A conventional pendulum is a mass suspended on a string that swings periodically. A torsion pendulum is a mass suspended on a string (or torsion fiber) that rotates periodically. When the mass of a torsion pendulum is rotated from its equilibrium position, the fiber resists the rotation and provides a restoring force that causes the mass to rotate back to its original equilibrium position. When the mass reaches its equilibrium position, it is moving quickly and overshoots. The fiber's restoring force, which is proportional to the rotation angle of the mass, eventually causes the mass to slow down and rotate back the other way. Because the restoring force of the torsion fiber is very small, a torsion pendulum can be used to measure extremely small forces affecting the test mass.