Recall the energy conservation law for solving a numerical problem. A rock of mass 20 kilograms is situated at the edge of a cliff at a height of 5 meters. After sliding down the cliff, its velocity is 9 meters per second. What will be the change in the internal energy of the rock? Here, the mass of the rock, the height of the rock from the ground, and the velocity of the rock after sliding down are known quantities, and the change in internal energy is an unknown quantity. Initially, the kinetic energy of the rock is zero, while the potential energy calculated from the weight and height of the rock is 980 joules. After sliding, the rock has zero potential energy and the kinetic energy calculated from the mass and velocity is 810 joules. Lastly, substituting the values of change in kinetic and potential energy in the law of conservation of energy equation gives the change in internal energy of the rock as 170 joules.