According to the law of static friction, the force of friction that resists a body's motion when it begins to descend down a surface is proportional to the normal which is perpendicular force that the body exerts on the surface. The following are the empirical rules of friction, Friction is independent of the contact surface.

• The frictional force always acts opposite to the tendency of the motion on the body that is resting over the surface of a fixed body in the direction tangential to the surface in contact under the action of external force.

• Friction always acts in the direction tangential to the common surfaces of contact between the two bodies.

• The frictional force prevents the motion of the body when the applied force is less than the limiting frictional force.

• At the start of the motion, the net externally applied force in the direction of motion is equal to the maximum frictional force, which is called limiting force.

• The normal reaction is proportional to the magnitude of the limiting friction.

• Limiting friction is independent of the contact area for the case when the normal reaction remains constant.

• Limiting friction depends on materials of the two surfaces of the bodies in contact and roughness between the surfaces.