A conservative force is one for which the work done in moving an object between two points is independent of the path taken, or equivalently, the work done in moving an object around a closed loop is zero. Gravitational force, electric force, and spring force are examples of conservative forces because the work done by them depends only on the initial and final positions. Frictional force, however, is a dissipative force and the work done by friction depends on the path length; the longer the path, the more work is done against friction. Work done by friction over a closed loop is generally non-zero and negative (energy is dissipated as heat).