Finding the Centre of Mass (Oxford AQA IGCSE Physics)
Revision Note
Written by: Leander Oates
Reviewed by: Caroline Carroll
Finding the Centre of Mass
The centre of mass of an object is
The point in an object at which the mass can be thought to be concentrated
For a symmetrical object of uniform density (such as a symmetrical cardboard shape), the centre of mass is located at the point of symmetry
The shape needs to have more than one line of symmetry
Centre of mass of a symmetrical object
For non-symmetrical objects, the centre of mass can be found using suspension
When an object is suspended from a point, the object will always settle with its centre of mass directly below the point of suspension
Centre of mass of a non-symmetrical object
The irregular shape (also called a plane lamina or thin lamina) 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
Worked Example
A student wishes to find the centre of mass of an isosceles trapezium.
Describe a suitable method for the student to use.
Answer:
Step 1: Determine the correct method to use
An isosceles trapezium is a symmetrical shape
However, it only has one line of symmetry
Therefore, the suspension method must be used
Step 2: Describe the method
Suspend the shape using a thread
Align a plumb line with the suspension thread and mark the plumb line on the shape (or use the line of symmetry as the first line)
Suspend the shape from a second point, align the plumb line and mark on the shape
Suspend the shape from a third point, align the plumb line and mark on the shape
The centre of mass is the point at which all three lines cross
Examiner Tips and Tricks
Make sure that you can describe how to find the centre of mass of a thin lamina with an irregular shape, as you may be asked to this in your exam.
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