Centre of Gravity
- The centre of gravity of an object is 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
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- 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
Exam Tip
Since the centre of gravity is a hypothetical point, it can lie inside or outside of a body.
The centre of gravity can also move, depending on the shape or the orientation of the object being considered.
For example, a human body’s centre of gravity is lower when leaning forwards than when standing upright. And you will notice your own centre of gravity shift backwards when you pick up your heavy school bag and put it on your back. This is why you instinctively lean forwards to carry it.
