A ball on a string is swung in a horizontal circle at a constant speed. Explain why, even though the ball moves at constant speed, it is still accelerating, and describe the direction of the force that causes this acceleration.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student claims that because the ball moves at constant speed around the circle, no resultant force can be acting on it. The student is incorrect.
Model answer (5 marks)
A ball moving in a horizontal circle at constant speed has a velocity that is a vector. The magnitude of the velocity (the speed) is constant, but its direction changes continuously as the ball moves around the circle. A change in velocity, even if the speed is unchanged, is a change in motion, i.e. acceleration. According to Newton’s second law, a net force is required to produce this acceleration. The force that keeps the ball moving in a circle is the centripetal force, which acts at every instant towards the centre of the circle.
Examiner tips
- Use the term ‘velocity is a vector’ to show understanding of magnitude and direction.
- Explain that changing direction = changing velocity = acceleration.
- State that a net force is required (Newton’s 2nd law).
- Identify the force as centripetal and give its direction (towards the centre).
Common mistakes
- Saying the ball is not accelerating because speed is constant.
- Confusing speed with velocity and ignoring direction.
- Failing to mention that the force must be towards the centre of the circle.
Mark scheme (5 marks)
- Velocity is a vector quantity, having both magnitude (speed) and direction
- The direction of the ball's motion is constantly changing as it moves around the circle
- A change in velocity (even with constant speed) means the ball is accelerating
- A resultant force is needed to cause this acceleration (by Newton's second law / F = ma)
- This force (centripetal force) acts towards the centre of the circle at all times
Key terms in this question
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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