Question

Why doesn't the boat move from the place when a person in it presses down on the side and why does it begin to move, if a person goes out of the boat and pushes it with the same force?

Answer 1

If a person presses down on the side from the inside of the boat, then by Newton's third law the boat with the same force puts pressure on a person and since the person is inside, then the resultant of these two forces is zero. If a person goes out of the boat, and pushes the boat being outside it, the boat will be only under one force of pressure - the force of counter pressure will be applied to the person (if he is on the ground, it actually means that the boat will "crush" the entire Earth).