P1.5.2P1. Motion, Forces and Energy
Turning effect of forces — Combined Science 0653 - Physics Revision Notes & Topical Questions
5 min read 8 parts Lesson 7 of 15 1 quiz
Lesson summary
P1.5.2 Turning effect of forces — COMBINED SCIENCE notes: Define the moment of a force, apply the principle of moments, and state the conditions for equilibrium.Moment = the turning effect of a force about a pivot.Equation — moment = force × perpendicular distance from the pivot,…
- Direction of turning
- Everyday examples
- Larger force
- Larger distance
- Several forces
- Unbalanced moments
Section 1 of 8Moment of a force
Moment
Moment the turning effect of a force about a pivot.
Equation
- Equation moment = force × perpendicular distance from the pivot, so M = F × d.
- The unit is the newton metre (N m), so convert centimetres to metres first.
Direction of turning
- Direction of turning a moment is clockwise or anticlockwise about the pivot.
Perpendicular distance
- Perpendicular distance the force must act at 90° to the line joining its point of application to the pivot.
Everyday examples
- Everyday examples a see-saw, a spanner, a door, a tap, scissors and a wheelbarrow.
Section 2 of 8Increasing the turning effect
Larger force
- Larger force a bigger force gives a bigger moment.
Larger distance
- Larger distance applying the force further from the pivot gives a bigger moment, so a smaller force is needed.
- A door is much easier to push open at the handle than next to the hinge.
- A long spanner turns a nut with less force than a short one.
Section 3 of 8Principle of moments
Statement
- Statement for a balanced object, the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot.
Equation
- Equation clockwise moment = anticlockwise moment, so F1d1 = F2d2.
Several forces
- Several forces add all the clockwise moments on one side and all the anticlockwise moments on the other.
- F2d2 = F1d1 + F3d3 when F2 is the only clockwise force.
Unbalanced moments
- Unbalanced moments the object turns in the direction of the larger moment.
1
Identify the pivot.
2
Find each force and its perpendicular distance from the pivot.
3
Calculate each moment and label it clockwise or anticlockwise.
4
Set the total clockwise moment equal to the total anticlockwise moment.
5
Solve for the unknown quantity.
Section 4 of 8Conditions for equilibrium
Equilibrium
Equilibrium a state with no resultant force and no resultant moment on the object.
Forces balanced
- Forces balanced the resultant force is zero, so the object does not accelerate.
Moments balanced
- Moments balanced the clockwise moment equals the anticlockwise moment, so the object does not turn.
Supported beam
- Supported beam the upward support forces equal the total downward weight.
Section 5 of 8Investigating the principle of moments
1
Fix an optical pin in a clamp stand and balance a metre ruler on it at its centre, using modelling clay if needed.
2
Hang unequal loads on cotton loops on either side of the pivot.
3
Adjust the distances until the ruler balances horizontally.
4
Record the masses and their distances from the pivot.
5
Calculate each force from F = mg and each moment from M = Fd, using kg and m.
6
Repeat with different loads and positions.
1Result
- Result in every case the anticlockwise moment equals the clockwise moment.
2Why the pivot is at the centre
- Why the pivot is at the centre the ruler's own weight then has no moment about the pivot.
3Reducing errors
- Reducing errors keep the loop lengths equal, avoid draughts, and repeat the readings to find a mean.
4Safety
- Safety wear goggles, secure the stand with a G clamp and put a soft mat below the masses.
Section 6 of 8More from the specification
1Moment
- Moment force multiplied by perpendicular distance from the pivot, with the unit newton metre
2Lamina experiment
- Lamina experiment hang an irregular lamina from a pin and use a plumb line to find its centre of gravity where lines intersect
3Principle of moments
- Principle of moments for equilibrium, clockwise moments equal anticlockwise moments about any pivot
4Toppling
- Toppling an object topples when the line of action of its weight falls outside the base,
- so a low centre of gravity and wide base improve stability;
- a child and an adult on a see-saw balance by sitting at different distances
5Moment
- Moment force multiplied by perpendicular distance from the pivot, with the unit newton metre
6Lamina experiment
- Lamina experiment hang an irregular lamina from a pin and use a plumb line to find its centre of gravity where lines intersect
7Principle of moments
- Principle of moments for equilibrium, clockwise moments equal anticlockwise moments about any pivot
8Toppling
- Toppling an object topples when the line of action of its weight falls outside the base,
- so a low centre of gravity and wide base improve stability;
- a child and an adult on a see-saw balance by sitting at different distances
9Gravity
- Force of attraction between objects with mass.
10Plane lamina
- A flat sheet of uniform thickness.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Moment | The turning effect of a force about a pivot, equal to force × perpendicular distance. |
| Pivot | The fixed point about which an object turns. |
| Principle of moments | For a balanced object, the clockwise moment equals the anticlockwise moment. |
| Equilibrium | No resultant force and no resultant moment on an object. |
| Newton metre | The unit of moment. |
| Clockwise | Turning in the direction of the hands of a clock. |
Section 8 of 8Exam technique
1
Using a distance that is not perpendicular to the force — use the perpendicular distance from the pivot.
2
Forgetting to convert cm to m — a moment in N m needs distances in metres.
3
Equating forces instead of moments — balance clockwise and anticlockwise moments about the pivot.
4
Saying balanced moments are enough for equilibrium — the resultant force must also be zero.
5
Ignoring the weight of the beam when the pivot is not at its centre.
Marked practiceQuizzes for this lesson
Last updated: