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

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

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Engineering Mechanics
According to the law of moments, if a number of coplanar forces acting on a particle are in equilibrium, then

The algebraic sum of their moments about any point in their plane is zero
Their algebraic sum is zero
The algebraic sum of their moments about any point is equal to the moment of their resultant force about the same point
Their lines of action are at equal distances

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