1. The problem statement, all variables and given/known data I encountered this past exam paper question in Hong Kong when I was giving tutorial to a high school student. I am confused with the answer provided by the official authority. The question is (in exact wordings): 2. Relevant equations 3. The attempt at a solution The friction f provides by the ground to propel the car is exactly cancelled by the component of weight down the inclined plane, hence the power is fv which is equal to mg(sin30°)v=1/2 mgv which is A. However the official answer is B. I have no idea how to arrive at the answer B and I believe the authority must have their own grounds. I even think that B is a "trap" choice when I first read this question. Any help is appreciated. Thanks.