Engineering Mechanics
Angle of friction is the

The ratio of minimum friction force to friction force acting when the body is in motion
The ratio of minimum friction force to the friction force acting when the body is just about to move
Ratio of limiting friction and normal reaction
Angle between normal reaction and the resultant of normal reaction and the limiting friction

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Engineering Mechanics
The loss of kinetic energy during inelastic impact, is given by (where m1 = Mass of the first body,m2 = Mass of the second body, and u1 and u2 = Velocities of the first and second bodies respectively.)

[m₁ m₂/2(m₁ + m₂)] (u₁ - u₂)²
[2(m₁ + m₂)/m₁ m₂] (u₁ - u₂)²
[m₁ m₂/2(m₁ + m₂)] (u₁² - u₂²)
[2(m₁ + m₂)/m₁ m₂] (u₁² - u₂²)

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