1. The problem statement, all variables and given/known data A 2.1 X 10^3 kg car starts from rest at the top of a driveway that is sloped at an angle of 20.0 degrees with the horizontal. An average friction force of 4.0 X 10^3 N impedes the car's motion so that the car's speed at the bottom of the driveway is 3.8 m/s. What is the length of the driveway? 2. Relevant equations Wnet - ΔKE = KEf - KEi Wnet = Fd cos θ Fnet = mgsinθ - Fk 3. The attempt at a solution I understand the basic idea of how to solve this problem. I think I would calculate Wnet using 1/2 mvf^2 (vi = 0); solve for Fnet and then substitute these values in to solve for d by rearranging Wnet = Fdcosθ. My problem is that the key defines two angles - θ and θ'; with θ=20 degrees and θ' = 0. I don't understand where θ' comes into the picture. I am a Chemistry teacher trying to teach an intro physics course and I need some concrete help!