Energy (which in the metric system is measured in joules, or kg.m²/s²) is of several varieties. What is important for racquet performance evaluation is the kinetic energy of the racquet and the ball.
Kinetic energy is the energy due to motion. When an object is moving, it has a kinetic energy measured in joules by the formula KE = ½ Mv². (KE is kinetic energy, M is the mass of the object in kilograms, and v is the velocity of the object in meters per second, m/s).
When a moving object collides with something stationary, like a ball hitting a racquet for example, it loses kinetic energy. How much is lost depends on the elasticity of the collision, which is indicated by the symbol c in our formulas (representing the coefficient of restitution).