Applied Mechanics and Graphic Statics
A body of weight ‘w’ placed on an inclined plane is acted upon by a force ‘P’ parallel to the plane which causes the body just to move up the plane. If the angle of inclination of the plane is ‘θ’ and angle of friction is ‘φ’, the minimum value of ‘P’, is

w cos (θ + φ)/cos φ
w sin (θ - φ)/cos φ
w sin (φ - θ)/cos φ
w sinθ cos(θ - φ)/sin φ

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Applied Mechanics and Graphic Statics
The following is not a law of static friction:

The force of friction always acts in a direction opposite to that in which the body tends to move
The force of friction depends upon the roughness of the surface
The magnitude of the limiting friction bears a constant ratio to the normal reaction between two surfaces
The force of friction is dependent upon the area of contact

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