Answer:
Friction and air resistance cause some of his kinetic energy to be “lost”. This makes him slow down.
Explanation:
The law of conservation of energy states that in absence of frictional forces, the mechanical energy of an object (given by the sum of its kinetic and potential energy) is conserved. In such a situation, the skateboarder would never stop his motion, because potential energy is continuously converted into kinetic energy and vice-versa, but the total energy remains the same so he would never stop.
In a real world, however, this is not true. In fact, in a real world some frictional force are present, in particular:
- friction: this force is due to the contact between the skateboard and the surface of the halfpipe, and its direction is always opposite to the motion of the skateboarder
- Air resistance: this force is due to the resistance opposed by the molecules of air that the skateboarder meets during his motion, and its direction is also opposite to the motion of the skateboarder
This two forces are said to be non-conservative forces, which means that they cause some of the mechanical energy of the skateboarder to be "lost", in the sense that it is dissipated as heat and it is no longer available for the skateboarder.
Therefore, the correct option is
Friction and air resistance cause some of his kinetic energy to be “lost”. This makes him slow down.
