Centre of Mass (OCR A Level Physics) : Revision Note

Katie M

Author

Katie M

Last updated

Centre of Mass

  • The centre of mass of an object is the point at which the weight of the object may be considered to act

  • The position of the centre of mass of uniform regular solid is at its centre

    • For example, for a person standing upright, their centre of mass 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 mass is located at the point of symmetry

Point of symmetry, downloadable AS & A Level Physics revision notes

The centre of mass of a shape can be found by symmetry

Stability

  • The position of the centre of mass of an object affects its stability

  • An object is stable when its centre of mass lies above its base

Stability, downloadable AS & A Level Physics revision notes

The object on the right will topple, as its centre of mass is no longer over its base

  • The wider base an object has, the lower its centre of mass and it is more stable

  • The narrower base an object has, the higher its centre of mass and the object is more likely to topple over if pushed

Stability and base, downloadable AS & A Level Physics revision notes

The most stable objects have wide bases and low centres of mass

Finding the Centre of Mass

  • When an object is suspended from a point, the object will always settle so that its centre of mass comes to rest below the pivoting point

  • This can be used to find the centre of mass of an irregular shape:

Finding Centre of Mass, downloadable AS & A Level Physics revision notes

Diagram showing an experiment to find the centre of mass of an irregular shape

  • The irregular shape (a plane laminar) is suspended from a pivot and allowed to settle

  • A plumb line (a lead weight) 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 or three different points

  • The centre of mass is located at the point where all three lines cross

Centre of gravity v 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 = mass × acceleration due to gravity, the centre of gravity does depend on the gravitational field

  • When an object is in space, its centre of gravity will be more towards the object with the larger gravitational field

    • For example, the Earth’s gravitational field on the Moon:

Centre of gravity vs centre of mass, downloadable AS & A Level Physics revision notes

The Earth’s stronger gravitational field pushes the Moons centre of gravity closer to Earth

Examiner Tips and Tricks

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

👀 You've read 1 of your 5 free revision notes this week
An illustration of students holding their exam resultsUnlock more revision notes. It's free!

By signing up you agree to our Terms and Privacy Policy.

Already have an account? Log in

Did this page help you?

Katie M

Author: Katie M

Expertise: Physics Content Creator

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

Download notes on Centre of Mass