AQA A-Level Physics Paper 1, June 2023: Question 28
1 mark · Medium difficulty · Multiple Choice
Calculate the frequency of rotation of two rotating discs given the speed of a pellet passing through them and the angular separation between the holes it creates.
Practise this questionQuestion
Question text
28 Two circular discs made of card rotate at constant speed on a common axle.
The discs are 2.00 m apart.
An air-gun pellet is fired parallel to the axle. The pellet makes holes in the discs.
The holes are separated by an angle of 45°.
The speed of the pellet between the discs is 300 m s−1.
How many revolutions does each disc complete in one second?
[1 mark]
A 19
B 118
C 740
D 1074
Mark scheme
Show the mark scheme
28 A 19
How to answer it
Rotating Discs and Pellet Speed Analysis
What this question tests
This question assesses your understanding of angular speed, linear kinematics, and periodic motion. You must combine the time taken for a projectile to travel a linear distance with the corresponding angular displacement of a rotating system to determine the frequency of rotation (revolutions per second).
Question 28: Multiple Choice Calculation
Determining Revolutions per Second
✅ Correct Answer
A: 19
💡 Key Knowledge
- Relating linear speed ( v ), distance ( s ), and time ( t ): t = s / v
- Converting angles between degrees and radians, or working directly with fractions of a full rotation ( 360° ).
- Frequency ( f ) is measured in revolutions per second ( s⁻¹ or Hz ).
🧠 Exam Technique
For multiple-choice calculations, always calculate the exact value first before looking at the options. Avoid rounding intermediate steps too early to prevent off-by-one errors in the final significant figures.
❌ Common Errors
- Forgetting to convert the angle of 45° into a fraction of a full circle (using 45 instead of 45/360).
- Inverting the time and speed relationship ( v × s instead of s / v ).
- Confusing revolutions per second (frequency) with angular speed in radians per second ( ω ).
📐 Step-by-Step Calculation
- Find the time ( t ) taken for the pellet to travel between the two discs:
Distance s = 2.00 m
Speed v = 300 m s⁻¹
t = s / v = 2.00 / 300 = 6.667 × 10⁻³ s - Determine the fraction of a full revolution completed in that time:
Angle rotated = 45°
Fraction of a circle = 45 / 360 = 1 / 8 = 0.125 of a revolution. - Calculate the frequency ( f ) in revolutions per second:
If 0.125 revolutions take 6.667 × 10⁻³ s , then the number of revolutions in 1 s is:
f = 1 / time for 1 revolution
Time for 1 full revolution = (6.667 × 10⁻³ s) / 0.125 = 0.05333 s
f = 1 / 0.05333 = 18.75 s⁻¹ - Round to appropriate significant figures:
Rounding 18.75 s⁻¹ to 2 significant figures gives 19 s⁻¹ , which matches option A.
Topics
Physics · 3.6 Further mechanics and thermal physics (A-level only)
Question and mark scheme from the AQA A-Level Physics examination, Paper 1, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.