Dry Friction

Dry friction is a force that opposes the relative motion between two solid surfaces in contact.

Consider a wooden crate resting on a rough, non-rigid horizontal surface that is being pulled horizontally to the right.

In equilibrium, the normal forces must act upward to balance the crate's weight, and the frictional forces act left to prevent the movement of the crate to the right.

The floor exerts an uneven distribution of normal and dry frictional force along the contact surface.

Close examination reveals microscopic irregularities at the interface, creating reactive forces at each contact point, contributing to frictional and normal force components.

Their resultants indicate their net effect, with the normal force acting at a distance y from the line of action of weight.

This position coincides with the centroid of the normal force distribution and balances the tipping effect caused by the applied force.

For example, if a force is applied at the height of h, then moment equilibrium about point O is satisfied only at a particular value of y.