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
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
- The force Ff acts tangent to the contact surfaces in the direction opposite to the motion.
- The force Fs is independent of the area of contact, provided the normal pressure is not very low nor high.
- 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.