# Kinetic Energy Equation conceptual question?

1. Jun 24, 2015

### scharry03

This is mostly a conceptual confusion I'm having, not a specific problem, so it didn't seem like it'd fit under hw problems.
I'm having trouble understanding when potential energy should or shouldn't be used in a energy conservation equation. When looking at a problem with a skier going down a hall, we were given his weight, initial velocity, slope of the hill, value of friction force, and distance traveled, and were asked to find his final velocity.
We found the net work done, added it to the initial kinetic energy, and found his final velocity through his final kinetic energy. We didn't use the potential energy of gravity at all, but later we have an equation that says initial and final potential energy of gravity should be on the respective sides of the equations. Why is it they weren't used for a problem of a skier skiing down a hill? Does the net work account for it?

Thanks!

2. Jun 24, 2015

### brainpushups

If there is a change in potential energy it needs to be considered. Of course, if the system is isolated and there is no change in potential energy then the kinetic energy is constant and you have a rather boring situation. If the system is not isolated the kinetic energy will change due to any net external work.

Right. The work done by gravity is equivalent to the (negative) change in gravitational potential energy. The work done by friction is not conservative and therefore does not have a potential energy.

Potential energy is just a convenient way to represent the work done by a conservative; if you find the net work this will equal the change in kinetic energy.

3. Jun 24, 2015

### scharry03

Awesome, that makes total sense. Thanks!