The law of conservation of energy states that energy is neither created nor destroyed, but can be converted from one form to another.
When the book was at the shelf, its kinetic energy was zero and its potential energy was maximum. As it fell, its potential energy stated converting to kinetic energy. Before the book directly hits the ground, its kinetic energy is maximum and its potential energy is zero.
Based on the above explanation, at the moment the book landed on the floor, ALL the potential energy was converted to kinetic energy, so the kinetic energy = 67 Joules.
