GCSE · Physics · Edexcel · Spec 1PH0
Centripetal force
Whirl a ball on a string at a perfectly steady speed. Is it accelerating? Most people say no. Physics says yes — and something has to keep pulling it.
Motion in a circle
View
Switch views. In each one, find the two arrows and the right angle between them.
The string pulls the ball in towards your hand at the centre. That pull — the tension — provides the resultant force towards the centre, at right angles to the ball's velocity.
From view to view the object changes. The rule for the two arrows doesn't.
Why a force is needed
Reason it through
The ball's speed never changes. So why does it need a force at all?
First link · your turn
The speed stays the same. What about the direction the ball is moving in?
Predict, then check
You're whirling a ball on a string, and we're looking down from above. Suddenly the string snaps.
The instant the string snaps, which path does the ball take (seen from above)?
How much force?
More force or less force?
Compared with before, does the object now need a larger or a smaller centripetal force?
Still to sort
Larger centripetal force needed (0)
Where the line is: Only one thing changes on each card — the others stay the same. That's what lets you judge which way the force goes.
Smaller centripetal force needed (0)
Each card changes just one thing and keeps everything else the same. Decide which way the centripetal force needed goes.
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
Going round at a steady speed still counts as accelerating — so something has to keep pulling towards the centre.
What you need to know
- Going round a circle means your direction keeps changing — so your velocity keeps changing, even at a steady speed.
- A changing velocity needs a resultant force. In circular motion it points towards the centre, and it's called the centripetal force.
- A real force always supplies it — tension, friction or gravity — and without it the object goes straight along the tangent.
The big picture
An object moving in a circle at a constant speed is still accelerating, because its direction — and so its velocity — keeps changing. The acceleration points towards the centre, so by Newton's second law a resultant force must act towards the centre: the centripetal force. A real force always supplies it, such as tension, friction or gravity. Acting at right angles to the motion, it changes the direction but not the speed. Remove it and the object carries straight on along the tangent. A higher speed, a tighter circle or a larger mass needs a larger centripetal force.
Key points
Worked example
Problem
A hammer thrower spins a heavy metal ball round on the end of a wire. The ball moves in a circle at a constant speed. Explain why the ball is accelerating, and state what provides the centripetal force.
⚠ Watch out
Saying an object going round a circle at a constant speed is not accelerating. Its speed is constant, but its direction isn't — so its velocity is changing and it is accelerating towards the centre.
Memory hook
“Velocity runs along, force points in.” The velocity arrow runs along the tangent; the force arrow points in to the centre, at right angles. Take the force away and the object follows its velocity arrow — straight off along the tangent.
Check yourself
Cover the page. The Moon travels round the Earth in a roughly circular path. Which way does its velocity point, which way does the resultant force point, and what supplies that force?
Flashcards
(13)What is the difference between speed and velocity?
Something goes round a circle at a steady speed. Which stays the same — speed or velocity?
Is an object moving in a circle at a steady speed accelerating? If so, which way?
What is the centripetal force?
Which law tells you that an acceleration needs a resultant force?
At what angle does the centripetal force act to the velocity — and what does that mean?
Ball whirled on a string: what supplies the centripetal force?
Car going round a bend: what supplies the centripetal force?
Satellite in orbit: what supplies the centripetal force?
Is centripetal force an extra force, on top of the others?
What is a tangent to a circle?
The string holding a whirling ball breaks. Which way does the ball go?
Name three changes that each mean a larger centripetal force is needed.
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.
Learning with Lightbulb is opening soon
You can use this lesson now. Join the waitlist and we'll let you know when the full Lightbulb experience is ready.
Keep me postedMore Edexcel GCSE Physics topics
- Acceleration equation
- Acceleration of free fall g = 10 m/s^2
- Activity decreases over time
- Advantages of high-voltage transmission
- All bodies emit radiation: temperature dependence
- Ammeter in series
- Amplitude, period, wave velocity, wavefront
- Analyse energy stores in system changes
- Atmospheric pressure variation with height
- Atomic/nuclear changes generate radiation
- Background radiation
- Balancing nuclear equations
How this lesson was checked. This Edexcel GCSE Physics (specification 1PH0)lesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 30 September 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.