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

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

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Strength of Materials
Coefficient of friction is the

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

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