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In the first screen you can see that if a person were to push a wall within a typical household the wall would not move while keeping themselves tractioned to the floor. If you push hard and do not traction yourself, you move back.

In screen two if you push a person with no space behind them they will move back, always, or at least have the potential. A 10 year old kid could push a 7'4", 680 lb. person back, but the wall doesn't move despite nothing being behind it. Here are the official questions:

  1. Why or how can the wall push you back?

  2. Why doesn't the wall move if there's nothing behind it to stop it?

  3. Why will a person always move when you push them, and they don't push back?

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Could you clarify your question in part 3? –  Dave Coffman Jun 24 at 21:58
When you push a person they move back usually before there's any chance of you moving back. Just imagine pushing a small kid ... do you see yourself moving back? –  Hanley Jun 24 at 22:01
+1 for the drawing. –  Max Ried Jun 24 at 23:02
Breaking a wall was indeed the most difficult challenge here: youtube.com/watch?v=hgdh4We8lLQ –  Count Iblis Jun 25 at 1:21

1 Answer 1

A wall is not just standing on the ground, its contact with the ground is made fast by foundations. Pushing against it is pushing against a system wall + ground. Since it is in addition a very rigid solid, it will transmit all of your push to the ground. You can probably be convinced that the resistance of this system to motion is much greater than yours.

Before considering pushing a person, consider pushing some heavy box which is just standing on the ground, and is stable enough (wider than it is tall). Above some minimum force that is needed to overcome friction with the ground, you'll be able to move it -- unless there is something behind, as you state, which will add its own resistance to this pushing motion (but wouldn't resist a pulling motion).

Now consider that the box has the same proportions as a human being: feet (ground contact) quite small compared to height. It will topple down before sliding along the ground.

This is what would happen with the person you're pushing, unless he is either prepared for it (feet apart, high muscle tone, as a wrestler would do e.g.) or unless he moves back to maintain his balance -- this is a postural reflex .

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