What is the principle of moments?

The principle of moments states that when a body is balanced, the total clockwise moment about the pivot equals the total anticlockwise moment about that pivot. A moment is the turning effect of a force, worked out as force times its perpendicular distance from the pivot, so moment equals force times distance. At N(A)-Level you use it to find a missing effort, load or distance on a lever or seesaw by setting the two moments equal.

How do you calculate a gear ratio, and what does it tell you?

The gear ratio is the number of teeth on the driven gear divided by the number on the driver gear. A ratio greater than 1 is a reduction: the driven gear turns more slowly but with a greater turning force, and the driven speed equals the driver speed divided by the ratio. Two meshing gears always turn in opposite directions, so designers add an idler gear to keep the same direction or to bridge a gap.

What does an idler gear do?

An idler gear is a gear placed between the driver and the driven gear. It does two useful jobs: it reverses the direction of the driven gear so that it turns the same way as the driver, and it bridges the distance between two shafts that are far apart. Crucially, an idler does not change the overall gear ratio, because the ratio depends only on the driver and driven gears, not on what is in between.

What is the difference between tension and compression in a structure?

Tension is a pulling or stretching force that tries to make a member longer; a member in tension is called a tie. Compression is a pushing or squeezing force that tries to make a member shorter; a member in compression is called a strut. Knowing which members are in tension and which are in compression tells a designer where to use thin, strong parts and where to use thick, stiff parts that will not buckle.

How can you make a structure more stable so it does not topple?

A structure is more stable when it has a wide base and a low centre of gravity, because its weight then stays over the base even when it is pushed or tilted. Putting heavier parts low down lowers the centre of gravity, and widening the base lets the structure lean further before its weight falls outside the base. Triangulation, fixing to a wall and adding feet are further ways to resist tipping and racking.

SingaporeDesign and Technology

Mechanisms and Structures: how N(A)-Level Design and Technology students use levers, linkages, gears, pulleys and structural members to change force and motion and carry loads, with the key calculations

A Singapore N(A)-Level Design and Technology (SEAB 7055) module overview of mechanisms and structures. Levers and the principle of moments, linkages, gears, pulleys and belts, and structures with triangulation and stability, including the core moment and gear-ratio calculations, with links to every dot point.

Generated by Claude Opus 4.87 min readSEAB-7055Updated 2026-06-13

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

Jump to a section

What this module covers
Levers and linkages
Gears and pulleys
Structures and stability
How this module is examined
Check your knowledge

What this module covers

Mechanisms and Structures is the most calculation-heavy part of N(A)-Level Design and Technology (SEAB 7055), and it rewards students who can both explain how a mechanism works and work out the numbers. Levers and linkages change force and the direction of motion, gears and pulleys change the speed and direction of rotation, and structures carry loads safely without bending or toppling. These ideas appear in the written paper as short structured questions, and they often show up inside the Design Project when a product has to move or support something. This overview links the three dot points and gathers the key formulae.

See the full set of dot points for this module under /sg-n-level/design-and-technology/syllabus/mechanisms-and-structures.

Levers and linkages

The foundation is the lever. The levers and linkages dot point names the three classes of lever, set by where the effort, load and pivot (fulcrum) sit, and applies the principle of moments to calculate the turning effect of a force. A moment is force times perpendicular distance from the pivot, and for balance the clockwise moment equals the anticlockwise moment. Linkages such as the reverse-motion and bell-crank linkage take that input and change the direction or nature of the motion, for example turning a push into a pull or sending motion round a corner.

Gears and pulleys

Gears and pulleys transmit rotary motion between shafts and trade speed for turning force. The gear ratio is driven teeth divided by driver teeth, and the driven speed is the driver speed divided by the ratio, so a reduction (ratio above 1) slows the output but increases torque. Meshing gears turn opposite ways; an idler restores the direction and bridges a gap without changing the ratio. A belt-and-pulley drive does a similar job using a belt: an open belt keeps the direction, a crossed belt reverses it, and a belt can slip to protect a mechanism from overload.

Structures and stability

Structures and stability covers how a structure carries loads and stays upright. Members are in tension (ties) or compression (struts); triangulation stiffens a frame so it does not rack; and shaping the material, for example folding a flat sheet or using a tube, adds strength without extra weight. Stability comes from a wide base and a low centre of gravity, which keep the weight over the base when the structure is pushed.

Calculating a moment and balancing a seesaw

A child of weight $300\ \text{N}$ sits $1.5\ \text{m}$ from the pivot of a seesaw. Find the moment they create, then find how far a second child of weight $450\ \text{N}$ must sit on the other side to balance it.

Step 1: Write the formula for a moment

A moment is force times perpendicular distance from the pivot:

\text{moment} = \text{force} \times \text{distance}

Step 2: Find the first child's moment

\text{moment} = 300 \times 1.5 = 450\ \text{N m}

Step 3: Apply the principle of moments

For the seesaw to balance, the anticlockwise moment equals the clockwise moment, so the second child must also create $450\ \text{N m}$ :

450 \times \text{distance} = 450

Step 4: Solve for the distance

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

The heavier child sits closer to the pivot ( $1.0\ \text{m}$ ) than the lighter child ( $1.5\ \text{m}$ ), which is exactly what you would expect, a larger force needs a shorter distance to give the same turning effect.

How this module is examined

Write the formula, then substitute. Moments and gear ratios both reward showing the formula before the numbers, and always finish with the correct unit.
Get direction right. Meshing gears reverse; an idler restores direction; an open belt keeps direction while a crossed belt reverses it.
Classify members and levers. Identify struts (compression) and ties (tension), and name the class of a lever from where the pivot, effort and load sit.
Explain trade-offs. A reduction gains turning force but loses speed; a lever lets a small effort move a large load only by moving through a longer distance.

Check your knowledge

Recall and calculation questions across the module. Attempt them, then check the worked solutions.

State the principle of moments. (2 marks)
A force of $20\ \text{N}$ acts at a perpendicular distance of $0.4\ \text{m}$ from a pivot. Calculate the moment. (2 marks)
A gear train has a driver gear with 15 teeth and a driven gear with 45 teeth. Calculate the gear ratio. (2 marks)
State the difference between tension and compression in a structure. (2 marks)
State two ways to make a structure more stable so it does not topple. (2 marks)

Solutions

Q1: When a body is balanced (in equilibrium), the total clockwise moment about the pivot equals the total anticlockwise moment about that same pivot.
Q2: $\text{moment} = \text{force} \times \text{distance} = 20 \times 0.4 = 8\ \text{N m}$ .
Q3: $\text{gear ratio} = \dfrac{\text{driven teeth}}{\text{driver teeth}} = \dfrac{45}{15} = 3$ (a 3:1 reduction).
Q4: Tension is a pulling or stretching force that tries to make a member longer (a tie); compression is a pushing or squeezing force that tries to make a member shorter (a strut).
Q5: Give it a wide base and a low centre of gravity (for example by putting heavy parts low down), so the weight stays over the base when the structure is pushed.

Sources & how we know this

Singapore-Cambridge GCE N(A)-Level Design and Technology (Syllabus 7055) — Singapore Examinations and Assessment Board (2026)