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:
Why or how can the wall push you back?
Why doesn't the wall move if there's nothing behind it to stop it?
Why will a person always move when you push them, and they don't push back?