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

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Engineering Mechanics
The static friction

All of these
Is independent of the area of contact, between the two surfaces
Always acts in a direction, opposite to that in which the body tends to move
Bears a constant ratio to the normal reaction between the two surfaces

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