-- Energy is never created or destroyed.
-- No energy is added to the pendulum during its swing.
-- If we ignore air resistance and friction, then no energy is lost
from the pendulum during its swing.
-- Therefore the total energy of the pendulum must be constant.
-- Any potential energy lost at any point in the swing
must show up as kinetic energy. If it had 484J at the top,
then it'll have 484J at the bottom.
