1.5.2Motion, Forces and Energy
Turning Effect OF Forces — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
5 min read 10 parts Lesson 45 of 91 1 quiz
Lesson summary
1.5.2 Turning Effect of Forces — IGCSE PHYSICS notes: Describe the moment of a force, use moment = force × perpendicular distance and apply the principle of moments.Moment = the turning effect of a force about a pivot.Pivot = the fixed point about which an object turns.Everyday e…
- What the symbols mean
- Increasing the distance reduces the force needed
- Door handle
- Extended case
- Both conditions are needed
- Improvements
Part 1 of 8Moment of a force
Moment
Moment the turning effect of a force about a pivot.
Pivot
Pivot the fixed point about which an object turns.
Everyday examples:
- Pushing a door about its hinges.
- Turning a spanner or a tap.
- A child on a see-saw.
- Using scissors, a screwdriver, a wheelbarrow or a crane.
Direction
- Direction a moment is clockwise or anticlockwise, judged like the hands of a clock.
Part 2 of 8The moment equation
Equation
- Equation moment = force × perpendicular distance from the pivot (M = Fd).What the symbols mean: moment (N m) · F = force (N) · d = perpendicular distance from the pivot to the line of the force (m)
Unit
- Unit the newton metre (N m); N cm is also possible if the distance is in cm.
The distance must be perpendicular to the line of action of the force.
Convert distances to metres unless the question asks otherwise.
Do not give the unit as N/m or joules, which are never accepted for a moment.
Part 3 of 8Factors affecting the moment
Factors affecting the moment
A larger force gives a larger moment.
A larger distance from the pivot gives a larger moment.
Increasing the distance reduces the force needed
- Increasing the distance reduces the force needed a long spanner loosens a nut more easily.
Door handle
- Door handle pushing far from the hinges needs much less force than pushing next to them.
Levers use this idea so that a small force far from the pivot can move a large load.
Part 4 of 8Levers in everyday use
| Example | Pivot | Force applied | How to get a bigger moment |
|---|---|---|---|
| Door | Hinges | Push on the handle | Push far from the hinges |
| Spanner | Nut | Hand on the handle | Use a longer spanner |
| See-saw | Centre support | Weights of the children | Heavier child sits closer to the pivot |
| Wheelbarrow | Wheel | Lift on the handles | Hold the handles at the far end |
| Scissors | Screw | Squeeze the handles | Cut near the pivot |
| Crowbar | Edge on the ground | Push on the end | Use a longer bar |
Part 5 of 8Principle of moments
Principle of moments
Principle of moments for a balanced object, the total clockwise moment about a pivot equals the total anticlockwise moment about the same pivot.
Equation
- Equation clockwise moment = anticlockwise moment, or F1d1 = F2d2 for one force on each side.
Extended case
- Extended case with several forces, the sum of the clockwise moments equals the sum of the anticlockwise moments.
Decide the direction of a moment by imagining that only that force acts on the beam and seeing which way it turns.
A beam pivoted at its centre of mass contributes no moment from its own weight.
- If the pivot is elsewhere, the weight of the beam acts at its centre and must be included.
Part 6 of 8Equilibrium
Equilibrium
An object is in equilibrium when there is no resultant force and no resultant moment.
Both conditions are needed
- Both conditions are needed balanced forces alone are not enough.
The sum of clockwise moments equals the sum of anticlockwise moments.
Part 7 of 8Verifying the principle of moments
1
Balance a metre rule on a pivot (an optical pin) at its centre, using modelling clay if needed to make it horizontal.
2
Hang a known mass on a cotton loop at a measured distance on one side.
3
Hang a second, unequal mass on the other side and move it until the rule balances horizontally.
4
Record both masses and their distances from the pivot.
5
Calculate each force from F = mg and each moment from M = Fd, converting g to kg and cm to m.
6
Repeat with different masses and show that clockwise moment = anticlockwise moment each time.
Improvements
- Improvements take repeat readings, keep cotton loops equal in length, avoid draughts and use a spirit level.
Safety
- Safety wear goggles in case the cotton snaps, and place a mat below to cushion falling masses.
Part 8 of 8More from the specification
1Moment of a force
- Moment of a force force multiplied by the perpendicular distance from the pivot, measured in newton metres
2Centre of gravity
- Centre of gravity the point where the whole weight appears to act;
- a freely hanging lamina comes to rest with its centre of gravity directly below the pivot
3Plumb line experiment
- Plumb line experiment hang a lamina from a clamp, draw vertical lines with a plumb line, and the intersection marks the centre of gravity
4Toppling
- Toppling an object topples when the vertical line through its centre of gravity falls outside its base, so a wide low base gives stability
5Moment of a force
- Moment of a force force multiplied by the perpendicular distance from the pivot, measured in newton metres
6Centre of gravity
- Centre of gravity the point where the whole weight appears to act;
- a freely hanging lamina comes to rest with its centre of gravity directly below the pivot
7Plumb line experiment
- Plumb line experiment hang a lamina from a clamp, draw vertical lines with a plumb line, and the intersection marks the centre of gravity
8Toppling
- Toppling an object topples when the vertical line through its centre of gravity falls outside its base, so a wide low base gives stability
9Gravity
- A force of attraction between objects with mass.
10Lamina
- A thin flat sheet.
11Position
- Location relative to a reference point.
12Topple
- Tip over because the weight acts outside the base.
Key terms
| Term | Meaning |
|---|---|
| Moment | The turning effect of a force about a pivot. |
| Pivot | The point about which something turns. |
| Perpendicular distance | Shortest distance from the pivot to the line of action of the force. |
| Newton metre (N m) | Unit of moment. |
| Principle of moments | In balance, clockwise moment equals anticlockwise moment. |
| Lever | A rigid bar that turns about a pivot. |
Avoid these mistakes
1
Measuring the distance along the lever when the force is at an angle — use the perpendicular distance.
2
Using cm in M = Fd and giving N m — convert to metres first.
3
Writing "the moments are equal" without saying clockwise and anticlockwise.
4
Stating only that forces are balanced for equilibrium — the moments must be balanced too.
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