Pearson Edexcel · GCSE · Physics
Course version: 1PH0 · Version from Sept 2016
Explain that for motion in a circle there must be a resultant force known…
Full specification wording
Topic 2 – Motion and forces: 2.21 Explain that for motion in a circle there must be a resultant force known as a centripetal force that acts towards the centre of the circle
An object moving in a circle at constant speed is still accelerating, because its velocity changes direction continuously.
Checked against the Pearson Edexcel 1PH0 specification (opens in a new tab) on .
- Point 20 of 32
Study notes
From the checked teaching notes for this specification point.
The idea
An object moving in a circle at constant speed is still accelerating, because its velocity changes direction continuously. Newton's first law therefore requires a resultant force, and for circular motion this resultant force is called the centripetal force. Its direction is always towards the centre of the circle, perpendicular to the instantaneous velocity, so it changes direction but not speed. The centripetal force is not an extra force: it is the name for whatever resultant force is provided, such as tension in a string, gravity on a satellite, or friction between tyres and road. If that resultant force disappears, the object travels in a straight line along the tangent instead of continuing round the circle.
What earns the marks
- States that an object moving in a circle has a changing velocity direction and so accelerates.
- Identifies the resultant force on an object in circular motion as the centripetal force.
- States that the centripetal force acts towards the centre of the circle.
- Explains that the centripetal force is provided by a real force such as tension, gravity or friction rather than being an additional force.
Common mistakes and how to put them right
Places the centripetal force pointing outwards along the radius.
Put it right: Direct it towards the centre of the circle.
Treats the centripetal force as a separate extra force added to tension or gravity.
Put it right: Identify it as the resultant of the forces already acting.
Claims that an object moving at constant speed in a circle has no acceleration.
Put it right: Note that the changing direction of velocity is itself an acceleration.
Exam tips
- Read the question's wording for the direction required and state clearly that the force points towards the centre.
- Name the real force providing the centripetal force in the context given, such as tension, gravity or friction.
- Link the force to the change in velocity direction rather than to a change in speed.
Try the lesson
The start of the Topic 2 – Motion and forces: 2.21 Explain that for motion in a circle there must be a resultant force known as a centripetal force that acts towards the centre of the circle lesson, made from checked teaching notes for this specification point.
Have a go first
A ball is swung in a circle on a string. In which direction does the resultant force on the ball act?
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