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

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Applied Mechanics and Graphic Statics
The characteristic of a couple is:

Algebraic sum of moments of forces, constituting a couple, about any point, is same
A couple can be never the balanced by a single force
Algebraic sum of forces, constituting a couple is zero
All listed here

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Applied Mechanics and Graphic Statics
The following statement is one of the laws of Dynamic friction

The magnitude of the kinetic friction bears a constant ratio to the normal reaction between two surfaces. The ratio being slightly less than that in the case of limiting friction
For moderate speeds the force of friction remains constant but decreases slightly with the increase of speed
The force of friction always acts in a direction opposite to that in which a body is moving
All of these

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