Applied Mechanics and Graphic Statics
‘u₁’ and ‘u₂’ are the velocities of approach of two moving bodies in the same direction and their corresponding velocities of separation are ‘v₁’ and ‘v₂’. As per Newton's law of collision of elastic bodies, the coefficient of restitution (e) is given by

e = v₂ - v₁/u₁ - u₂
e = v₁ - v₂/u₂ + u₁
e = u₂ - u₁/v₁ - v₂
e = v₁ - v₂/u₂ - u₁

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Applied Mechanics and Graphic Statics
The following is not a law of static friction:

The force of friction depends upon the roughness of the surface
The force of friction is dependent upon the area of contact
The force of friction always acts in a direction opposite to that in which the body tends to move
The magnitude of the limiting friction bears a constant ratio to the normal reaction between two surfaces

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