Science 7–10 · Year 7
Pulleys: measuring effort and rope distance for fixed and movable pulleys
Physical sciences — Forces (NSW Stage 4 focus area)
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The idea
A movable pulley halves the force needed to lift a load but doubles the length of rope pulled, so the work done stays the same.
What you need
- single pulleys on hooks, 3
- cord 2 m, 1
- 500 g mass with hook, 1
- 0 to 10 N newton meter, 1
- retort stand with a high crossbar or a hook in a door frame, 1
- metre rule, 1
How to do it
- Lift the 500 g mass directly with the newton meter and record the force; this is the load's weight.
- Set up a single fixed pulley on the crossbar, pass the cord over it, tie one end to the load and hook the meter to the other. Pull down steadily to lift the load 20.0 cm; record the force and how far the meter end moves.
- Set up a single movable pulley: tie the cord to the crossbar, pass it under a pulley attached to the load and up to the meter. Lift the load 20.0 cm; record the force and the distance the meter moves.
- Combine a fixed and a movable pulley (two supporting rope sections) and repeat.
- For each set-up calculate effort x distance moved by the effort, and load x height lifted, in joules, and compare.
What you should see
Direct lift: 4.90 N. Fixed pulley: about 4.90 N plus a little friction, and the rope moves 20 cm. Movable pulley: about half of the load plus the movable pulley's own weight, (4.90 N + pulley weight) / 2, plus friction, and the rope moves 40.0 cm. Two-section system: the same effort and 40 cm. Useful work is the same in each case, 0.98 J to lift the load 20 cm; the measured effort x distance comes out higher because of friction in the pulley bearings and, for the movable pulley, the work done lifting the pulley itself.
What changes
- What you change
- pulley arrangement (number of rope sections supporting the load)
- What you measure
- effort force (N) and distance the effort moves (cm)
- What you keep the same
- same load
- same 20.0 cm lift
- steady pull at constant speed
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- A pulley reduces the work needed, not just the force.
- A single fixed pulley halves the force.
- The load's weight changes when it hangs from a pulley.
Safety card
Hazards
- load falling if the cord slips
- stand tipping
Controls
- clamp the stand or use a fixed hook
- keep feet clear of the load
Note
No hazardous chemicals or naked flames are used. Complete the school's risk assessment for the activity before the lesson; the NSW Department of Education Science safety and compliance page points to CSIS 1.7 (Risk assessment – a pre-requisite for risk control) for how to carry it out.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7–10 Syllabus (2023)SC4-FOR-01SC4-WS-01SC4-WS-05
- Australian Curriculum v9AC9S7U04AC9S7I03AC9S7I04
Sources
The pages the author read to write this activity.