As the book gets faster it gains more kinetic energy and loses more potential energy. Just before it hits the floor all the gravitational potential energy has been converted to kinetic energy.

The energy of a thermodynamic system that is NOT either the kinetic energy or gravitational potential energy of the system as a whole is known as Internal Energy. The internal energy is associated ...

It lifts large bricks using electric motors, thereby creating gravitational energy. When power needs to be discharged back to the grid, the bricks are lowered, harvesting the kinetic energy.