Activity

Pulleys to lift a person or heavy load

Summary
Use various numbers of pulleys to pull up a person or concrete block. Compare the force needed with a single fixed pulley or a composite pulley set-up.
Science content
Physics: Motion and Forces, Newton’s Laws, Gravity (K, 2, 6)
Physics: Simple and complex Machines (5)
Lessons activity is in
Materials
  • strong bar to support the system e.g. playground equipment or tree branch
  • rope, two long lengths and four short lengths
  • 5 pulleys, strong enough to hold double or triple the weight of the largest student/load
  • for pulling up people: swing seat or wooden disc with rope loop to support a person
  • for pulling up concrete blocks: two blocks stacked and strapped together, with a rope loop ("load")
Procedure

1. Try lifting a friend/concrete load using one fixed pulley:
Attach one pulley to the supporting bar with a short length of rope. Feed the long rope through this pulley and tie it off at the seat/concrete block.
Pull on the free end of the rope to try and raise the person/load up. This will most likely be hard, and not possible for some students.
The pulley allowed you to pull in one direction and move the friend the opposite direction - it simply changed the direction of the force.

2. Try pulling a person up using four pulleys:
Tie two pulleys to the supporting bar with two short lengths of rope. Add a loop of rope to the seat or load, and through the eye hooks of two pulleys, while knotting the rope between the pulleys to keep them spaced apart (see photos).
Feed the long rope through one of the upper pulleys, then through a lower pulley, then through the second upper pulley, then through the second lower pulley, then finally tie the rope off at the supporting bar. Pull the whole system down to reach the ground, then tie off the other end of the long rope to the seat or concrete blocks.
Pull on the free end of the rope to raise the person/load upwards. Make sure students are pulling hand over hand, so that the rope never runs out (act as a brake on the rope act where they are holding it). Do not allow them to pull the person or load too high.
The point is for them to feel the force difference between this pulley set up and with the one pulley. It will be much easier to pull up with the composite pulley system. This is because there are now four ropes pulling the person/load up (count them with the students) and they share the force. You only need to pull on the free rope with 1/4 of the force of the one-pulley system. BUT you need to pull four times the length of rope through the pulleys to raise the load by the same amount. The total amount of work is the same with each pulley system: a product of the force and the distance over which the force is exerted.

Look at photos of cranes which have multiple cables extending from the load, allowing the machinery to lift a greater load with the same force from a motor. (More cables will need to be pulled through though.)

Notes

This activity requires calm students and some care in running it.
Students can get pinched if they think they can hold more weight than they can, and let the rope run out. The teacher should act as a brake and hold the rope past where the child is holding it.

Students should pull each other up (as pulling yourself up leads to the rope running out and you falling backwards), unless an adult is always standing behind the student.

Grades taught
Gr K
Gr 1
Gr 2
Gr 3
Gr 4
Gr 5
Gr 6
Gr 7
Teaching site
Carnarvon Elementary
Gordon Elementary Science Club
ingridscience afterschool
Kerrisdale Elementary
Tyee Elementary