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?

The skateboarder eventually stop due to frictional force.

Explanation:

The Newton's first law of motion or law of inertia says that an object will remain in its previous state until and unless the unbalanced force acts on it.

When the skateboarder reaches the bottom of the hill, the skateboarder will slow until she stops. It slows down due to frictional force which is acting in opposite direction. It is a contact force and it acts between two surfaces.

Hence, the frictional force will act between the ground and the skateboard which reduces the speed of the skateboarder.