AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
The weight of an object may be considered to act at a single point…
Full specification wording
Gravity: The weight of an object may be considered to act at a single point referred to as the object’s ‘centre of mass’.
Weight is the gravitational force on an object, and for many calculations you can treat this force as acting at one point: the centre of mass.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 9 of 11
Study notes
From the checked teaching notes for this specification point.
The idea
Weight is the gravitational force on an object, and for many calculations you can treat this force as acting at one point: the centre of mass. This point is the balance point of the object. For a uniform ruler, it lies at the midpoint; for a symmetrical shape, it lies on every axis of symmetry. If you support an object at its centre of mass, it balances because the total clockwise moment equals the total anticlockwise moment. This idea lets you draw a single downward arrow from that point when solving moment or equilibrium problems, rather than drawing many small arrows. It also explains why a low centre of mass improves stability.
What earns the marks
- Weight is the force of gravity acting on an object and is measured in newtons (N).
- The centre of mass is the single point at which the whole weight of an object may be considered to act.
- For a uniform regular object, the centre of mass is at its geometric centre; for a symmetrical object, it lies on the axis or axes of symmetry.
- An object suspended freely comes to rest with its centre of mass directly below the point of suspension.
- When taking moments, you can treat the entire weight as a single force acting vertically downwards through the centre of mass.
- A lower centre of mass makes an object more stable because the line of action of its weight stays within its base for a larger tilt.
Common mistakes and how to put them right
Confusing mass and weight.
Put it right by labelling the quantity and unit.
Thinking the centre of mass must lie inside the material of the object.
Put it right by recalling examples such as a ring or a horseshoe, whose centre of mass is in the empty space at the centre.
Drawing the weight arrow from the geometric centre of an irregular object.
Put it right by finding the centre of mass experimentally, for example by suspending the object and using a plumb line.
Exam tips
- When drawing force diagrams, show weight as one arrow acting vertically downwards from the centre of mass, and label it W or weight.
- In moment questions, state clearly that you are taking the weight to act at the centre of mass, then use the perpendicular distance from the pivot to that point.
- For stability questions, refer to the line of action of weight falling inside or outside the base, not just to the height of the object.
Try the lesson
The start of the Gravity: The weight of an object may be considered to act at a single point referred to as the object’s ‘centre of mass’. lesson, made from checked teaching notes for this specification point.
Have a go first
What name is given to the single point at which the weight of an object may be considered to act?
Listen
Weight is the gravitational force on an object, and for many calculations you can treat this force as acting at one point: the centre of mass. This point is the balance point of the object. For a uniform ruler, it lies at the midpoint; for a symmetrical shape, it lies on every axis of symmetry. If you support an object at its centre of mass, it balances because the total clockwise moment equals the total anticlockwise moment. This idea lets you draw a single downward arrow from that point when solving moment or equilibrium problems, rather than drawing many small arrows. It also explains why a low centre of mass improves stability. A strong answer earns marks for these points. Weight is the force of gravity acting on an object and is measured in newtons (N). The centre of mass is the single point at which the whole weight of an object may be considered to act. For a uniform regular object, the centre of mass is at its geometric centre; for a symmetrical object, it lies on the axis or axes of symmetry. An object suspended freely comes to rest with its centre of mass directly below the point of suspension. A common slip: Confusing mass and weight: mass is the amount of matter in kilograms (kg) and is constant, while weight is a force in newtons (N) that depends on gravitational field strength. Instead, labelling the quantity and unit. Exam tip: When drawing force diagrams, show weight as one arrow acting vertically downwards from the centre of mass, and label it W or weight.
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