# Laws of Friction in Engineering Mechanics

## Laws of Friction:

Prof. Coulomb, after extensive experiments, gave some laws of friction, which may be grouped under the following heads:

### 1. Laws of Static Friction:

The laws of static friction state that:

i. The force of friction always acts in a direction, opposite to that in which the body tends to move if the force of friction would have been absent.

ii. The magnitude of the force of friction is exactly equal to the force, which tends to move the body.

iii. The magnitude of the limiting friction bears a constant ratio to the normal reaction between the two surfaces, Mathematically,

F/R = Constant
where,
F = Limiting friction and R = Normal reaction.

iv. The force of friction is independent of the area of contact between the two surfaces.
v. The force of friction depends upon the roughness of the surfaces.

### 2. Laws of Dynamic Friction:

The laws of dynamic friction state that:

i. The force of friction always acts in a direction, opposite to that in which the body is moving.

ii. The magnitude of kinetic friction bears a constant ratio to the normal reaction between the two surfaces. But this ratio is slightly less than that in the case of limiting friction.

iii. For moderate speeds, the force of friction remains constant. But it decreases slightly with the increase of speed.