Restitution, Momentum, Collisions

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When two particles collide, of course we know that momentum is conserved. On top of this, particles also obey Newton’s Law of Restitution: where is the coefficient of restitution. The coefficient is taken to be a fixed constant for collisions between any two bodies. We can then write down two equations from the law of conservation of momentum and Newton’s Law of Restitution and solve them simultaneously fo find the velocities of the two particles after the collision. For example, if:

From the first diagram, the momentum is

Therefore, from the Law of Conservation of Momentum,

From Newton’s Law of Restitution,

We solve the simultaneous equations,



(1)+7*(2) gives


Sub this value of into (2) to get

Since the collision is inelastic and kinetic energy is lost. The initial kinetic energy is

The final kinetic energy is

So of energy is lost.

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