Centre of Gravity (Edexcel IGCSE Physics)

Spec point — 1.31P · know that the weight of a body acts through its centre of gravity (physics only)

Centre of gravity

• The centre of gravity of an object is defined as: The point through which the weight of an object acts

• For a symmetrical object of uniform density, the centre of gravity is located at the point of symmetry

  • For example, the centre of gravity of a sphere is at the centre

Finding the centre of gravity of symmetrical objects

The centre of gravity of a regular shaped object can be found using symmetry

• The centre of gravity of an irregular object can be found using suspension

• The irregular shape is suspended from a pivot and allowed to settle
• A plumb line (weighted thread) is then held next to the pivot and a pencil is used to draw a vertical line from the pivot (the centre of mass must be somewhere on this line)
• The process is then repeated, suspending the shape from two additional points
• The centre of mass is located at the point where all three lines cross

Exam Hint Since the centre of gravity is a hypothetical point, it can lie inside or outside of a body. The centre of gravity will constantly shift depending on the shape of a body. For example, a human body’s centre of gravity is lower when learning forward than when standing upright

However, when you are drawing force diagrams, always draw the weight force as if it were acting from the centre of gravity of the object!