Engineering Mechanics
If u₁ and u₂ are the velocities of two moving bodies in the same direction before impact and v₁ and v₂ are their velocities after impact, then coefficient of restitution is given by

(u₂ + u₁)/(v₂ + v₁)
(u₁ - u₂)/(v₁ - v₂)
(v₂ - v₁)/(u₁ - u₂)
(v₁ - v₂)/(u₁ - u₂)

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Engineering Mechanics
The total momentum of a system of masses (i. e. moving bodies) in any one direction remains constant, unless acted upon by an external force in that direction. This statement is called

Newton's second law of motion
Principle of conservation of energy
Newton's first law of motion
Principle of conservation of momentum

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