# (4/10) Friction’s role in Statics?

#### What is friction?

It is a force that resists the movement of two contacting surfaces that slide relative to one another.

Fs = μsN

μs — Coefficient of Static Friction

N — Normal reaction force

Fs — Static Friction

On the verge of sliding, the Normal force N and friction force Fs combine to create a resultant Rs. The angle Θs that Rs makes with N is called the angle of static friction. N acts at a distance x to the right of the line of action of W.  The distance is necessary to balance the tipping effect caused by P.

For example, if P is applied at a height h from the surface to the moment about point O is satisfied if

Wx=Ph As shown in the above diagram, the resultant force at the surface has a line of action parallel to the applied force.

### Characteristics of Dry Friction

1. The force Ff acts tangent to the contact surfaces in the direction opposite to the motion.
2. The force Fs is independent of the area of contact, provided the normal pressure is not very low nor high.
3. Fs is generally greater than Fk, but they may be roughly similar during extremely low velocity.

Key point to note: Ff is not effected by the Surface area in contact, as noticed in the formula.

For a more detailed understanding of the subject you may visit the previous topic.