Engineering Mechanics
Angle of friction is the

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

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Engineering Mechanics
The total momentum of a system of masses (i. e. moving bodies) in any one direction remains constant, unless acted upon by an external force in that direction. This statement is called

Newton's second law of motion
Newton's first law of motion
Principle of conservation of momentum
Principle of conservation of energy

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