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Motion and Forces

Circular motion and centripetal force

  • 3 things to know
  • Paper 5

Overview

Why velocity changes in circular motion at constant speed, what a centripetal force is and where it acts, and gravity, friction and tension as the forces providing it.

What you need to know

Answer each one before you reveal it.

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Key term

A car drives around a roundabout at a steady 8 m/s. Compare its speed and its velocity during the turn.

Key term

A ball on a string is whirled in a horizontal circle at a steady rate. Describe how its velocity changes.

Key term

Explain why an object cannot move along a circular path unless a resultant force acts on it.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Explain why an object moving in a circle at a constant speed has a changing velocity.

    Model answer

    Its direction of motion changes continuously, and velocity depends on direction as well as on speed.

  2. 2

    A satellite orbits the Earth at a steady speed. State whether its velocity is constant and give a reason.

    Model answer

    Its velocity is not constant, because the direction of travel keeps changing.

  3. 3

    Is an object moving around a circular path at constant speed accelerating? Explain your answer.

    Model answer

    Yes, because its velocity changes direction, and any change of velocity is an acceleration.

  4. 4

    A car drives around a roundabout at a steady 8 m/s. Compare its speed and its velocity during the turn.

    Model answer

    The speed stays at 8 m/s, but the velocity changes because the direction changes.

  5. 5

    A ball on a string is whirled in a horizontal circle at a steady rate. Describe how its velocity changes.

    Model answer

    The magnitude of the velocity stays the same while its direction changes at every point of the circle.

  6. 6

    What is meant by a centripetal force?

    Model answer

    It is the resultant force acting towards the centre of a circular path.

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Key Concepts of Physics
Motion and Forces
Conservation of Energy
Waves

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ALength is measured in metres, with the symbol m.
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