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

The force of friction always acts in a direction opposite to that in which a body is moving
All of these
For moderate speeds the force of friction remains constant but decreases slightly with the increase of speed
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

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

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