SEAB Original-style practice: A crowbar is used to lift a load. The load of 600\ N is 0.10\ m from the pivot, and the effort is applied 0.80\ m from the pivot on the other side. (a) Calculate the effort needed to just lift the load. (b) Calculate the mechanical advantage of the crowbar.

(a) Apply the principle of moments about the pivot. For balance, the clockwise moment equals the anticlockwise moment: $effort × effort distance = load × load distance$ $E × 0.80 = 600 × 0.10$ $E = (60)/(0.80) = 75\ N$ (b) Mechanical advantage is load divided by effort: $MA = loadeffort = (600)/(75) = 8$ What markers reward: the principle of moments set out as effort times effort distance equals load times load distance, correct substitution giving an effort of 75 N, and mechanical advantage as load over effort giving 8 (a number, with no unit).

SingaporeDesign and TechnologySyllabus dot point

How do levers multiply force, and how does the principle of moments let us calculate the effort a lever needs?

Apply the principle of moments to levers, classify the three orders of lever, and calculate effort, load and mechanical advantage

A focused answer to the O-Level Design and Technology outcome on levers. The three orders of lever, the principle of moments, mechanical advantage, and worked moment calculations.

Generated by Claude Opus 4.89 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 apply the principle of moments to levers, classify the three orders (classes) of lever, and calculate effort, load and mechanical advantage. This is one of the calculation outcomes in the technology strand, so you must be able to set up and solve a moments equation and find mechanical advantage, as well as recognise lever types in real tools.

The answer

What a lever is

A lever is a rigid bar that turns about a fixed point called the pivot (or fulcrum). A force called the effort is applied to move a force called the load. Levers let a small effort move a large load, or move a load a larger distance, depending on the arrangement. They are the simplest and most common mechanism.

The principle of moments

The moment of a force is its turning effect about the pivot:

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

measured in newton metres ( $\text{N m}$ ). The principle of moments states that when a lever is balanced (in equilibrium), the total clockwise moment about the pivot equals the total anticlockwise moment:

\text{effort} \times \text{effort distance} = \text{load} \times \text{load distance}

This equation lets you calculate any one quantity if you know the others. It is the key tool for lever problems.

Mechanical advantage

Mechanical advantage (MA) measures how much a lever multiplies force:

MA = \frac{\text{load}}{\text{effort}}

It has no unit (it is a ratio). An MA greater than 1 means the effort is smaller than the load, so the lever makes the job easier. A crowbar with MA of 8 lets a 75 N effort lift a 600 N load. MA also equals the ratio of effort distance to load distance for an ideal lever.

The three orders of lever

Levers are classified by the order of pivot, load and effort along the bar:

First order (class 1). Pivot in the middle, between effort and load. Examples: seesaw, scissors, crowbar, pliers.
Second order (class 2). Load in the middle, between pivot and effort. Examples: wheelbarrow, nutcracker, bottle opener. These always give an MA greater than 1.
Third order (class 3). Effort in the middle, between pivot and load. Examples: tweezers, fishing rod, the human forearm. These have an MA less than 1 but move the load a greater distance and faster.

A quick way to remember: in class 1 the pivot is in the middle, in class 2 the load is in the middle, in class 3 the effort is in the middle.

Worked example

A wheelbarrow (a second-order lever) carries a load of $400\ \text{N}$ whose centre is $0.40\ \text{m}$ from the wheel (the pivot). The handles are gripped $1.2\ \text{m}$ from the wheel. Find the effort needed to lift the handles and the mechanical advantage.

Step 1: Identify the pivot, load and effort distances

The pivot is the wheel. The load of $400\ \text{N}$ acts $0.40\ \text{m}$ from the pivot. The effort is applied at the handles, $1.2\ \text{m}$ from the pivot.

Step 2: Apply the principle of moments

For balance, effort times effort distance equals load times load distance:

E \times 1.2 = 400 \times 0.40

Step 3: Solve for the effort

E \times 1.2 = 160 \implies E = \frac{160}{1.2} = 133\ \text{N}

So an effort of about $133\ \text{N}$ lifts the $400\ \text{N}$ load.

Step 4: Find the mechanical advantage

MA = \frac{\text{load}}{\text{effort}} = \frac{400}{133} = 3.0

The wheelbarrow has a mechanical advantage of $3$ , so it makes the load three times easier to lift. This is why the long handles (large effort distance) and the load placed near the wheel give such a useful force multiplication.

Examples in context

Example 1. Scissors as a first-order lever. The pivot sits between the effort (your fingers on the handles) and the load (the material at the blades). Long handles and short blades give an effort distance larger than the load distance, so the scissors multiply your force to cut tough material. Moving the material closer to the pivot increases the cutting force further, which is why you cut thick card near the hinge.

Example 2. A bottle opener as a second-order lever. The lip of the opener under the cap is the load, sitting between the pivot (the edge resting on the cap) and the effort (your hand on the handle). Because the effort distance is much larger than the load distance, a small hand force produces a large prying force on the cap, giving a high mechanical advantage that pops the cap easily.

Try this

Cue. A seesaw has a $300\ \text{N}$ child $1.5\ \text{m}$ from the pivot. Where must a $450\ \text{N}$ child sit to balance it? Answer: by moments, $300 \times 1.5 = 450 \times d$ , so $d = 450/450 = 1.0\ \text{m}$ from the pivot.
Cue. A lever lifts a $200\ \text{N}$ load with an effort of $50\ \text{N}$ . State the mechanical advantage. Answer: $MA = \text{load}/\text{effort} = 200/50 = 4$ (no unit).
Cue. Classify a wheelbarrow and tweezers by lever order. Answer: a wheelbarrow is second order (load in the middle); tweezers are third order (effort in the middle).

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.

Original6 marksA crowbar is used to lift a load. The load of

600\ \text{N}

0.10\ \text{m}

from the pivot, and the effort is applied

0.80\ \text{m}

from the pivot on the other side. (a) Calculate the effort needed to just lift the load. (b) Calculate the mechanical advantage of the crowbar.

Show worked answer →

(a) Apply the principle of moments about the pivot. For balance, the clockwise moment equals the anticlockwise moment:

\text{effort} \times \text{effort distance} = \text{load} \times \text{load distance}

E \times 0.80 = 600 \times 0.10

E = \frac{60}{0.80} = 75\ \text{N}

(b) Mechanical advantage is load divided by effort:

MA = \frac{\text{load}}{\text{effort}} = \frac{600}{75} = 8

What markers reward: the principle of moments set out as effort times effort distance equals load times load distance, correct substitution giving an effort of 75 N, and mechanical advantage as load over effort giving 8 (a number, with no unit).

Original5 marks(a) State the principle of moments. (b) For each of the three orders (classes) of lever, name the arrangement and give one example tool or device.

Show worked answer →

(a) The principle of moments states that when a body is balanced (in equilibrium) about a pivot, the sum of the clockwise moments about the pivot equals the sum of the anticlockwise moments about the same pivot. A moment is force times perpendicular distance from the pivot.

(b) First order (class 1): the pivot (fulcrum) is between the effort and the load, for example a seesaw, scissors or a crowbar. Second order (class 2): the load is between the pivot and the effort, for example a wheelbarrow or a nutcracker. Third order (class 3): the effort is between the pivot and the load, for example tweezers, a fishing rod or the human forearm.

What markers reward: the principle of moments stated correctly with equilibrium, and each of the three orders correctly described by which of pivot, load and effort is in the middle, with a valid example for each.