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Physics Simple machines
Class 7 Intermediate About 20 min Premium

Pulleys: direction, mechanical advantage and efficiency

Lift loads with a fixed pulley, a movable pulley and a block and tackle, and compare the effort, mechanical advantage and efficiency.

Core concept

Pulleys change the direction of a force, and more supporting strands reduce the effort needed

Topic: Simple machines

A pulley is a wheel with a rope running over it. A single fixed pulley does not reduce the effort, but it changes its direction: you pull down to lift a load up, which is easier. A movable pulley moves with the load, so two strands of rope share the load and the effort is roughly halved. A block and tackle uses several pulleys, so even more strands share the load. Mechanical advantage (MA) is load ÷ effort. Velocity ratio (VR) is how far you pull the rope ÷ how far the load rises, which equals the number of supporting strands. Efficiency = MA ÷ VR × 100%. It is always less than 100% because some effort is wasted lifting the moving pulleys and overcoming friction: A force that slows things down when two surfaces rub together. All key words.

Key words explained

What you will learn

  • Describe how a fixed pulley changes the direction of a force.
  • Calculate mechanical advantage, velocity ratio and efficiency of a pulley system.
  • Explain why real machines are less than 100% efficient and how to improve them.

Before you start

  • Know that force is measured in newtons (N).
  • Know what a lever's mechanical advantage means.

Materials

  • Loads of 20 N to 80 N
  • Spring balance to measure the effort
  • Strong string

Equipment

  • Single pulley, two-pulley blocks and a support stand
  • Metre rule

Procedure

  1. With the single fixed pulley, record a measurement and note which way you pull.
  2. Change to the movable pulley and the block and tackle, recording a measurement with each.
  3. With the block and tackle, record a light load (20 N) and a heavy load (80 N). Compare the efficiency.
  4. Oil the pulleys (no friction) and record again.

How this simulation is simplified

  • The rope has no weight and does not stretch.
  • Each pulley wastes the same friction force, and the moving pulley block weighs a fixed amount.
  • The load is raised by 20 cm each time.
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