1.5 (d) Centre of mass

The centre of mass is the point where we can assume all the mass of the object is concentrated.

This is a useful simplification because we can pretend gravity only acts at a single point in the object, so a single arrow on a diagram can represent the weight of an object.

The centre of mass for objects with a regular shape is in the centre.


To find the centre of mass of simple objects, such as a piece of card, follow these steps:

  1.  Hang up the object.
  2. Suspend a mass from the same place.
  3.  Mark the position of the thread.
  4. The centre of mass is somewhere along the line of the thread.
  5. Repeat steps 1 to 3 with the object suspended from a different place.

The centre of mass is where the two lines cross.

Centre of mass

The terms “center of mass” and “center of gravity” are used synonymously in a uniform gravity field to represent the unique point in an object or system which can be used to describe the system’s response to external forces and torques. The concept of the center of mass is that of an average of the masses factored by their distances from a reference point. In one plane, that is like the balancing of a seesaw about a pivot point with respect to the torques produced.