your hand. Doesn't this mean Newton's third law is wrong in these cases?

See this question. http://physics.stackexchange.com/q/93599/ — mmesser314, May 28 '14 at 03:33
Not much related, but the strong form of Newton's third law doesn't hold in special relativity. — jinawee, May 28 '14 at 08:27
@Floris Specifically, that there is no conservation law for hurt. Like entropy, it always increases... — David Richerby, May 28 '14 at 15:00
Your thinking is correct! (also first post add a correct point about the objects on which forces act) The amazing and subtle detail you missed is MASS. Every object as you described will have same forces acting on it but objects will have different masses -> so by the 2nd law they will move differently. The mass of the broken peace of wall was smaller than yours -> that's why you stayed there and the wall moved there :D Simple. That is why all Newton law's have to be always applied together. They form a coherent system. — Asphir Dom, May 28 '14 at 18:16
possible duplicate of With Newton's third law, why are things capable of moving? — senshin, May 28 '14 at 18:28
Does this answer your question? If Newton's third law is true, why can we sink in sand? — knzhou, Apr 12 '20 at 17:42
Some intuition should help. If you punched the wall at 2000N, ideally you will feel 2000N hitting back at your fist as well, that's why fist hurts or breaks as well along with the wall. If Newton's third law was not acting here, wall would have broke nor deformed with that force but your fist would feel "nothing". — Meenie Leis, Feb 07 '23 at 12:15

Rocketmagnet · Accepted Answer · 2017-09-13T09:38:54.327

Despite 11 answers to this question already, I don't feel that any have answered the question well.

(Note: This answer is simplified and assumes the punch is slow enough to ignore inertia and relativity)

Firstly, let's look at force at the atomic level. This is where the force is really happening. The forces that we feel in everyday life are generally the forces between atoms and molecules (intermolecular forces). I'll use Helium atoms as an example, because they're easy to draw. When two He atoms get close together, their electron shells overlap and cause them to repel each other. Note that you never get a situation where one atom repels, and the other does nothing, or one repels and one attracts. Always they both repel each other, or both attract each other, and both atoms feel the same magnitude force, in exactly opposite directions.

Force between Helium atoms

The force they feel is a function of the distance between them. The force between them behaves basically like a spring. In the illustration above, the two atoms are repelling each other, and will accelerate away from each other. As they move apart, the force decreases, until at a certain point, it reaches zero, and we consider them not to be 'touching' any more.

Now imagine we start with one atom stationary, and throw another atom at it. When the moving atom gets close enough to the stationary one, they will feel the force of repulsion. Both will accelerate based on the force between them. They accelerate in opposite directions, so the stationary atom accelerates and flies off, while the moving one decelerates to a stop.

Molecules behave in a similar way towards each other.

Since a wall is made up of molecules, it behaves pretty much like the force between molecules, except in a solid object, neighboring molecules are bonded together, meaning that when you push them closer together, they repel, and when you pull them further apart, they attract. The wall is basically a very stiff spring. When you push on a wall, it bends.

Pushing on a bendy wall

Bending is the only way it can push back on you. Bending means that some of the molecules in the wall are pushed closer together, and some are pulled further apart. The harder you push, the more it bends. It bends just so that it's pushing back on you as hard as you're pushing. If you're pushing with a constant force, everything is in equilibrium, and all the force vectors acting on each molecule add up to zero, so nothing is accelerating.

If you push hard enough, you'll manage to stretch some molecules far enough apart that their bond breaks. At that point the force between them drops to zero. Now those molecules are not in equilibrium, and they will accelerate away from each other.

If you push hard enough, and the wall breaks, it's no longer bending, it's accelerating away from your hand, just like the atoms in the example above. As it accelerates away, the force between your hand and the wall decreases and reaches zero when your hand and the wall are no longer 'touching'.

Punching a wall

When you punch a wall, the forces you and the wall are feeling are entirely made up of the forces between atoms and molecules. So whether the wall stands or falls, Newton's 3rd law holds the whole time. The wall can only push back on your hand to the extent that it can bend without breaking.

But what if I push really hard on the wall?

The answer is you can't. You can put a lot of effort into the punch, but if you were to measure the actual force applied to the wall, it would increase up to the point, then the wall would break, then the force would drop back down to zero.

