a ball is a kind of object & rolling is a kind of motion ->  a ball rolling is an example of an object moving  a ball rolling is an example of an object moving & moving changes potential energy into kinetic energy and heat energy ->  a ball rolling will change the potential energy of the ball into kinetic energy and heat energy  a ball rolling will change the potential energy of the ball into kinetic energy and heat energy & down the hill is a kind of direction & a metal ball is a kind of ball -> a metal ball rolling down the hill will change the potential energy into kinetic energy and heat energy