# Center of mass

(Redirected from Center of gravity) Diagram of an educational toy that balances on a point: the CM (C) settles below its support (P). Any object whose CM is below the fulcrum will not topple.

Center of mass or barycenter of a system is the average position of all the mass in a system.

In a rigid body, the centre of mass is always in the same place. In a loose collection the center of mass may be in space, as it is in the Solar System. Also, as planets' orbits change slightly, so will the position of the barycenter.

A simple way to understand the concept of barycenter is to push a supermarket trolley. If you hold only one corner and push, it will only go in a straight line when you push through the centre of mass. Otherwise a turning force is created. This shows that the mass acts as if it were only in one place.

The center of gravity is a closely related term. It is the point where an object can be balanced perfectly by a sharp point holding it up under the object. It is the center of mass acting in a gravitational field.

## History

The concept of center of mass was first introduced by the ancient Greek physicist, mathematician, and engineer Archimedes of Syracuse. Archimedes showed that the torque (turning force) exerted on a lever by weights resting at various points along the lever is the same as what it would be if all of the weights were moved to a single point — their center of mass.

Archimedes was the first person to work out ways of finding the center of mass in various objects.

## Relation between the two terms

The center of mass is often called the center of gravity because any uniform gravitational field g acts on a system as if the mass M of the system were concentrated at the center of mass R.