This lesson explains Circular Motion for Cambridge IGCSE Physics 0625. It covers Supplement content for Extended candidates. Focus on the cause-and-effect explanation and the exact quantities being compared. Work through the example before testing the same skill without notes.
Why is an object moving in a circle accelerating?
Velocity is a vector. It has size and direction. An object on a circular path keeps changing direction, so its velocity changes even when its speed stays constant. A changing velocity means acceleration. That acceleration needs a resultant force, and this force acts towards the centre of the circle.
There is no centripetal force equation in 0625. You only need the definitions and the qualitative relationships below.
| Quantity | Symbol | Unit |
|---|---|---|
| Speed | m/s | |
| Mass | kg | |
| Radius of path | m | |
| Force towards centre | N |
The syllabus asks for three relationships, each with the other two quantities kept constant:
| Change | Effect on force needed |
|---|---|
| Increase speed (, constant) | Force increases |
| Increase radius (, constant) | Force decreases |
| Increase mass (, constant) | Force increases |
What provides the centripetal force?
The centripetal force is not a new force. It is an existing force acting towards the centre. Name the actual force in your answer. For a planet orbiting the Sun, gravity provides it. For a car cornering, friction between tyres and road provides it. For a ball whirled on a string, tension provides it. Cut the string and the ball flies off along the tangent, not outwards. There is no real “outward” force in the 0625 model, so never write “centrifugal force”.
Worked example
A 0.20 kg ball is whirled on a string in a horizontal circle at constant speed.
(a) Explain why the ball is accelerating. [2] (b) State the direction of the resultant force on the ball and name the force that provides it. [2] (c) The speed doubles while mass and radius stay the same. State what happens to the force in the string. [1]
Solution. (a) Velocity is a vector, so it includes direction. The direction changes continuously, so the velocity changes. Changing velocity is acceleration, even at constant speed. (b) The resultant force acts towards the centre of the circle. The tension in the string provides it. (c) The force increases. (Stating “increases” earns the mark; no formula exists at this level to quantify it.)
Original marking points
- B1: velocity is a vector / has direction, and the direction changes.
- B1: changing velocity = acceleration (accept “accelerates towards the centre”).
- B1: force directed towards the centre of the circle.
- B1: tension (in the string) named.
- B1: force increases.
These marking points belong to this original example. They are not an official Cambridge mark scheme.
This example is assessed through explanation rather than calculation, so precise wording matters.
Common errors and how to correct them
- Writing “centrifugal force”. It is not part of the required 0625 explanation. Fix: name the real inward force (tension, friction, or gravity).
- Saying the speed changes. Speed is constant; velocity changes because direction changes. Fix: always pair “velocity” with “direction” in your sentence.
- Predicting the ball flies outwards when released. It moves along the tangent in a straight line (Newton’s first law). Fix: sketch the tangent before answering.
- Reversing the radius relationship. Larger radius at the same speed and mass needs less force. Fix: memorise the three-row table above as a set.
- Vague direction statements. “Inwards” alone can lose the mark. Fix: write “towards the centre of the circle”.
How to apply this in an exam
For any “explain why it accelerates” question, use this three-sentence chain: velocity is a vector → direction changes, so velocity changes → changing velocity means acceleration. Each sentence maps to a marking point. This explanation can be tested in several contexts, so keep each causal step visible.
Where this skill matters
Circular motion is Supplement content assessed through the Extended route. Paper 2 can test “which force provides the centripetal force?” and the speed-radius-mass relationships. Paper 4 can ask the explain-the-acceleration question, often linked to satellite orbits from Space Physics, so revise the two together. It has no Paper 5/6 practical, and Core candidates are not assessed on this Supplement content. Schools may teach it late in Form 4 or early Form 5; if your school rushed it, the marking points above cover everything 0625 can ask.
Key concepts in Circular Motion
Work through each concept below. Every page explains the idea, the common exam mistakes and the calculation steps that earn marks.
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