Technologies K–10 · Years 7–8

Pulley systems: measuring mechanical advantage and efficiency

Engineering Technologies and Systems focus area (NSW Technology 7–8, 2023); Design and Technologies: Knowledge and understanding, Technologies context: Engineering principles and systems (ACARA v9)

Practical, model not builtLow risk

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The idea

A pulley system with n rope strands holding the load needs, ideally, 1/n of the load's weight as effort but n times the rope length, and friction makes the real effort larger.

What you need

1 single fixed pulley, 1 single movable pulley and a pair of double pulleys for a 4-strand system
Retort stand, boss head and clamp, fixed to the bench
500 g slotted mass on a hanger
10 N spring balance reading to 0.1 N
Metre ruler and nylon cord

How to do it

Weigh the load with the spring balance and compare with 0.500 kg × 9.80 N/kg.
Lift the load with the single fixed pulley, pulling slowly and steadily; read the effort and measure the rope pulled for a 0.20 m lift.
Repeat with the single movable pulley (2 strands) and the 4-strand system, noting how many pulley wheels the rope passes over in each.
Weigh the movable pulleys on their own.
Work out the actual mechanical advantage (load ÷ effort) and the efficiency (actual ÷ ideal) for each.
Choose a system for lifting a 20 kg bag of potting mix onto a shelf and justify it with the data.

What you should see

The load weighs 4.90 N. Ideal efforts are 4.90 N, 2.45 N and 1.23 N for 1, 2 and 4 strands, and the rope pulled for a 0.20 m lift is 0.2 m, 0.4 m and 0.8 m. Measured efforts are larger than ideal because of friction and because the effort also lifts the movable pulleys, so efficiency is below 100% and tends to fall as pulleys are added. Work in (effort × rope pulled) is always at least the work out (load × 0.20 m). The learner knows it worked when the fixed pulley's effort is close to the load and the 4-strand effort is above a quarter of the load but well below half.

What changes

What you change: number of supporting strands (1, 2, 4)
What you measure: effort (N) and rope pulled (m)
What you keep the same: the same 500 g load
a 0.20 m lift
slow steady pulling
the same spring balance

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Pulleys reduce the work needed (they reduce the force but increase the distance).
More pulleys always help (each adds friction and weight).
A single fixed pulley halves the effort (it only changes the direction of the pull).

Safety card

Low riskLearners carry it out

Hazards

A falling mass hurting feet
The stand tipping

Controls

Keep the load over the bench and no higher than 0.6 m
Clamp the stand to the bench

Note

No chemicals or heat. Record the activity on RiskAssess (riskassess.com.au).

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Technology 7–8 Syllabus (2023), NESA. Current: taught from 2026. Code read from the outcomes page on 2026-09-22.TE4-MSC-01
Technology Mandatory 7–8 Syllabus (2017), NESA. Outgoing: replaced by the Technology 7–8 Syllabus (2023) from 2026 and not available after December 2027. Code read from the official syllabus document (DOCX) on 2026-09-22.TE4-8EN
Australian Curriculum v9AC9TDE8K03

Sources

The pages the author read to write this activity.

All Concept Studio activities