Friction

 Friction force

 When there is a relative motion between two surfaces that are in contact with each other, or when there is such a temptation, the force acting on it against the contact surface is the force of friction

When a force p is applied to the mass m on the rough surface, gradually increasing from zero to zero, the friction force F is applied to the mass by the rough surface. To some extent the magnitude of F is equal to the magnitude of P. That is, the mass remains stationary because the total force is zero up to that limit.

In this case the active friction force is the static friction force. As the magnitude of F gradually increases, the value of F is maximal at one point

  At this point the object is in equilibrium very close to moving

 This state is called the limiting equilibrium state.

In this case the maximum frictional force exerted on the object is called F maximum and it is called the limiting friction force F limit.

Slightly increasing the value of P by or slightly above this maximum friction force causes the object to start moving.

Where M(mu) is the coefficient of friction between the rough surface and the object

                                                  

Thus   M (Mu) has no units

The magnitude of the limiting friction force does not depend on the rough surface and the area of ​​the contact object.

As the force on the smooth surface is gradually increased from zero to a plane parallel to the plane, the mass begins to move along the plane, as the smooth surface does not exert any frictional force against the motion.

Causes for friction

Major causes for friction are considered as intermolecular attraction- like electro-static forces of particles between two surfaces  and magnetic forces between them. we have to goto the particle level to identify them.

Example Question