Centre of Gravity (CIE AS Physics)

• The centre of gravity of an object is defined as:

The point at which the weight of the object may be considered to act

• For example, for a person standing upright, their centre of gravity 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 gravity is located at the point of symmetry

The centre of gravity of different shapes

The centre of gravity of a shape can be found by symmetry

 

Stability

• The position of the centre of gravity of an object affects the object's stability
• An object is stable when its centre of gravity lies above its base

Centre of gravity of stable and unstable objects

The object on the right will topple, as its centre of gravity is no longer over its base

• The wider the base of an object
• • The lower its centre of gravity
  • The more stable the object is
• The narrower the base of an object
  • The higher its centre of gravity
  • The less stable the object is

Centre of gravity of narrow and wide based objects

The most stable objects have wide bases and low centres of gravity 

Centre of gravity versus 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 is the product of mass and the acceleration due to gravity, W = mg, the centre of gravity does depend on the gravitational field

• When an object is in space:
  • Its centre of gravity will be closer to the object with larger gravitational field
  • Its centre of mass will be close to its geometric centre

Centre of gravity and centre of mass of the moon

The Earth’s stronger gravitational field pushes the Moons centre of gravity closer to Earth