1. The problem statement, all variables and given/known data A skier, mass m = 85 kg including skis, poles and clothes, skis directly down a slope with an angle Φ = 21° to the horizontal. His body is rigid, the poles are not touching the snow. He is travelling at a constant speed of 71 kph in a straight line. The coefficient of kinetic friction is 0.21. In all of the following, think about the units and sign, and it probably helps to draw a free body diagram. a)What is the rate at which gravity is doing work on the skier? (Here, 'skier; includes skis, poles and clothes.) (in kW) b)What is the rate at which friction is doing work on him? c)What is the rate at which air resistance is doing work on him? 2. Relevant equations Work done by gravity= mgdcos(x) kW= J/s F= ma 3. The attempt at a solution I got the work equation from a textbook, but I think the problem is rate. Subbing in the values gives 85x 9.8m/s^2 x 19.7222 m/s x cos(11) = 1624.6111 kg/s Converting this to kW, you get 161.109. But this isn't really a rate.... How do you figure out the rate? is it a derivative of some sort?