SEAB Original-style practice: A spanner is used to undo a nut. A force of 50\ N is applied at the end of the spanner, 0.20\ m from the nut, at right angles to the spanner. (a) Define the moment of a force. (b) Calculate the moment about the nut. (c) State the unit.

(a) The moment of a force is the turning effect of the force about a pivot, equal to the force multiplied by the perpendicular distance from the pivot to the line of the force. (b) Moment = force × distance = 50 × 0.20 = 10\ N m. (c) The unit is the newton metre (N m). What markers reward: the definition including perpendicular distance, the calculation force times distance, and the unit newton metre.

SingaporePhysicsSyllabus dot point

How does friction affect motion, and how do forces make objects turn?

Describe friction and its effects, and calculate the moment of a force about a pivot

Describe friction and its useful and wasteful effects, define the moment of a force, use moment = force times distance, and apply the principle of moments to a balanced beam at N(A)-Level.

Generated by Claude Opus 4.88 min answerUpdated 2026-06-06

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking

SEAB wants you to describe friction and its useful and wasteful effects, to define the moment of a force as its turning effect, to use moment $=$ force $\times$ distance, and to apply the principle of moments to a balanced object. The big idea is that forces can slow motion (friction) and can turn objects about a pivot (moments).

The answer

Friction

Friction is a force that opposes motion between two surfaces that are touching. It always acts in the direction that opposes the movement, or the tendency to move.

Friction can be useful: it lets us walk without slipping, lets car tyres grip the road, and lets brakes slow a wheel. It can also be wasteful: it slows moving parts in a machine and produces heat, so energy is transferred away as thermal energy. We reduce unwanted friction with lubrication (oil), smoother surfaces, or rollers and ball bearings.

Air resistance and drag

Air resistance is a kind of friction between an object and the air it moves through. Like other friction it opposes motion and grows as the object moves faster. It is why fast vehicles are shaped to be streamlined.

The turning effect of a force: moments

A force can make an object turn about a fixed point called a pivot. The turning effect is called the moment of the force:

\text{moment} = \text{force} \times \text{perpendicular distance from the pivot}

The unit is the newton metre ( $\text{N m}$ ). A bigger force, or a longer distance from the pivot, gives a bigger turning effect. This is why a long spanner undoes a tight nut more easily than a short one, and why a door handle is placed far from the hinge.

The principle of moments

When an object is balanced (in equilibrium and not turning), the turning effects balance:

\text{total clockwise moment} = \text{total anticlockwise moment}

This is the principle of moments. It lets you find an unknown force or distance on a balanced beam, such as a seesaw or a metre rule pivoted at its centre.

Worked example

A child of weight $300\ \text{N}$ sits $1.5\ \text{m}$ from the pivot of a seesaw. Where must a child of weight $450\ \text{N}$ sit on the other side to balance it?

Step 1: Write the anticlockwise moment

Take the $300\ \text{N}$ child as anticlockwise: moment $= 300 \times 1.5 = 450\ \text{N m}$ .

Step 2: Apply the principle of moments

For balance, clockwise moment $=$ anticlockwise moment, so the $450\ \text{N}$ child must give $450\ \text{N m}$ .

Step 3: Write the clockwise moment with the unknown distance

450 \times d = 450

Step 4: Solve for the distance

d = \frac{450}{450} = 1.0\ \text{m}

The heavier child must sit $1.0\ \text{m}$ from the pivot, closer in than the lighter child.

Examples in context

Example 1. Walking and slipping. Friction between your shoe and the ground pushes you forward as you push back on the ground. On ice there is almost no friction, so your foot slips and you cannot push off. Rough soles increase friction and grip.

Example 2. A wheelbarrow. A wheelbarrow is a lever with the wheel as the pivot. Lifting the handles far from the wheel gives a large moment, so a small lifting force can balance the moment of a heavy load placed close to the wheel. This is why the load is carried near the front.

Try this

Cue. A force of $20\ \text{N}$ acts $0.30\ \text{m}$ from a pivot at right angles. Find the moment. [2 marks] Moment $= 20 \times 0.30 = 6.0\ \text{N m}$ .
Cue. Give one useful and one wasteful effect of friction. [2 marks] Useful: it lets car tyres grip the road; wasteful: it slows moving parts in an engine and wastes energy as heat.
Cue. A $6.0\ \text{N}$ weight is $0.50\ \text{m}$ left of a pivot. A weight $W$ is $0.30\ \text{m}$ to the right and the beam balances. Find $W$ . [3 marks] $W \times 0.30 = 6.0 \times 0.50 = 3.0$ , so $W = \dfrac{3.0}{0.30} = 10\ \text{N}$ .

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original4 marksA spanner is used to undo a nut. A force of

50\ \text{N}

is applied at the end of the spanner,

0.20\ \text{m}

from the nut, at right angles to the spanner. (a) Define the moment of a force. (b) Calculate the moment about the nut. (c) State the unit.

Show worked answer →

(a) The moment of a force is the turning effect of the force about a pivot, equal to the force multiplied by the perpendicular distance from the pivot to the line of the force.

(b) Moment $= \text{force} \times \text{distance} = 50 \times 0.20 = 10\ \text{N m}$ .

What markers reward: the definition including perpendicular distance, the calculation force times distance, and the unit newton metre.

Original4 marksA uniform metre rule is balanced at its centre. A

2.0\ \text{N}

weight hangs

40\ \text{cm}

to the left of the pivot. A second weight

W

hangs

20\ \text{cm}

to the right of the pivot and the rule balances. (a) State the principle of moments. (b) Calculate

W

Show worked answer →

(a) When an object is balanced, the total clockwise moment about the pivot equals the total anticlockwise moment about the pivot.

(b) Anticlockwise moment $= 2.0 \times 0.40 = 0.80\ \text{N m}$ . For balance, clockwise moment $= 0.80\ \text{N m}$ , so $W \times 0.20 = 0.80$ , giving $W = \dfrac{0.80}{0.20} = 4.0\ \text{N}$ .

What markers reward: the principle of moments stated correctly, equating clockwise and anticlockwise moments, and solving for $W = 4.0\ \text{N}$ (using consistent metre units).

What this dot point is asking

The answer

Friction

Air resistance and drag

The turning effect of a force: moments

The principle of moments

Examples in context

Try this

Exam-style practice questions

Related dot points