A toy car rolls down a hill at with a height of h and goes through a loop with a radius r. What is the height the toy car has to start off from rest at, if everything is frictionless and is able to make it throught the loop. I believe the answer it 2r because of the energy involved. If everything is frictionless, then the toy car must start from the height of the top of the loop. My professor wants equations, but i don't know how to break this one down. Thanks for the help.