Moments and turning effectsIB MYP Physics: Revision notes
Section 1
The moment of a force
A force can make an object turn about a fixed point called the pivot. The turning effect is the moment of the force:
moment = force × perpendicular distance from the pivot
Moment is measured in newton metres (N m). The distance must be measured at right angles to the line of action of the force.
Example: a force of 80 N at right angles, 0.25 m from the pivot, gives 80 × 0.25 = 20 N m.
To increase a moment, either increase the force or push further from the pivot. Pushing further from the pivot is usually easier.
Section 2
Clockwise and anticlockwise moments
A moment turns an object either clockwise (like the hands of a clock) or anticlockwise.
When there are several forces, add up all the clockwise moments and all the anticlockwise moments separately. If one total is bigger, the object turns in that direction.
Section 3
The principle of moments
An object is balanced (not turning) when:
total clockwise moment = total anticlockwise moment
This is the principle of moments.
Worked example: child P weighs 300 N and sits 2.0 m from the pivot of a see-saw. Child Q weighs 400 N. How far from the pivot must Q sit to balance it?
- Anticlockwise moment = 300 × 2.0 = 600 N m
- Clockwise moment = 400 × d
- 400 × d = 600, so d = 1.5 m.
The heavier child sits closer to the pivot.
Use the perpendicular distance from the pivot, not the length of the whole object, and keep clockwise and anticlockwise moments separate.
Section 4
Levers and everyday turning effects
A lever turns about a pivot so that a small force can make a large moment.
- A spanner: a longer handle gives a larger moment for the same force, so the nut is easier to turn.
- A see-saw: children of different weights balance by sitting at different distances from the pivot.
- A door: the handle is far from the hinge, so only a small force is needed.
- A crowbar or wheelbarrow: the load is close to the pivot and the force is applied far away.
Section 5
Centre of mass and stability
The centre of mass is the point where the whole weight of an object appears to act.
An object is stable while the vertical line through its centre of mass falls inside its base. If it is tilted so far that the line falls outside the base, the weight produces a moment about the edge and the object topples.
To make an object more stable:
- give it a low centre of mass (a heavy bottom)
- give it a wide base
A tall, narrow object with a high centre of mass, such as a loaded double-decker bus on its top deck, topples more easily.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Moments and turning effects
- A mechanic in São Paulo uses a spanner 0.25 m long to loosen a wheel nut. She pushes at right angles to the end of the spanner with a force of 80 N, but the nut does not turn.She slides a long pipe over the spanner so that she can push 0.50 m from the nut with the same force. Explain why this helps her to turn the nut.2 marks
- Two children sit on a see-saw that has its pivot at the centre. Child P weighs 300 N and sits 2.0 m from the pivot on the left. Child Q weighs 400 N and sits on the right. The see-saw is balanced horizontally.Child Q now moves so that she sits 2.0 m from the pivot. Explain what happens to the see-saw.2 marks
- A student investigates balancing a uniform metre rule that is pivoted at its centre, so the weight of the rule itself has no turning effect. A 4.0 N load is hung 0.30 m to the left of the pivot. She hangs different weights on the right and moves each one until the rule balances. A 2.0 N weight balanced at 0.60 m from the pivot, a 3.0 N weight at 0.40 m and a 6.0 N weight at 0.20 m.State the independent variable, the dependent variable and one control variable in this investigation.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).