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5.6.7Forces

Circular Motion

5 min read 3 parts Lesson 43 of 67 Unit quiz

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Section 1 of 3Speed and Velocity in Circles

Circular motion can look simple, but the direction of motion keeps changing, which means the object is accelerating.

Speed
  • Speed The magnitude of velocity, which can stay constant while an object moves around a circle.
Velocity
  • Velocity A vector quantity, so its direction changes continuously as the object moves around the circle.
Acceleration
  • Acceleration A change in velocity, including a change of direction, so the object is always accelerating towards the centre.
Direction of acceleration
  • Direction of acceleration The acceleration points towards the centre of the circle, not along the direction the object is moving.

Section 2 of 3Forces in Circular Motion

A resultant force is needed to keep an object moving in a circle, and it must always act in the same way.

1
An object moving in a circle at constant speed has a resultant force acting towards the centre of the circle.
2
This force is called the centripetal force, and it always acts at right angles to the velocity.
3
If the centripetal force is removed, the object moves off in a straight line along a tangent to the circle.
4
Centripetal force does no work on the object, because it never changes the object's speed.

Section 3 of 3Exam Checklist & Tips

5.6.7 – Key Points for the Exam

  • Centripetal force A resultant force acting towards the centre of a circle that keeps an object moving in a circle.
  • Tangent A straight line that touches a circle at a single point and shows the direction of motion at that point.
  • Resultant force The single force that results from all the forces acting on an object together.
  • Constant speed A speed that does not change over time, even when the direction of motion changes.
  • Acceleration The rate of change of velocity, which includes changes in direction.

Common Mistakes

  • Saying an object moving in a circle at constant speed has zero acceleration, when its direction is changing.
  • Drawing the centripetal force pointing outwards from the centre of the circle, when it acts inwards.
  • Describing centripetal force as an extra force added on top of the forces already acting on the object.
  • Stating that an object flies outwards because of the centripetal force, when the object moves off in a straight line once that force is removed.
Physics 0625 & 9702 SUVAT Mastery Graph Interpretation

Constant Acceleration Equations (SUVAT) & Slopes

v = u + at  |  s = ut + ½at2  |  v2 = u2 + 2as  |  s = ½(u + v)t

  • Displacement-Time Graph: Gradient = Velocity (curve = accelerating).
  • Velocity-Time Graph: Gradient = Acceleration; Area under curve = Distance travelled / Displacement.
  • Free Fall in vacuum: a = g ≈ 9.81 m/s2 downwards (air resistance negligible).