Centre of MassOxford AQA IGCSE Physics: Revision notes

The centre of mass of an object is the single point at which the mass of the object may be thought to be concentrated. For calculations of balance, stability and motion, we can treat the whole object as if all its mass acts at this one point.

For a thin, flat, irregularly shaped object (a lamina), the centre of mass can be found practically:

1. Suspend the lamina freely from a point near its edge so it can swing
2. Once it comes to rest, hang a plumb line from the same suspension point and mark the vertical line it makes on the lamina
3. Repeat from a second (and ideally a third) different suspension point, drawing a new vertical line each time
4. The centre of mass is where the lines cross

A freely suspended object always comes to rest with its centre of mass directly below the point of suspension. This is because, at rest, there is no resultant moment acting on it — any other position would create a turning effect due to gravity acting through the centre of mass.

For a symmetrical object, the centre of mass lies along the axis (or lines) of symmetry.

The position of an object's centre of mass affects its stability:

• A lower centre of mass, and a wider base, make an object more stable and harder to topple
• A higher centre of mass, or a narrow base, make an object less stable and easier to topple