Explain why you move forward and the boat moves backward when you jump from a boat to a pier?

Answer 1

The force used when you step from the boat creates an equal reaction on the boat, which has very little friction with the water. If the boat were on the beach, friction with the sand would prevent it from moving very much.

Force Pairs

The forces two objects exert on each other are called force pairs. The forces in a force pair act in opposite directions and are always equal in size. To jump higher, you must push harder on the ground. Then the ground pushes harder on you.

You might think that if force pairs are equal in size and act in opposite directions, they must cancel out. But remember that the forces in force pairs act on different objects. When you jump, you exert a force on Earth, and Earth exerts a force on you. One force in the force pair acts on Earth, and the other force acts on you. These forces don't cancel out because they act on different objects. Equal and opposite forces cancel out only if they act on the same object.

Action and Reaction

According to the third law of motion, forces always act in pairs. For example, when you push on a wall, the wall pushes back on you. One force of the force pair is called the action force, and the other force is the reaction force. Your push on the wall is the action force, and the wall pushing back on you is the reaction force. For every action force, there is a reaction force that is equal in size, but in the opposite direction.