GCSE · Physics · Edexcel · Spec 1PH0
Circular motion at constant speed
Whirl a bag round your head at a perfectly steady speed. Physics says it is accelerating the whole time, and that something is pulling it inwards. Here's why.
Physics · Circular motion
View
Step round the circle ↑
At the top the ball is moving right. The velocity arrow points along the tangent (at right angles to the string); the string pulls the ball towards the centre.
A ball on a string, whirled round on a smooth table at a steady speed, seen from above. Step round the circle: watch the blue velocity arrow, then the red force arrow.
Physics · Speed or velocity?
Same motions, two questions
Put each motion in the right column.
Is the object getting faster or slower?
Still to sort
Changing (0)
Something about the motion is different from one moment to the next.
Where the line is: For velocity, a change of direction alone is enough to count as changing.
Not changing (0)
Exactly the same from one moment to the next.
Where the line is: For velocity, both the speed AND the direction must stay the same.
Sort the six motions by speed first. Then switch to velocity and sort them again. Watch which ones move.
Circular motion · the reasoning
Reason it through
Why does something going round in a circle at a steady speed need a force at all?
First link · your turn
Think about the direction of motion as the object goes round. What keeps happening to it?
Predict, then check
Same ball on a string, on a smooth table, seen from above. Ignore friction.
The ball is at the top of the circle, moving to the right, when the string snaps. Which path does the ball take?
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
Why anything going round in a circle needs a force pulling it towards the centre.
What you need to know
- Velocity is speed in a given direction, so changing direction changes the velocity.
- An object moving in a circle at a constant speed has a constantly changing velocity, so it is accelerating.
- At any instant the velocity points along the tangent to the circle, the way the object is moving.
- The resultant force on an object moving in a circle points towards the centre of the circle.
- If that force is removed, the object carries on in a straight line along the tangent.
The big picture
Velocity is speed in a given direction. An object moving in a circle at a constant speed keeps changing direction, so its velocity keeps changing, which means it is accelerating. An acceleration needs a resultant force, and for circular motion that force points towards the centre of the circle: the tension in a string for a whirled ball, gravity for the Moon or a satellite. Remove the force and the object carries on in a straight line along the tangent.
Key points
Worked example
Problem
A fairground ride carries a rider round a flat, horizontal circle at a constant speed. A student says: “The rider's speed isn't changing, so the rider isn't accelerating.” Is the student right? Explain your answer.
⚠ Watch out
Saying an object going round a circle at a constant speed “isn't accelerating” or “has a constant velocity”. The speed is constant but the direction isn't, so the velocity changes. And the force points towards the centre, never outwards.
Memory hook
Same length, new direction. The velocity arrow never grows or shrinks, it just keeps turning, and the only thing that can turn it is a pull towards the centre.
Check yourself
Without looking back: why is a ball whirled round at a steady speed accelerating, which way does the resultant force on it point, and where does it go if the string breaks?
Flashcards
(8)What is velocity?
Can an object with a constant speed have a changing velocity?
Why is an object moving in a circle at constant speed accelerating?
At any instant, which way does the velocity point for an object moving in a circle?
Which way does the resultant force act on an object moving in a circle at constant speed?
What provides the force towards the centre for a ball whirled on a string?
What provides the force towards the centre for the Moon or a satellite orbiting Earth?
What happens to an object moving in a circle if the force towards the centre is removed?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
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