When a collection of objects act together as a single entity, then the motion of that entity can be described by the position of the centre of mass of the system of objects. For example, when describing the motion of a car travelling down the highway, it is not neccessary to worry about the location of each wheel, or the engine, or the passengers, etc. Instead a single point is sufficient, the point located at the centre of mass of all the components which make up the car.

The location of the centre of mass can be calculated if the mass m_i and location (x_i,y_i) of each component is known. By definition the centre of mass is located at (x_cm,y_cm) where