Nonconservative forces and kinetic/potential energy.

[Crusaderking1]

Homework Statement

A 60.0-kg skier starts from rest at the top of a ski slope of height 66.0 m.

A) If frictional forces do −1.02×104 J of work on her as she descends, how fast is she going at the bottom of the slope?

Answer is 30.9 m/s

B)Now moving horizontally, the skier crosses a patch of soft snow, where the coefficient of friction is 0.21. If the patch is of width 70.0 m and the average force of air resistance on the skier is 180 N, how fast is she going after crossing the patch?

C) After crossing the patch of soft snow, the skier hits a snowdrift and penetrates a distance 2.2 m into it before coming to a stop. What is the average force exerted on her by the snowdrift as it stops her?

Homework Equations

work of non-conservative forces = KEf -KEi

The Attempt at a Solution

For B)

I did this.

worknc = Kef-KEi

work done by air = -180 * 70.0 = -12600 J
work done by friction is (0.21)(60.0)(9.8)(70.0)cos180 = -8643.6 J

-21243.6 = 30v^2 - .5(60.0)(30.9)^2

v = 15.6 (I actually received 15.7, but plugged into a previous part, it seems that 15.6 is more accurate).

C) I did:

Fsnow(2.2) = -.5(60.0)(15.6)^2

F=3.39*10^3


Are these right? non-conservative means only friction and air, and I think the only force in part C is the snow.


Thanks!

 

[anathema]

Your solutions for parts B and C seem to be correct. The work done by non-conservative forces includes both friction and air resistance, so your approach for part B was correct. For part C, the only force acting on the skier is the snowdrift, so your solution is also correct. Keep up the good work!