Circular motion
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 1 worked example, 9 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Going round in circles
- Why it needs a force
- Picture it
- What provides the force
Read the transcript of the teaching part
Last time
Before we start: last time, in Moments, balance and stability, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Going round in circles
Spin a ball round on a string. The speed can be steady, yet the ball is always accelerating. Its direction keeps changing, and a change of direction is a change of velocity.
Why it needs a force
An object moving in a circle at constant speed keeps changing direction, so its velocity keeps changing. A changing velocity is an acceleration, so there must be a resultant force. That force always points towards the centre of the circle. We call it the centripetal force. It is at right angles to the motion, so it changes the direction but not the speed.
Picture it
Look at the object going round. The velocity points along the circle, along the tangent. The force always points inwards, to the centre. If the force vanished, the object would fly off in a straight line along the tangent.
What provides the force
For a ball on a string, the force is the tension in the string. For a car on a bend, the force is the friction between the tyres and the road. For the Moon going round the Earth, and for any satellite, the force is gravity. The force is not a new kind of force. It is always one of the forces we already know.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Circular motion · Circular motion.
Chapter 1.5: Forces
- Balanced forces and F = ma 6 min
- Newton's third law 5 min
- Friction, drag and terminal velocity 5 min
- Thinking, braking and stopping distance 6 min
- Circular motion 5 min
- Springs: load–extension graphs 5 min
- Moments, balance and stability 5 min
Also for O Level Physics 5054
Which topics come up most in O Level Physics 5054 past papers, counted from every paper since 2004.