What is not setting moments equal for balance?

To find an unknown distance or force, set clockwise moments equal to anticlockwise moments.

SingaporeDesign and TechnologySyllabus dot point

How do levers let a small effort move a large load, and how do you calculate the turning effect?

Describe the three classes of lever and use the principle of moments to calculate the turning effect of a force

A clear answer to the N(A)-Level D&T outcome on levers. The three classes of lever, how linkages change direction of motion, and using the principle of moments to calculate the turning effect of a force.

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
The answer
Examples in context
Try this

What this dot point is asking

SEAB wants you to describe the three classes of lever, explain how levers and linkages change forces and motion, and use the principle of moments to calculate the turning effect of a force. This is one of the few topics with a real calculation, so showing the formula and working clearly is where the marks are.

The answer

What a lever is

A lever is a stiff bar that turns about a fixed point called the pivot (or fulcrum). You apply an effort to move a load. A lever lets a small effort move a large load, or move a load a larger distance, depending on how it is arranged.

The three classes of lever

The class depends on what lies in the middle:

Class 1. The pivot is between the effort and the load. Examples: a seesaw, scissors, a crowbar.
Class 2. The load is between the pivot and the effort. Examples: a wheelbarrow, a bottle opener.
Class 3. The effort is between the pivot and the load. Examples: tweezers, the human forearm.

A simple memory aid is the order of the middle item: pivot, load, effort for classes 1, 2 and 3.

The principle of moments

A moment is the turning effect of a force:

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

measured in newton metres (N m). The principle of moments says that when something balances, the clockwise moments equal the anticlockwise moments:

F_1 \times d_1 = F_2 \times d_2

This is how a small force far from the pivot can balance a large force close to it, which is the secret of how levers give a mechanical advantage.

Linkages

A linkage is a set of connected bars (links) that pass on or change motion. Linkages can reverse the direction of a movement (a reverse-motion linkage), change push into a turn, or move two points together. They let a single input control movement in a chosen direction, useful in toys, tools and folding products.

Worked example: effort to lift a load with a lever

A crowbar (a class 1 lever) lifts a 600 N load that is 0.2 m from the pivot. The effort is applied 1.2 m from the pivot on the other side. Find the effort needed to balance the load.

Step 1: Write the principle of moments

For balance, anticlockwise moment equals clockwise moment:

F_{\text{effort}} \times d_{\text{effort}} = F_{\text{load}} \times d_{\text{load}}

Step 2: Put in the numbers

F_{\text{effort}} \times 1.2 = 600 \times 0.2

Step 3: Work out the load moment

600 \times 0.2 = 120 \text{ N m}

Step 4: Solve for the effort

F_{\text{effort}} = \frac{120}{1.2} = 100 \text{ N}

So only 100 N of effort balances the 600 N load, because the effort acts far from the pivot. The lever gives a mechanical advantage.

Examples in context

Example 1. A wheelbarrow. This class 2 lever puts the load between the wheel (pivot) and the handles (effort). Lifting at the handles, far from the wheel, lets a person raise a heavy load with much less effort, exactly as the principle of moments predicts.

Example 2. A reverse-motion linkage in a toy. Two links pivoting on a central point make one end go up when the other goes down. A designer uses this so a single push makes a toy figure's arms move in opposite directions.

Try this

Q1. State the formula for the moment of a force. [1 mark]

Cue. Moment = force times distance from the pivot (in N m).

Q2. A force of 20 N acts 0.5 m from a pivot. Calculate the moment. [2 marks]

Cue. Moment = 20 times 0.5 = 10 N m.

Q3. A 200 N load sits 0.3 m from a pivot. What effort is needed 0.6 m from the pivot to balance it? [3 marks]

Cue. Set moments equal: effort times 0.6 = 200 times 0.3 = 60, so effort = 60 divided by 0.6 = 100 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.

Original5 marksA child sits 1.5 m from the pivot of a seesaw and weighs 300 N. (a) Calculate the moment the child creates about the pivot. (b) A second child of 450 N sits on the other side. How far from the pivot must they sit to balance the seesaw?

Show worked answer →

(a) Moment = force times distance from the pivot:

moment = 300 N times 1.5 m = 450 N m.

(b) To balance, the moments on each side must be equal. So 450 N times distance = 450 N m, giving distance = 450 divided by 450 = 1.0 m.

What markers reward: the formula moment = force times distance, the correct moment of 450 N m in part (a), and in part (b) setting the two moments equal (the principle of moments) to find a distance of 1.0 m, with the unit N m.

Original4 marksName the three classes of lever and give one everyday example of each.

Show worked answer →

Class 1: the pivot (fulcrum) is between the effort and the load. Example: a seesaw, scissors, or a crowbar.

Class 2: the load is between the pivot and the effort. Example: a wheelbarrow or a bottle opener.

Class 3: the effort is between the pivot and the load. Example: tweezers, or a person's forearm lifting a weight.

What markers reward: the correct arrangement for each class (what lies between what) and a sensible everyday example for each. Mixing up class 2 and class 3 is the common error.