Newton's 3rd law doesn't mean that everything is indestructible.

Added:

If you haven't already discovered Veritasium's excellent YouTube channel, you should. He has a good video helping us to understand Newton's Third Law:

Thanks. Exactly everything I needed to know, and you went beyond that as well. Very interesting. — Jason, May 28 '14 at 22:52

Earth is a Spoon · Answer 2 · 2014-05-28T14:44:16.990

20

Nice question. It's a common confusion among many beginning students. When I push something, shouldn't it stay still as there's an equal and opposite reaction to counterbalance my force?

The Answer: The two forces in question act on two different bodies.

The resistance force of wall has nothing to do with its equal and opposite reaction. The reaction is acting on hand, not the wall itself to prevent its own motion.

edited May 28 '14 at 14:44

answered May 28 '14 at 03:28

Earth is a Spoon

4,978

1

So essentially. If your hand hits a wall hard enough to break the wall, the equal and opposite reaction force will likely be enough to break your hand. The key concept being, the hand applies the force and the hand suffers the reaction. Right? – Jodrell May 28 '14 at 16:17
@Jodrell Yes, that's correct. :) – Earth is a Spoon May 28 '14 at 16:23
So if someone punches a wall, and the wall breaks, the force is opposite and equal. But doesn't this sort of depend on how long the force acts on the wall? For example, if you ram a wall with an object with a constant force, the object will keep going after it smashes through the wall; so the bits of wall that were destroyed are no longer applying any force to the object, nor is the object applying any force to the bits of broken wall. This is correct, yes? – Jason May 28 '14 at 16:53
@Jason The moment you hit the wall, you suffer from the reaction at the same instant. For long duration force, it's better to think on differentials to avoid confusion. The destroyed part was destroyed after exerting reaction. – Earth is a Spoon May 28 '14 at 16:58
1

@SachinShekhar If you hit a wall, in the ideal case of a perfectly rigid wall what you claim is true, but it is not true if the wall breaks. This is simple to see , because energy is lost if the wall breaks, therefore the work done by the force on the punch and that due to the force of the wall resisting the punch will not be equal. – auxsvr May 28 '14 at 17:24
1

@auxsvr Newton's Third Law talk about forces, not work done by forces. Certainly, you're not invalidating rocket propulsion (which is in motion and yet follows Newton's Third Law). Are you? – Earth is a Spoon May 28 '14 at 17:29
@SachinShekhar Relative motion is irrelevant here, I'm talking about the magnitude and the type of the force. What you claim is true only in the case of forces of interaction that are conservative, which is obviously not the case for a broken wall, since heat is emitted. – auxsvr May 28 '14 at 17:47
@auxsvr If heat is emitted, it'd be counterbalanced with different displacement magnitudes. The force will always be equal and opposite. It fails only in the situation when force propagation isn't quick (Relativistic ju-ju). – Earth is a Spoon May 28 '14 at 18:00
@SachinShekhar For example, assume you pull a string that is not elastic. The tension will not be the same along its length. This is easy to see, especially where it breaks, where the tension is zero. – auxsvr May 28 '14 at 18:02
@auxsvr You aren't getting importance of Newton's Third Law even in non-ideal macroscopic world. If it was wrong, the sum of internal forces wouldn't be zero causing spontaneous acceleration of anything. – Earth is a Spoon May 28 '14 at 18:19
@SachinShekhar In the case of the broken wall actually there exists acceleration due to the heat emitted, but normally it is negligible. The difference with the internal forces of many solids is that they are of electrostatic nature, namely conservative, until the bond is broken. – auxsvr May 28 '14 at 19:53
@auxsvr I am talking about internal forces of system, not solids. – Earth is a Spoon May 28 '14 at 21:03
@SachinShekhar When you punch a wall, the forces exerted are mostly electrostatic. An ideal classical gas, for example, can be described by the hard sphere model, in which the forces are also conservative. – auxsvr May 28 '14 at 21:05
@auxsvr No, I am talking generally for your non-ideal macroscopic world where friction works too. – Earth is a Spoon May 28 '14 at 21:06
@auxsvr, I think you are confusing Newton's Third Law of Motion with Conservation of Energy. – Jodrell May 29 '14 at 08:11

score 5 · Answer 3 · answered May 28 '14 at 03:31

5

If the wall breaks, that just implies that it was not strong enough to resist the force of the push you tried to apply. It also means that you did not manage to apply the full force, as the wall broke before you reached that level.

