Home · A-Level 9702 · Forces, density and pressure · Turning effects · Practice
AS · Practice questions · Turning effects of forces

Where you push matters.

Six original Cambridge-style questions on moments, forces applied at an angle, the torque of a couple, the centre of gravity, and balancing a beam.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Perpendicular distance, every time.

01
Analysis
[2 marks]

Define the moment of a force about a point, and give its SI unit.

  • The moment is the force multiplied by the perpendicular distance from the point to the line of action of the force. ✓
  • Its unit is the newton metre (N m). ✓
02
Analysis
[2 marks]

A door handle is 0.80 m from the hinge. A person pushes with 25 N perpendicular to the door at the handle. Find the moment about the hinge.

  • Moment = F × d = 25 × 0.80 = 20 N m. ✓
03
Analysis
[3 marks]

A force of 60 N is applied at the end of a 0.50 m lever, at an angle of 40° to the lever. Find the moment of the force about the other end.

  • Perpendicular distance = 0.50 sin 40° = 0.50 × 0.643 = 0.321 m. ✓
  • Moment = 60 × 0.321 = 19 N m. (Equivalently 60 × 0.50 × sin 40°.) ✓
04
Analysis
[2 marks]

A steering wheel is turned by two forces of 8.0 N applied at opposite ends of a diameter of 0.36 m, forming a couple. Find the torque of the couple.

  • Torque of a couple = one force × the separation = 8.0 × 0.36 = 2.9 N m. ✓
  • (Do not multiply by two; the formula already accounts for both forces.) ✓
05
Analysis
[3 marks]

Describe an experiment to find the centre of gravity of an irregular flat lamina using a plumb line.

  • Hang the lamina freely from a pin through one hole and let it settle; hang a plumb line from the same pin and mark the vertical line. ✓
  • Repeat from a second hole to get another line. ✓
  • The centre of gravity is the point where the two lines cross. ✓
06
Analysis
[3 marks]

A uniform beam of weight 12 N and length 2.0 m is pivoted 0.50 m from its left end. Find the moment of the beam's weight about the pivot, and state whether it acts clockwise or anticlockwise about the pivot.

  • The weight acts at the centre of gravity, the midpoint, which is 1.0 m from the left end. ✓
  • This is 1.0 − 0.50 = 0.50 m to the right of the pivot. ✓
  • Moment = 12 × 0.50 = 6.0 N m, acting clockwise (the heavy side is to the right of the pivot). ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.