# Law of Conservation of Energy of a skier

1. Dec 6, 2003

### tongtong

A skier of mass 55.0 kg slides down a slope 11.7 m long, inclined at an angle theta to the horizontal. The magnitude of the kinetic friction is 41.5 N. The skier's initial speed is 65.7 cm/s and the speed at the obttom of the slope is 7.19 m/s. Determine the angle theta from the law of conservation of energy. Air reistance is negligible.

The answer is 17.5 degrees, but the work is hard for me to start at.
From what I've learned, we use Et1 = Et2, and I can find theta from 11.7 sin theta which is delta y. I get 12.9 degrees from this. I know this is incorrect, and the friction force needs to be added in somewhere, I just don't know where.

2. Dec 7, 2003

### Staff: Mentor

If it wasn't for friction, the mechanical energy (KE + PE) would be conserved. The change in KE + PE (a decrease, of course) is equal to the work done by friction on the skier. (Don't forget that work is negative if the force is opposite the displacement.)