The above should be slightly modified to take into account static vs dynamic friction. Static friction (without movement) is higher than the dynamic friction when the object first starts to move. This concept is familiar to anyone who has tried braking on a slippery road. With low brake force, the tyres keep rolling, and the contact point with the road does not move with respect to the road. If the brake force exceed the maximum friction the tyres can provide then the wheels lock up, and it suddenly feels as if the car shoots forward. At that point it is better to release the brakes and try again. Modern cars with ABS systems do this automatically, many times per second, and you can feel it as a juddering during an emergency stop.

The same can be true for the wall: the force to break the wall may be stronger than that required to push the pieces further apart.

answered May 28 '14 at 03:31

hdhondt

10,898

I have locked up the brakes several times on ice and I have never felt that "the car shoots forward". But it does feels strange when the car is moving sideways. – LDC3 May 28 '14 at 03:34
1

@LCD3 You need to deliberately try it: apply the brakes gently, and gradually increase pressure. At the point where the wheels lock, the brakes suddenly lose effectiveness, and that makes it feel as if the car shoots forward. The reverse effect happens when you accelerate until the engine hits the rev limiter: it feels as if you've hit a wall. – hdhondt May 28 '14 at 03:52
2

Sorry, it never felt like the "car shoots forward". It just never slowed down. – LDC3 May 28 '14 at 03:55
1

How is this addressing Newton's Third Law? – Earth is a Spoon May 28 '14 at 10:28
@Sachin Newton's third law is obeyed, because you can only apply as much force as the wall will take. If you try to apply more force, you can't because the wall disappears. Hence action = reaction. – hdhondt May 28 '14 at 11:03
1

Fully irrelevant (wrong) logic. – Earth is a Spoon May 28 '14 at 11:08
@LDC3 If you are stopping, and then stop stopping it will feel as though you speed up, even if you don't. Consider when you're in a car, and you have full control of the brakes. Start braking rather hard, and then release the brake. It will feel as if you just shot forward, the same feeling you get in the car when your brakes lock on ice. – Cruncher May 28 '14 at 14:13
1

Your car isn't really speeding up while on ice (unless maybe it's on a downhill). It's just that you expect the car to slow (and to feel thrown forward against your seatbelt), but it's missing, and you think the car is therefore accelerating because you're missing the cues that it's slowing down. Same thing with a hit baseball "speeding up" on a bounce -- it simply didn't slow down as [much as] expected (perhaps due to spin being transferred to forward speed), and people interpret that as "speeding up". – Phil Perry May 28 '14 at 14:26

score 3 · Answer 4 · answered May 28 '14 at 14:54

The wall cannot react completely by stopping the blow, but this is not the only way that it can react. It will transfer as much of the blow's energy back to your hand as possible, which is why your hand hurts. But once the wall has reached its limits there, it has to drain off the excess in other ways. Many objects can do this by moving -draining off the energy of the blow as kinetic energy- which is part of how the famous demonstration with the hanging metal balls works. But the wall is anchored in place, so it can't move. Objects can also deform to at least some degree, but the wall is likely made of material that cannot deform very much, so while a little energy still goes into deforming the wall, that can only go so far.

Another possibility is to break apart. Once this happens, at least some of the pieces are no longer anchored in place, and a lot of the energy of the blow can be transferred to them as kinetic energy. This is why small pieces of the wall go flying all over the place instead of staying in a neat pile near the remains of the wall.

Some energy also gets released to the air, first as the sound of the blow hitting the wall, and then as the sound of the wall breaking. Realistically speaking, some of the energy will also be released as heat. None of these is going to be a huge factor, compared to what gets transferred back into your arm or into the wall's destruction. But these remaining factors, and others, help to account for the energy that escapes the wall-fist system. Eventually, it all adds back up.

Another thing to consider is that once the wall breaks, it's no longer in the blow's path, which prevents the blow from applying any further force. This isn't so different from what happens when the object moves without breaking.

Lastly, as other answers have pointed out, Newton's Third Law is not universal. It's actually more of a special case. It happens to be a very large special case -it works well enough in our particular corner of the universe to be useful in everyday life situations- but there do exist cases where it doesn't hold. This just isn't one of them.

