A ball is attached to a string and swung in a vertical circle at constant speed. Explain why the ball moves in a circular path rather than continuing in a straight line, and describe the direction of both the velocity and the resultant force acting on the ball at any point in its circular path.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
A ball in a vertical circle would, by Newton’s first law, continue in a straight line unless a force acts on it. The tension in the string supplies a force that is always directed towards the centre of the circle. This inward (centripetal) force continuously changes the direction of the ball’s velocity, keeping it on a circular path. At any point the velocity is tangent to the circle, while the resultant force (the tension) is perpendicular to the velocity and points towards the centre.
Examiner tips
- Use Newton’s first law to justify why a force is needed
- State that tension is the centripetal force
- Describe velocity as tangent and force as radial
- Show the force is always perpendicular to velocity
Common mistakes
- Confusing the direction of velocity with the direction of the force
- Forgetting that the force is always towards the centre
- Not mentioning Newton’s first law or inertia
Mark scheme (5 marks)
- Without a force the ball would continue in a straight line (Newton's first law / inertia)
- The tension in the string provides a resultant force that acts towards the centre of the circle
- This inward (centripetal) force continuously changes the direction of the ball, keeping it on a circular path
- The velocity of the ball at any point is directed along the tangent to the circle at that point
- The resultant force (centripetal force) is always directed towards the centre of the circle, perpendicular to the velocity
Key terms in this question
circular path · velocity · resultant force
Related
- All Eduqas A-Level Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Circular motion (A-Level only) questions
- A roller-coaster carriage travels at constant speed around a circular loop. Expl…
- A car travels at constant speed around a circular roundabout. Explain why the ca…
- A stone is tied to a string and swung in a horizontal circle at constant speed. …
- A satellite travels at constant speed in a circular orbit around the Earth. Expl…