Centre of Gravity (Edexcel IGCSE Physics (Modular))
Revision Note
Written by: Katie M
Reviewed by: Caroline Carroll
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
Examiner Tips and Tricks
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!
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