Moments and centre of gravity Edexcel International GCSE Physics revision
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In plain words
A moment is the turning effect of a force: how hard it tries to spin something round a pivot. A long spanner makes a nut easier to undo because the same push is further from the pivot. Something balances when the clockwise moments equal the anticlockwise ones.
Three things to know
- Moment = force × perpendicular distance from the pivot. Its unit is N m.
- When an object is balanced, the total clockwise moment about the pivot equals the total anticlockwise moment.
- An object is more stable with a low centre of gravity and a wide base. It topples when its centre of gravity moves outside its base.
Worked example
A 4 N weight sits 30 cm to the left of a pivot. Where must a 6 N weight go on the right to balance it?
- Anticlockwise moment = 4 × 30 = 120 N cm.
- For balance, 6 × d = 120.
- d = 20 cm from the pivot.
Tips and tricks
- The distance is measured at right angles from the pivot to the line of the force.
- In balance questions write "clockwise moments = anticlockwise moments" before any numbers. It's the sentence the mark scheme is looking for.
It lands in your notebook with its questions as flashcards.
Moments and centre of gravity: 6 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
A spanner 0.20 m long is pushed at its end with a force of 50 N. What is the moment?
Moment = 50 × 0.20.
Why is it easier to open a door by pushing far from the hinge?
Moment = force × distance, so a longer distance gives more turning effect.
A beam is balanced. Which statement must be true?
That is the principle of moments.
Where is the centre of gravity of a uniform metre rule?
For a uniform object it is at the middle.
A 6 N weight hangs 40 cm from a pivot. What weight 60 cm from the pivot on the other side balances it?
6 × 40 = W × 60, so W = 4 N.
Which change makes a table lamp more stable?
It lowers the centre of gravity and widens the base.
Quiz
6 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
4 questions, 10 marks. Write your answers on paper, then check them.
Moments and centre of gravity
Edexcel International GCSE Physics 4PH1 · 10 marks · papermunch.org
Name ______________________________ Date ______________
A 40 N force acts 0.25 m from a pivot. Calculate its moment.[2]
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10 N m. 40 × 0.25.
A child weighing 300 N sits 2.0 m from the pivot of a see-saw. How far from the pivot must a 400 N child sit on the other side to balance it?[3]
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1.5 m. 300 × 2.0 = 400 × d, so d = 600 ÷ 400.
A uniform metre rule is pivoted at its centre. A 2.0 N weight hangs 30 cm to the left of the pivot. What weight must hang 20 cm to the right to balance it?[3]
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3.0 N. 2.0 × 30 = W × 20, so W = 60 ÷ 20.
Explain why a racing car is built low and wide.[2]
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A low centre of gravity and a wide base make it stable, so it is less likely to topple when cornering.
Answers: Moments and centre of gravity
- 1. 10 N m. 40 × 0.25.
- 2. 1.5 m. 300 × 2.0 = 400 × d, so d = 600 ÷ 400.
- 3. 3.0 N. 2.0 × 30 = W × 20, so W = 60 ÷ 20.
- 4. A low centre of gravity and a wide base make it stable, so it is less likely to topple when cornering.