Applied Mechanics and Graphic Statics
The angle of friction is:

The force of friction when the body is in motion
The force of friction at which the body is just about to move
The angle between the normal reaction and the resultant of normal reaction and limiting friction
The ratio of the friction and the normal reaction

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

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