Symbol e. A measure of the elasticity of colliding bodies. For two spheres moving in the same straight line, e=(v2−v1)/(u1−u2), where u1 and u2 are the velocities of bodies 1 and 2 before collision (u1 > u2) and v1 and v2 are the velocities of 1 and 2 after impact (v2 > v1). If the collision is perfectly elastic e=1 and the kinetic energy is conserved; for an inelastic collision e < 1.