Centres of Mass (AQA A Level Maths: Mechanics)

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Uniform Rods & Laminae

What is meant by the centre of mass of an object?

  • The centre of mass of an object is the point at which the weight of the object may be considered to act
  • For a uniform object the centre of mass is at the centre of the object where the lines of symmetry intersect
    • For a uniform rod this will be at its midpoint
    • For a uniform rectangular lamina this will be where the diagonals intersect

  • For a non-uniform object the centre of mass is not necessarily at the centre of the object

4.1.3 Centres of Mass Diagram 1, downloadable Edexcel A Level Mechanics revision notes

How can I solve problems involving uniform rods?

  • If you are told that a rod is uniform then you can draw the weight at the midpoint of the rod
  • If a rod lies on a support or peg then there will be a normal reaction force which acts perpendicular to the rod at that point
  • If the rod is suspended by strings or cables then there will be tensions in the strings which keep the rod in place

Worked example

4.1.3_WE_Centre of Mass_1

4-1-3-centres-of-mass-example-solution-1

Examiner Tip

  • If there are two supports with unknown reaction forces then choosing the pivot to be at one of the supports will help to find the force at the other support. The same method works with strings too.

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Dan

Author: Dan

Expertise: Maths

Dan graduated from the University of Oxford with a First class degree in mathematics. As well as teaching maths for over 8 years, Dan has marked a range of exams for Edexcel, tutored students and taught A Level Accounting. Dan has a keen interest in statistics and probability and their real-life applications.