Strength of Materials
Pick up wrong statement about friction force for dry surfaces. Friction force is

proportional to normal load between the surfaces
proportional to velocity of sliding
dependent on the materials of contact surface
independent of the area of contact surfaces

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Strength of Materials
Limiting force of friction is the

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

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