Centre of Mass (AQA A Level Physics): Revision Note

Centre of mass

Centre of mass definition

• The centre of mass of an object is the point at which the weight of the object may be considered to act

• The position of the centre of mass of uniform regular solid is at its centre

  • For example, for a person standing upright, their centre of mass is roughly in the middle of the body behind the navel, and for a sphere, it is at the centre

• For symmetrical objects with uniform density, the centre of mass is located at the point of symmetry

Stability

• The position of the centre of mass of an object affects its stability

• An object is stable when its centre of mass lies above its base

• The wider base an object has, the lower its centre of mass and it is more stable

• The narrower base an object has, the higher its centre of mass and the object is more likely to topple over if pushed

Centre of gravity v centre of mass

• In a uniform gravitational field, the centre of gravity is identical to the centre of mass

• The centre of mass does not depend on the gravitational field

• Since weight = mass × acceleration due to gravity, the centre of gravity does depend on the gravitational field

• When an object is in space, its centre of gravity will be more towards the object with the larger gravitational field

  • For example, the Earth’s gravitational field on the Moon: