AQA A-Level Physics Paper 1, June 2024: Question 28

1 mark · Medium difficulty · Multiple Choice

Identify the direction of the frictional force acting on a small mass placed on a rotating horizontal turntable.

Practise this question

Question

A multiple-choice question showing a top-down view of a horizontal turntable rotating clockwise about a centre O. A small mass P is positioned on the turntable. Dashed lines connect P to O, and extend horizontally to points Q and R, and vertically to point S. Students must choose from options A (from P to O), B (from P to Q), C (from P to R), or D (from P to S) to identify the direction of the frictional force on the mass.
Question text

28 A small mass is placed at P on a horizontal turntable. The turntable rotates clockwise with

a constant angular speed about a vertical axis through its centre O.

The mass remains at rest relative to the turntable.

What is the direction of the frictional force on the mass at the instant shown?

[1 mark]

A from P to O

B from P to Q

C from P to R

D from P to S

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer is A, corresponding to 'from P to O', with an assessment objective of AO1.

28 A from P to O AO1

How to answer it

Direction of Frictional Force in Circular Motion

What this question tests

This question assesses your understanding of circular motion dynamics (AO1), specifically the requirement for a centripetal force to maintain an object in a circular path, and your ability to identify the physical origin of that force (friction) and its orientation relative to the centre of rotation.

Question 28

Exam Breakdown & Analysis

✅ Correct Answer

A: from P to O

Awarded 1 mark for selecting option A.

💡 Key Knowledge

  • Any object moving in a circle at a constant speed experiences acceleration directed towards the centre of the circle (centripetal acceleration).
  • By Newton's Second Law, a resultant force—the centripetal force—must act in the exact same direction as the acceleration: directly towards the centre ( O ).
  • On a horizontal turntable, friction between mass P and the surface is the external horizontal force providing this necessary centripetal force.

🧠 Exam Technique

  • Ignore distractors: The mention of "clockwise rotation" is extra information that can confuse students into thinking about velocity or tangential vectors.
  • Anchor to the centre: Always locate the centre of rotation ( O ). For any object performing uniform circular motion, any force acting as the centripetal force must point straight along the radius towards O .

❌ Common Errors

  • Tangential confusion (Options B or C): Students often mix up centripetal force with linear velocity or linear momentum and incorrectly choose directions tangential to the circle ( P to Q or P to R ).
  • Centrifugal misconception (Option D): Choosing P to S assumes a "fictitious outward force" pushes the mass away from the centre, which contradicts real inertial forces in an inertial reference frame.

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 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.