Full of misconceptions. You can't confuse Newton's Third Law with energy manipulations. The wall would exactly exert exactly equal and opposite reaction on hand. Heat, sound are consequences after that. When wall breaks, your subsequent force application may be less, but opposite reaction would always be equal. It doesn't lost as Kinetic Energy. — Earth is a Spoon, May 28 '14 at 16:40
I said that the reaction was equal: it always adds up in the end. But I don't understand how your comment interacts with conservation of energy: if the energy lost as heat and sound does not come from the force of the blow, then where does it come from? — The Spooniest, May 28 '14 at 22:26
Don't mix Force and Energy. Both work independently and are related with other variables too. — Earth is a Spoon, May 29 '14 at 00:01

score 2 · Answer 5 · answered May 28 '14 at 03:30

No, it does not mean that Newton's 3rd law is not correct. The wall pushed back (your hand hurts), but the force you applied broke the wall and pushed pieces forward. I will try to list the forces.

Hand pushes wall - wall pushes back
hand moves wall - wall resists moving
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ $ - sound is made

Did I miss anything?

score 1 · Answer 6 · answered May 28 '14 at 03:53

When a body exerts a force on another body, both being in contact, the other body also exerts an equal and opposite force at their contact.

The first body may push the second and make it move, which is because of a non-zero net external force. But the force the two bodies are exerting on each other (internally) at their contact are equal and opposite. The first block is able to move the second block because the external force one first block is greater than that on the second block.

The internal forces inside a body cancel out from Newton's third law.

score 1 · Answer 7 · answered May 28 '14 at 07:29

You can't apply to a wall the force bigger than it can withstand. If the wall breaks at some certain amount of newtons of the force applied then it follows that you applied this certain force. And of course the reaction of the wall equals the force applied.

score 0 · Answer 8 · answered May 28 '14 at 08:18

Almost off-topic, it's worth mentioning that Newton's laws only apply in a Galilean reference frame, which is rather utopic (making Newton's law an approximation of the reality… but what else is Physics anyway?)

In any case, other answers were right: the wall has a reaction from you (and it may break) and also applies a force on you (you may feel pain in your hand).

auxsvr · Answer 9 · 2014-05-28T17:14:00.690

Newton's third law is not always correct, contrary to what you may have heard. It is correct in the context of newtonian mechanics, because we assume then that point particles are described only by their mass, and symmetry and conservation of momentum of the system imply that the third law must apply for the case of a closed system, which the universe is defined to be. It does not apply in the case of macroscopic bodies in general, because they deform themselves, as you described; energy is dissipated, hence the system is not closed. Also, it does not apply in the case of electromagnetism, because the field carries part of the momentum of the system.

MycrofD · Answer 10 · 2014-05-29T09:32:43.090

0

(As suggested in Classical Mechanics by H. Goldstein, 3rd edition in chapter 1 : Survey of the Elementary Particles )

No, the third famous law is not always valid. As pointed out above, in the case of electromagnetism, take for an example, two charged particles A and B are in motion.

B is just travelling perpendicular to the path of A and is right on the axis of A's motion.

You can calculate Coulomb's force for one due to another. But try finding the magnetic force due to one on the other. You will find the Lorentz force (sum of electric and magnetic forces) on one is not equal to the other.

Voila! Newton's third law violated!!

Well, if the fields concept is taken into account, and it has to be, then the third law is improved and protected: no violation. But excluding that, we can say it is not a strong law.

edited May 29 '14 at 09:32

answered May 28 '14 at 14:29

MycrofD

553

Good someone brings it up. The reason is that you are forgetting the momenta on the fields. Nevertheless, it is not addressing the problem at hand. – Davidmh May 28 '14 at 15:01
I don't think this is correct, since Newton's law is saved by the fields, as pointed out by @Davidmh. Admittedly, it is somewhat of a matter of taste – Danu May 28 '14 at 15:13
Which is why we have 'strong law of action and reaction' and a 'weak law of action and reaction'! – MycrofD May 29 '14 at 05:02
And in the wiki page we have this.
"Newton used the third law to derive the law of conservation of momentum;[33] from a deeper perspective, however, conservation of momentum is the more fundamental idea (derived via Noether's theorem from Galilean invariance), and holds in cases where Newton's third law appears to fail, for instance when force fields as well as particles carry momentum, and in quantum mechanics."
– MycrofD May 29 '14 at 05:21
of course..saved by fields.. but the point is newton's third law is not complete on its own,right. We both wanna explain the same thing. We need the fields to save it. So, i think i made my point clear. And now i hope you feel my answer is correct. And this had to be brought. It is not completely off-topic, is it? @Davidmh I hope the downvote is removed. Thank you. – MycrofD May 30 '14 at 16:32

