Question

A skateboarder is skating back and forth on the halfpipe as seen below. As he skates his energy transforms from potential energy to kinetic energy and back again. According to the law of conservation of energy he should keep moving forever, but eventually he comes to a stop. Why does he eventually stop moving? Question 8 options: Friction and air resistance cause some of his kinetic energy to be “lost”. This makes him slow down. The law of conservation of energy does not apply to this situation. One side of the ramp is shorter than the other so he slows down. The mass of the skateboarder causes him to have less kinetic energy than potential energy so he slows down. Save

Answer 1

The skateboarder stops due to friction and air resistance, which convert kinetic energy into forms like heat, rather than a violation of the conservation of energy.

Step-by-step explanation:

The skateboarder eventually stops moving not because of a violation of the law of conservation of energy, but due to friction and air resistance. These forces cause energy to be dissipated as heat, thereby reducing the mechanical energy of the skateboarder-system. Although energy is conserved in the sense that it changes form, the kinetic energy which allows the skateboarder to move is converted into forms that don't contribute to the motion of the skateboarder, such as thermal energy.

In an ideal scenario without friction or air resistance, the skateboarder would indeed continue moving indefinitely, with potential energy and kinetic energy converting back and forth perfectly. However, in real-life situations, such processes are not 100% efficient, rendering the perpetual motion observed in a perfect conservation scenario impossible.

Answer 2

Friction and air resistance cause some of his kinetic energy to be “lost”. This makes him slow down.

Step-by-step 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.