AQA AS Level Physics Paper 2, June 2025: Question 23

1 mark · Medium difficulty · Multiple Choice

Determine which diagram correctly shows a uniform disc in equilibrium when acted upon by weight, tension, and a couple.

Practise this question

Question

Multiple choice question showing four diagrams (A, B, C, D) of a circular disc acted upon by three forces: tension T in a vertical thread, and horizontal forces P and Q forming a couple at the top and bottom edges. In A, T = 25 N is tangential to the right edge, P points left at the top, Q points right at the bottom. In B, T = 50 N is on the right edge. In C, T = 10 N is on the left edge. In D, T = 50 N is tangential to the left edge, P points left at the top, and Q points right at the bottom.

Mark scheme

Show the mark scheme Mark scheme indicating the correct answer for question 23 is option D, showing the diagram where T = 50 N acts upwards tangentially on the left side of the disc, force P acts to the left at the top, and force Q acts to the right at the bottom.

How to answer it

AQA AS Physics • Mechanics • Moments & Equilibrium

Equilibrium of a Suspended Disc Under a Couple

What this question tests

This question tests the two essential conditions required for a rigid body to be in complete static equilibrium:

  • Translational Equilibrium: The vector sum of all external forces must equal zero ( ΣF = 0 ).
  • Rotational Equilibrium: The sum of clockwise moments about any pivot must equal the sum of anticlockwise moments ( Στ = 0 ).
  • Understanding how to identify the moment of a couple and balance it using a single force acting at a perpendicular distance from the centre of mass.

Question 23 Analysis

Multiple Choice • 1 Mark • AO2 (Application of Knowledge)

✅ Correct Answer: Option D

The correct diagram is D, where the thread provides an upward tension of T = 50 N tangential to the left-hand rim of the disc.

Mark Scheme Breakdown:
• 1 mark for selecting D.

💡 Key Knowledge

  • Weight acts downwards through the centre of gravity ( W = 50 N at the centre).
  • Couple: A pair of equal and opposite coplanar forces whose lines of action do not coincide.
    Torque of couple = Force × perpendicular distance = F × 2r .
  • For total equilibrium, both linear forces and moments must be individually balanced.

🔨 Step-by-Step Derivation

  1. Step 1: Balance Vertical Forces (Translational Equilibrium)
    The uniform disc has a weight of W = 50 N directed downwards.
    The only vertical force balancing weight is the upward tension T in the thread.
    ΣFvertical = T - W = 0 ⇒ T = 50 N .
    Outcome: Options A ( T = 25 N ) and C ( T = 10 N ) are immediately eliminated.
  2. Step 2: Determine the Direction and Magnitude of the Couple's Torque
    Force P = 25 N acts to the left at the top rim.
    Force Q = 25 N acts to the right at the bottom rim.
    Taking moments about the central axis (radius r ):
    • Moment of P = 25 × r (anticlockwise)
    • Moment of Q = 25 × r (anticlockwise)
    Total anticlockwise torque from couple = 25r + 25r = 50r (anticlockwise).
  3. Step 3: Balance Rotational Moments About the Centre
    The weight W acts through the centre, producing zero moment about the centre.
    Therefore, the tension T = 50 N must provide a clockwise moment of 50r to achieve rotational equilibrium:
    • If T is on the right rim (Option B): Pulling upwards produces an anticlockwise moment ( 50 × r ), adding to the couple. Result = unbalanced ( 100r anticlockwise).
    • If T is on the left rim (Option D): Pulling upwards produces a clockwise moment ( 50 × r ), exactly canceling the couple!

🧠 Exam Technique • Elimination Strategy

  • Two-stage filter: Always split 2D equilibrium problems into forces first, then moments.
  • Filtering by vertical forces ( ΣFy = 0 ) takes 5 seconds and halves your choices (down to B and D).
  • Use your finger or pen to simulate rotation: place your finger at the centre of the circle, push up where T acts, and observe which way it turns. You need a clockwise rotation to counteract the couple.

❌ Common Traps & Misconceptions

  • Confusing Couple with Single Force: Thinking the tension needs to equal the value of force P or Q ( 25 N ), leading students to pick A.
  • Wrong Rotation Sense: Incorrectly assuming an upward pull on the right produces a clockwise moment. It turns the wheel anticlockwise!
  • Ignoring Weight: Forgetting that the disc itself has mass and weight ( 50 N ), which acts at the centre of gravity even though it is not explicitly drawn with an arrow.

Topics

Physics · 3.4 Mechanics and materials

Question and mark scheme from the AQA AS Level Physics examination, Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.