Did you ever see one of those wonderous machines with the big wheel...
...and the many weights on hinges aranged along the rim of the wheel? When one of the weights reaches its tipping point as the wheel turns on its axis the little weight pivots on its hinge, gains speed and slams into the wheel, imparting its energy to the spinning wheel, thus keeping the wheel moving perpetually, using only the gravity of the earth for power. Except that it does not work. You don't get something for nothing.