Question

Think of a skateboarder riding down a hill. Once the skateboarder reaches the bottom of the hill, if she does not manually add motion, the skateboarder will slow until she stops. Why does the skateboarder eventually stop?

Answer 1

From first law of motion, law of inertia, we know that an object remains in the state of rest or motion unless acted upon by an unbalanced external force. The skateboard riding down a hill when reaches the bottom slows down due to frictional force acting in the opposite direction. Frictional force is a contact force which acts between two surfaces against the direction of  motion of one surface over the other. Here, frictional force acts between skateboard and ground. This reduces the speed of the skateboard and it eventually stops.

Answer 2

The wheels give off energy to the ground because of the friction until they run out, think of the wheels as a battery and the ground as a device that needs power, it just sucks out the power