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Edexcel A Level Physics · Topic 6 Further Mechanics · Paper 1
Spec 5.2.2
OCR A Level Physics A · Module 5 Newtonian World and Astrophysics · Papers 1 and 3
Circular Motion Practical: How Speed Affects Radius
Studying OCR A? See the OCR version of this page (spec 5.2.2).
Studying Edexcel? See the Edexcel version of this page (spec 105–107).
In this lesson you'll learn to: explain how the tension in a string provides the centripetal force on a whirling bung,
name the variables, work out the speed from timed orbits, use F = mv²/r to explain the shape of the graph, say what the
gradient of r against v² represents, and evaluate the method.
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Prefer to watch? Scroll to the bottom for the video on this topic
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A video walkthrough is coming soon — it will appear at the bottom of this page
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Start here — have a play
Don't read anything yet. Change the speed of the bung and watch the radius. What happens to the hanging masses' force? Record at least four speeds, then look at the shape of the graph.
Speed of the bung
Tension stays the same: 100 g of hanging masses
▲ Change the speed, record your results and build the graph
Seen the shape of the graph? Now explain it.
The notes below give the method, the variables, suggested results and the F = mv²/r explanation.
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Revision notes
Modelling an orbit with a bung on a string
Not a required practical. The weight of the masses provides the tension in the string. The tension provides the centripetal force on the bung, acting towards the centre of the circle, and models the force of gravity on an orbiting object. The string is free to move through the tube, so the radius changes until it is correct for the speed.
Set the radius with the clip, then whirl the bung so the clip stays just below the tube.
- Independent variable: the speed of the bung.
- Dependent variable: the radius of the circle.
- Control variables: the hanging masses (so the tension stays the same); the mass of the bung.
Method
Set the radius with the clip, then whirl the bung so the clip stays just below the tube. Time 10 orbits, three times, and find the mean time for one orbit, T. Then v = 2πr ÷ T. Repeat for other radii. Wear eye protection.
Suggested results: 20 g bung, 100 g of hanging masses.
⭐ Why the graph is a curve
The hanging masses keep the centripetal force F constant. F = mv²/r, so r = (m/F) v². The radius is proportional to v², not v. A graph of r against v² is a straight line through the origin with gradient m/F.
Biggest error: keeping the speed steady, so that the radius stays the same (accept reaction time when timing). To improve it: practise first and watch the clip; time more orbits; or film it and time from the video.
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How to write it in the exam
Explain why the radius increases when the speed increases
The hanging masses keep the centripetal force constant →
F = mv²/r, so r = (m/F) v² →
so a higher speed needs a bigger radius, with r proportional to v².
the forcethe equationthe result
Worked example · checking the force
A 20 g bung moves in a circle of radius 0.60 m. Ten orbits take 7.0 s. The hanging masses are 100 g. Calculate the centripetal force on the bung and compare it with the weight of the masses.
- T = 7.0 ÷ 10 = 0.70 s
- v = 2πr ÷ T = 2π × 0.60 ÷ 0.70 = 5.4 m/s
- F = mv² ÷ r = 0.020 × 5.4² ÷ 0.60 = 0.97 N
- Weight = 0.100 × 9.81 = 0.98 N. They agree, so the tension provides the centripetal force.
Worked example · the gradient
A student plots r against v² for the same bung and masses. Calculate the gradient they should expect.
- r = (m/F) v², so the gradient is m/F
- gradient = 0.020 ÷ 0.98 = 0.020 s² m⁻¹
📘 Now practise it
Fill in the variables, complete the results table, plot the graph, then write your conclusion and evaluation, with answers.
Edexcel A Level Physics · Topic 6 Further Mechanics · practical worksheet OCR A Level Physics A · Module 5 Newtonian World and Astrophysics · practical worksheetThe free practical worksheet for this lesson is on its way.
✅ Can you do it? Tick as you go
0% complete🎉 Nice work! You've ticked off every objective for this practical.
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Prefer to watch? Here's the whole thing
Video walkthrough — coming soon
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it.
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it. The video is on its way.
🎬
Video coming soon
The walkthrough for this lesson is being filmed and will appear right here. Until then, the animation and the notes above cover every mark.
Now put it into practice
Complete the results table, plot the graph and evaluate the method, with answers.
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