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AQA GCSE Physics · Topic 4.5 Forces · Physics (Triple) only
Edexcel International GCSE Physics · Section 1 Forces and motion · Physics only
Moments — Turning Forces and Balance
Triple only Moments, levers and gears (4.5.4) are Physics (Triple) only
— Combined Science students can skip this page.
Physics only Moments (1.30P–1.33P) are Physics only — Double Award
students can skip this page.
In this lesson you'll learn to: define the moment of a force and use M = F d; find the
perpendicular distance from the pivot to the line of action; use the principle of moments to balance
an object; and use the centre of gravity to find an unknown weight.
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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. Turn the bike pedal: when does the same push give the biggest turning effect, and when does it give none at all? Then test yourself on three pedals, balance the seesaw, and use a rope and a force meter to find the weight of a plank.
Turn the crank
The foot always pushes straight down with 100 N. The crank is 0.17 m long.
Left side · anticlockwise
Right side · clockwise
Pull up on the rope
moment = force × perpendicular distance M = F d (N m). The perpendicular distance is measured from the pivot to the line of action of the force, at right angles to it. Balanced (in equilibrium): total clockwise moment = total anticlockwise moment.
Physics With Kate · physicswithkate.com — GCSE & International GCSE Physics
It's not how long the lever is that counts: it's the perpendicular distance to the line of the force.
Moment = force × perpendicular distance.
And when it's balanced, clockwise moments = anticlockwise moments. The notes show you how to set out a moments calculation for full marks.
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Revision notes
The moment of a force
A moment is the turning effect of a force about a pivot.
⭐ Learn this word for word (it's a long one)
The moment of a force is the force multiplied by the perpendicular distance between the pivot and the line of action of the force.
moment = force × perpendicular distance M = F d M in N m, F in N, d in m.
- The line of action is the line the force acts along, extended in both directions.
- The perpendicular distance is measured from the pivot to that line, at right angles to it. It is not always the length of the spanner, crank or beam.
- Distances must be in metres: 25 cm = 0.25 m, 600 mm = 0.60 m.
Which has the largest moment?
Balanced: the principle of moments
⭐ Learn this
When an object is balanced (in equilibrium), the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot.
So a small force a long way from the pivot can balance a large force close to it: a 400 N child 1.5 m from the pivot balances a 600 N adult 1.0 m from it (400 × 1.5 = 600 × 1.0 = 600 N m).
Centre of gravity
The weight of an object acts through its centre of gravity. For a uniform plank, beam or ruler, that's the middle. That's how you find an unknown weight: balance the plank with a known force, then use the principle of moments with the weight acting at the centre.
A beam on two supports
A heavy object on a beam resting on two supports: the support nearer the object pushes up with the bigger force. Move the object towards a support and that support's upward force increases, while the other one's decreases. The two upward forces always add up to the total weight.
Levers and gears AQA
A lever is a force multiplier: a small effort a long way from the pivot gives a large force close to it (a crowbar, a wheelbarrow, a spanner). Gears transmit a turning effect from one shaft to another. A small gear driving a larger gear makes the larger one turn more slowly but with a bigger moment.
⚠ Trap — perpendicular, and in metres
Use the perpendicular distance to the line of action, not the length of the object. Convert cm and mm to m before you multiply, or your moment is 100 or 1000 times too big. A moment's unit is N m.
The set-up that gets the marks
Taking moments about the pivot: the object is balanced, so
total clockwise moment = total anticlockwise moment, so
F₁ × d₁ = F₂ × d₂.
say where you take moments state the principle then substitute
🗣 Words to use in a written answer
Writing an “explain” answer? Tick each one off as you use it.
How to write it in the exam
A spanner is 25 cm long. A force of 60 N is applied at right angles to the end of the spanner. Calculate the moment of the force on the nut.
d = 25 cm = 0.25 m. M = F × d = 60 × 0.25 = 15 N m
A child of weight 400 N sits 1.5 m from the pivot of a seesaw. How far from the pivot, on the other side, must an adult of weight 600 N sit to balance it?
Balanced, so total clockwise moment = total anticlockwise moment: 600 × d = 400 × 1.5 = 600 N m, so d = 600 ÷ 600 = 1.0 m
A light beam 240 cm long rests on supports A and B at its ends. A crate rests on the beam 600 mm from A. The upward force from B is 0.45 kN. (a) Calculate the weight of the crate. (b) Calculate the upward force from A.
(a) Distances in metres: 0.60 m and 2.40 m. Taking moments about A: W × 0.60 = 0.45 × 2.40, so W = 1.08 ÷ 0.60 = 1.8 kN (1800 N). (b) Total up = total down: FA + 0.45 = 1.8, so FA = 1.35 kN
A uniform plank is hinged at one end. It is held level by a rope 2.0 m from the hinge, pulling up with a force of 40 N. The centre of the plank is 1.0 m from the hinge. Calculate the weight of the plank.
The weight acts at the centre of gravity, 1.0 m from the hinge. Balanced, so W × 1.0 = 40 × 2.0, so W = 80 N
Explain why a door handle is put as far from the hinges as possible. (2 marks)
The perpendicular distance from the pivot (the hinges) is larger, so the same force gives a larger moment — or a smaller force is needed to open the door.
✅ Check your answer
Distances in metres · the perpendicular distance used · say where you take moments · write “clockwise = anticlockwise” before you substitute · unit N m for a moment, N for a force.
📘 Now do it in your workbook
The moment in words and symbols, write your own moment question, which has the largest moment, the conditions for equilibrium, and the beam-on-two-supports exam question.
Edexcel iGCSE Forces and Motion Workbook · page 16 AQA GCSE Physics Workbook · FORCES · page 8✅ Can you do it? Tick as you go
0% complete🎉 Nice work! You've ticked off every objective for this topic.
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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.
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Video coming soon
The walkthrough for this lesson is being filmed and will appear right here. Until then, the notes and the free workbook page above cover every mark.
Now put it into practice
Page 16 of the Forces and Motion workbook is free: the definition, which has the largest moment, equilibrium and the beam exam question.
Page 8 of the Forces workbook is free: the definition, which has the largest moment, equilibrium and the beam exam question.
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