When force is exerted over a distance ,work is done by force. Forces can be classified as conservative or non conservative. The work done by a non conservative force depends on the path over which the force acts. Friction is an example of a non conservative force because the amount of work that must be done to overcome friction depends on the path taken .consider a person pushing a crate across a warehouse floor as shown in the diagram Describe how the normal force changes as the mass of the crate increases.