Turning effects of forces
Moments and their everyday turning effects, the principle of moments and the centre of gravity.
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Before you begin
A force can turn an object about a pivot as well as push it along. The lessons start with the moment of a force, its turning effect, in doors, spanners and wheelbarrows, and calculate it from the force and its perpendicular distance from the pivot. They then use the principle of moments to solve balance problems and show where an object’s own weight acts: at its centre of gravity.
Be comfortable with: forces, mass and weight, because a beam’s weight is one of the forces that turns it.
Learning goals
- Describe a moment as a force's turning effect in everyday examples
- Apply moment = force × perpendicular distance from the pivot
- Apply the principle of moments to new situations and problems
- Treat the weight of a body as acting at a single point, its centre of gravity
Syllabus statements covered
- describe the moment of a force about a pivot in terms of its turning effect and relate this to everyday examples
- recall and apply the relationship moment of a force (or torque) = force × perpendicular distance from the pivot to new situations or to solve related problems
- state the principle of moments for a body in equilibrium
- apply the principle of moments to new situations or to solve related problems
- show an understanding that the weight of a body may be taken as acting at a single point known as its centre of gravity
Lessons
Work through them in order.
- Moments and turning effectsExplain everyday turning effects about a pivot and use moment = force × perpendicular distance to find a moment, a force or a distance.
- Principle of moments and centre of gravityState the principle of moments and use it, with each weight acting at its centre of gravity, to find an unknown force, distance or weight on a balanced beam.
Practise and check
Topic reference
Quick reference
| Idea | Relationship | Note |
|---|---|---|
| moment of a force | moment = force × perpendicular distance from the pivot, M = F × d | unit N m; d to the line of action, at 90° |
| force through the pivot | moment is zero | d = 0 |
| principle of moments | sum of clockwise moments = sum of anticlockwise moments | about the same pivot, for a body in equilibrium |
| support force | upward forces = downward forces | the pivot’s force has no moment about the pivot |
| centre of gravity | the whole weight acts at one point | the middle of a uniform beam or rule |
Core knowledge to remember
- The moment of a force about a pivot is its turning effect. Moment of a force = force × perpendicular distance from the pivot.
- Principle of moments: for a body in equilibrium, the sum of the clockwise moments about a pivot is equal to the sum of the anticlockwise moments about the same pivot.
- The weight of a body may be taken as acting at a single point known as its centre of gravity.
Common mistakes
- Using the distance to where a force is applied instead of the perpendicular distance to its line of action.
- Multiplying by a distance in centimetres and writing the answer in N m.
- Using scale readings as distances instead of measuring from the pivot.
- Leaving out the beam’s own weight when the pivot is not at its centre of gravity.
- Adding the loads on one side before multiplying by their different distances.