A stone is tied to a string and swung in a horizontal circle at constant speed. Explain why the stone is accelerating even though its speed does not change, and describe the direction of both the acceleration and the resultant force acting on the stone.

Eduqas A-Level Physics — 1.5 Circular motion (A-Level only) · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A student swings a stone tied to a string in a horizontal circle above their head. The stone completes each circle at a steady, constant speed.

Model answer (5 marks)

The stone’s speed is constant but its velocity vector changes direction as it moves around the circle. Because velocity is a vector, a change in direction means a change in velocity, i.e. an acceleration. The acceleration is directed towards the centre of the circle (centripetal). The only force on the stone is the tension in the string, which acts in the same direction – towards the centre of the circle.

Examiner tips

  • Use the term ‘centripetal acceleration’ to show understanding of direction.
  • Mention that speed is constant but velocity changes – key to explain acceleration.
  • State that the tension is the resultant force and points towards the centre.
  • Keep the answer concise and use correct UK spelling.

Mark scheme (5 marks)

  1. Velocity is a vector quantity, so it has both magnitude (speed) and direction
  2. The direction of the stone's motion is constantly changing as it moves in a circle
  3. Because velocity is changing (direction changes), the stone is accelerating
  4. The acceleration is directed towards the centre of the circle
  5. The resultant force (tension in the string) also acts towards the centre of the circle

Key terms in this question

acceleration · resultant force

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