Dynamics

Coefficient of Restitution

The ratio of the speed of separation to the speed of approach in an impact, from 0 to 1.

In a direct impact, e = (v₂′ − v₁′)/(v₁ − v₂): how fast the bodies move apart afterwards relative to how fast they closed. A value of 1 is a perfectly elastic impact in which kinetic energy is conserved; 0 is a perfectly plastic one in which the bodies move off together.

Momentum conservation gives one equation for the two unknown final velocities, and the coefficient of restitution gives the second. Kinetic energy is not conserved unless e = 1, so an energy balance across a real impact gives the wrong answer.

How much of the FE Civil exam is Dynamics?