score 0 · Answer 11 · answered May 28 '14 at 14:43

For example, a person punches a wall and the wall breaks. The wall wasn't able to withstand the force, nor provide equal force in opposite direction to stop the punch.

Two separate events happened here.

First, your muscles accelerated your fist. The equal and opposite reaction was the rest of your body moving backwards. Since your body weighs a lot more than your arm it moves backward a lesser amount.

Now your fist has kinetic energy. When this is delivered to the wall, the wall will move away based on the mass x velocity of your fist vs. the inertia of the wall. Factor in the crush/breakability of the wall (and your fist) and we will eventually reach equilibrium. The debris (and/or blood) on the floor represents the expended energy of the event, but if you add everything up (and I do mean everything )it will all be centered in the same place as before.

Your body doesn't have to accelerate backward. You can lean into a punch and everything moves forward. In this case, the opposite reaction is your shoes rotating the earth behind you. — Cruncher, May 28 '14 at 15:18

score 0 · Answer 12 · edited Jun 04 '20 at 16:03

For example, a person punches a wall and the wall breaks. The wall wasn't able to withstand the force, nor provide equal force in opposite direction to stop the punch.

If the force was indeed equal, wouldn't the punch not break the wall? I.e., like punching concrete, you'll just hurt your hand. Doesn't this mean Newton's third law is wrong in these cases?

Nothing says that walls must provide enough force to stop a punch. In this case, the person's fist will carry on moving, through the space which the wall used to occupy, until some other object exerts a force on it.

For example, once the person's arm becomes fully extended and resists being stretched, that may stop their fist. Alternatively, if the punch was really strong, their whole body may get dragged along (and the Earth will accelerate a little in the opposite direction, like when someone jumps).

The fist will be slowed down, in direct proportion to the amount of force needed to break the wall, since force = mass * acceleration (remember that slowing down is still an acceleration!)

score 0 · Answer 13 · answered May 28 '14 at 20:18

0

I think that this third law is all about Conservation of Energy. When a person punches a wall and the wall breaks; some of the energy is transfered to the wall and the wall breaks because of not having strength to the punch force. Some of the energy turns to heat and internal energy, the effects of which is not that much obvious to the person, and some of the energy turns back to the person's body and turns to his internal energy which he feels some pain in his hand. Imagine if the person breaks his hand while punching the wall. That is because his bones do not have strength to dissipate the energy which comes back to his body. All is the matter of energy.

answered May 28 '14 at 20:18

enthu

101

Newton's third law is about equal & opposite forces, not energy. – Kyle Kanos May 29 '14 at 01:56
You are right, but, is not force multiplied by distance equal to work or energy? That why I think energy and force are somehow related to each other. – enthu May 30 '14 at 16:14
$W=\int \mathbf F\cdot d\mathbf x$ is the proper definition of work and in some cases, $\mathbf F=-\nabla V$, so certainly the two are related. However, the question is about Newton's 3rd law, not energy so what you say here seems fairly irrelevant if not wrong. – Kyle Kanos May 30 '14 at 16:17

score 0 · Answer 14 · answered Jun 17 '14 at 08:25

The reason it appears that Newton's 3rd law does not apply in your example, is that there is some confusion and wrong assumptions going on. It should be obvious that a wall can only provide a "reactive" force up to its "breaking limit," which depends on its dimensions and composition. Obviously, a thin wall made of sheet-rock will not "resist" as much as a thick wall made of steel! There is also an assumption that the wall is somehow "held in place." Otherwise, if the wall is allowed to move, the reactive force will be limited to the time the fist is in contact with the wall. In other words, if the wall is strong enough to resist the force (and immobile), it will provide a reactive force equal and opposite to the applied force.

Is Newton's third law always correct?

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