There is a limited amount of fossil fuels (our main source of energy for powering our appliances and cars) on Earth. Why? What is an example of a "renewable" energy source, and why is it considered "renewable"?

Asolid cylinder and a cylindrical shell have the same mass, same radius, and turn on frictionless, horizontal axles. the cylindrical shell has light-weight spokes connecting the shell to the axle. a rope is wrapped around each cylinder and tied to blocks of equal masses that are held the same height above the ground. both blocks are released simultaneously. the ropes do not slip. which block hits the ground first? or is it a tie?