# Theoretical Problem - F=ma

When a force acts on an object, the object exerts on equal and opposite force - Newton's third law. Imagine this situation:

I am breaking down a wall with a hammer. I begin hitting the wall gently and this wall doesn't break. I then hit harder and the force exerted on the wall increases. As I'm hitting the wall harder, the wall begins to crumble and break.

What I want to know is:

- Does the wall have a certain threshold value at which point it cannot exert a force opposite and equal to the force I hit with and begins to crumble? The harder I hit, the more I exceed its threshold value and the more it crumbles and breaks.

-Why does the wall not fall down completely if I exert a force just above this theorized threshold value? Is it due to friction forces in the planes of atoms? Is it due to the composition of the wall itself? Or is it due to the way atoms are arranged in the wall?

tiny-tim
Homework Helper
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Hi I'm clever! Welcome to PF!

The horizontal reaction force doesn't really matter …

the wall breaks because of vertical forces …

the hammer makes the wall bend slightly, that produces a vertical force, and if that force is too great, the wall breaks.

Interesting thoughts. A wall crumbling has very little to do with the 'equal and opposite force' however. The wall begins to crumble once enough damage has been done that it cannot support its own weight. Damage begins to be done once the force (only considering the 'force' is a simplification... but its okay) you apply is greater than the structural forces in the material (e.g. the forces between clumps of sheet-rock, or dry-wall, or whatever).

The equal and opposite nature of forces is (in most situations) a fundamental property of nature---not the material. There is no strength of hitting the wall at which there would no longer be an equal and opposite force. At a certain strength, however, the wall would give way.

Yes zhermes, completly Ok.
Let'sjust remenber that the same damage caused to the wall has been "trying t be done" to the hummer. However , since the particles of the hummer are much more strongly related it does not affect them as much as it does on the particles of the wall whome are less strong related. After times of doing walls by the same hummer it will became less usefull. So:
- there is not "seuil" limit to the reaction,the 3 rd law always exists.
-the reaction is equal to the action in value opposant in direction.

sophiecentaur
Gold Member
2020 Award
Interesting thoughts. A wall crumbling has very little to do with the 'equal and opposite force' however.

I don't think a wall can pick or choose whether or not to go along with Newton III. If you hit the wall, the wall hits back. What can happen, though is that the force you actually exert can be less, if it starts to move away from your hammer and not all the momentum is lost (i.e. transferred to the Earth) But even in that case, there will be an equal and opposite force on hammer and wall.

If you try to 'break' a sheet of paper that's floating past you, using a hammer, you just can't get enough force on the paper. It will just accelerate along with the head of the hammer and you won't feel any reaction force. You can't transfer much energy into the air by punching it with a hammer, either, for the same reason.

To break something, it's usually necessary to keep one bit stationary (clamped in some way) and to exert a force on another bit of it. That will maximise the force. If you are a martial arts specialist, you can whizz your sword so fast through the air that you can cut a hair floating in front of you. You impose such a high acceleration on the tiny mass that the force is enough to go clean through it.

The equal and opposite nature of forces is (in most situations) a fundamental property of nature---not the material. There is no strength of hitting the wall at which there would no longer be an equal and opposite force. At a certain strength, however, the wall would give way.
same
I don't think a wall can pick or choose whether or not to go along with Newton II...the force you actually exert can be less...But even in that case, there will be an equal and opposite force on hammer and wall.

sophiecentaur