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!
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$\begingroup$ If gravity does no work, why is it called a gravity car ? I just never saw one before.. school-for-champions.com/science/… $\endgroup$– user108787Commented Nov 13, 2016 at 3:30
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1$\begingroup$ 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? $\endgroup$– Chet MillerCommented Nov 13, 2016 at 13:16
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1 Answer
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Gravity does work on the falling block, which then does work on the car (through some gearing). We could detail it a bit more and say that the block actually does work on the wheels that are being turned, which then in turn cause the friction from the ground to do work on the car.
The work done by gravity is what we call change in potential energy.
