AQA GCSE Physics Foundation (8463), Foundation tier · Forces
Practise Moments, levers and gears. 7 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
A force can make an object turn about a pivot. You need to calculate moments with M = F d, use the balance of clockwise and anticlockwise moments, and explain how levers and gears transmit the turning effect of forces. This is in GCSE Physics only, not in Combined Science.
Key facts
M = F d (moment = force × perpendicular distance from the pivot)
Units: M in N m, F in N, d in m
Balanced: total clockwise moment = total anticlockwise moment
Greater distance from the pivot means a smaller force is needed for the same moment
Lever = force multiplier when the effort is further from the pivot than the load
Small gear drives large gear: larger moment, slower rotation
Large gear drives small gear: smaller moment, faster rotation
Meshing gears turn in opposite directions
Notes
Moments
diagram
A force, or a system of forces, can cause an object to rotate about a pivot.
The turning effect of a force is called the moment of the force.
moment of a force = force × distance: M = F d
M in newton-metres (N m), F in newtons (N), d in metres (m).
d is the perpendicular distance from the pivot to the line of action of the force. The moment is biggest when you push at right angles, far from the pivot.
Moment = F × d, with d measured at right angles to the force.
Balanced objects
diagram
If an object is balanced, the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot.
Example: a 300 N child sits 2.0 m from the pivot of a see-saw (moment 600 N m). A 400 N child balances it by sitting 600 ÷ 400 = 1.5 m from the pivot on the other side.
Balanced: 300 × 2.0 = 600 N m anticlockwise equals 400 × 1.5 = 600 N m clockwise.
If the moments are not balanced, the object turns in the direction of the larger total moment.
Levers
A simple lever transmits the rotational effect of a force.
The effort and the load each have a moment about the pivot. If the effort is further from the pivot than the load, a small effort can move a large load: the lever is a force multiplier.
Examples: a crowbar, a wheelbarrow, a spanner, a bottle opener.
Gears
diagram
Gears are wheels with teeth that interlock, so one gear turning makes the next one turn. Neighbouring gears turn in opposite directions.
Where the teeth meet, the forces on the two gears are the same size. The bigger gear has a larger radius, so the moment on it is larger.
The small gear turns 3 times for each turn of the big gear. The big gear turns the other way, more slowly, with a bigger moment.
Small gear driving a big gear: the big gear turns more slowly but with a bigger moment. This is how the first (lowest) gear in a car gives a large turning effect for pulling away.
Big gear driving a small gear: the small gear turns faster but with a smaller moment.
How to answer each type of question
Calculate a moment
2 marksGrade 4
Check the distance is the perpendicular distance from the pivot, in metres.
Write M = F d and substitute.
Give the unit N m.
Example. A mechanic pushes at right angles on a spanner with a force of 45 N, 0.20 m from the centre of a nut. Calculate the moment of the force about the nut.
Show the model answerHide the model answer
M = 45 × 0.20 (1) M = 9.0 N m (1)
Don’t lose marks
Using the length along a slanted bar instead of the perpendicular distance from the pivot to the line of action of the force.
Using distances in cm when the answer should be in N m.
Adding the clockwise and anticlockwise moments together instead of setting them equal.
Forgetting that neighbouring gears turn in opposite directions.
Saying gears 'create more energy'. They change the speed and the moment, but they do not give out more energy than is put in.
More tips
Memory tricks
Moment = force × perpendicular distance to the pivot. Say 'perpendicular' every time.
See-saw sense check: the heavier person must sit closer to the pivot.
Gears: bigger driven gear = slower but bigger turning effect.
Longer handle, less effort: that is why door handles are fixed far from the hinges.
Exam technique
M = F d must be recalled.
In balance questions, write 'total clockwise moment = total anticlockwise moment' first, then substitute.
In lever and gear 'explain' questions, use the word 'moment' and link the distance from the pivot to the size of force needed.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Give examples of turning effects of forcesOpening a door, turning a spanner and a see-saw all involve forces that cause rotation.
Grade 4
Calculate a moment using M = F dMoment (N m) = force (N) × perpendicular distance from the pivot (m).
Grade 5
Find a force or distance from a momentF = M ÷ d and d = M ÷ F.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Write down the equation that links distance (d), force (F) and moment of a force (M).
M = F d
State whether it is easier to crack the nut by placing it close to the hinge, or far from the hinge (near the handles).
Close to the hinge.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) A force is applied further from the pivot of a lever, without changing its size. What happens to the moment of the force? Tick (✓) one box.[1]
It increases
It decreases
It stays the same
It becomes zero
(b) Which two of these are examples of a lever? Tick (✓) two boxes.[2]
A pair of scissors
A brick
A see-saw
A ball
(c) Write down the equation that links distance (d), force (F) and moment of a force (M).[1]
Show the answer and mark scheme
(a)Answer: It increases
(b)Answer: A pair of scissors; A see-saw
(c)Answer: M = F d
M = F d
Question 2Medium4 marks
A student uses a pair of nutcrackers to crack a nut. The nutcrackers act as a lever with the hinge at one end and the hands squeezing the handles at the other end.
(a) State whether it is easier to crack the nut by placing it close to the hinge, or far from the hinge (near the handles).[1]
(b) Explain your answer, using the idea of moments.[3]
Show the answer and mark scheme
(a)Answer: Close to the hinge.
close to the hinge
(b)Answer: The moment of the hands’ force about the hinge is the same; the nearer the nut is to the hinge, the smaller its distance, so the bigger the force on the nut (force = moment ÷ distance).
the hands apply the same moment about the hinge (same force at the same distance from the hinge)
the moment of the force between the nut and the nutcracker about the hinge must balance this moment, and moment = force × distance
so when the nut is closer to the hinge (smaller distance), the force on the nut is bigger for the same effort, making it easier to crack