Differences between bouncing balls, springs, and rods

When one hard steel ball collides with another, kinetic energy is conserved, even if the balls have different diameters. Why is kinetic energy conserved in such a collision, given that kinetic energy is not conserved when two unequal length steel springs or rods collide? Experimental results with bouncing balls, springs, and rods are presented, which reveal the answer. For colliding springs and rods a significant fraction of the initial kinetic energy is retained after the collision as vibrational energy in the longer spring and rod. When two hard balls collide, a negligible fraction of the initial energy is converted to vibrational energy because the collision time is much longer than the transit time of an acoustic wave across each ball due to the fact that the contact region of a hard spherical ball is much softer than the rest of the ball. © 2008 American Association of Physics Teachers. DOI: 10.1119/1.2948778