Question

when you walk on the sidewalk, how do you know the sidewalk is exerting a force on your feet? because if the sidewalk werent exerting a force, your foot would fall into the sidewalk. because you can see the sidewalk moving when it exerts a force. rewatch

Explanation:

According to Newton's third law of motion, for every action, there is an equal and opposite reaction. When you walk on the sidewalk, you exert a downward force (action) on the sidewalk due to your weight. The sidewalk, in turn, exerts an upward normal force (reaction) on your feet. If the sidewalk didn't exert this force (normal force), there would be no force to counteract the force of gravity pulling your foot down, and your foot would fall into the sidewalk (similar to how you would fall through if there was no solid surface). Also, the sidewalk is a rigid object and does not visibly move when it exerts a normal force on your feet (unlike some deformable objects where you might see a small deformation, but even then, it's not a visible'moving' in the sense of the whole sidewalk shifting position as described in the second option).

Answer:

Because if the sidewalk weren't exerting a force, your foot would fall into the sidewalk.