# How does a gravity car work (and where does the energy come from)?

So I am doing an experiment with gravity car, basically dropping a hanging mass and spins the axle using a pulley system. And then I thought although the work formula W=Fd doesn't prove that gravity does work on the car, the kinetic energy comes from the change in gravitational potential energy. How is the energy transferred if gravity doesn't do work on the car? What is the relationship between energy transfer and work done? Do they contradict each other or what? Thanks!

• If gravity does no work, why is it called a gravity car ? I just never saw one before.. school-for-champions.com/science/… – user108787 Nov 13 '16 at 3:30
• Draw a free body diagram of the wheel and axle, including the part of the rope going up to the pulley. Then, do a moment balance on the wheel and axle, including the effect of the tension in the rope and the torque imposed on the wheel by the forward frictional force from ground. Neglect the rotational inertia of the wheel and the friction in the bearings. How is the tension in the rope related to the forward frictional force from the ground? – Chet Miller Nov 13 '16 at 13:16