You are here

Activity

Pulleys to pull someone up

Summary: 
Use various numbers of pulleys to pull a person up, and compare the force and distance of rope used in each case.
Science content (2016 curriculum): 
Physics: Motion and Forces, Newton’s Laws, Gravity (K, 2, 6)
Physics: Simple and complex Machines (5)
image
image
image
Lessons activity is in: 
Materials: 
  • strong bar to support the system
  • rope, one long length and three short lengths
  • 4 pulleys, strong enough to hold double or triple the weight of the largest student
  • swing seat with rope loop, or a rope loop, to support a person
Procedure: 

1. Try pulling a friend up using one pulley (see first photo):
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.
Pull on the free end of the rope to raise the swing seat and your friend upwards. This will most likely be hard, and maybe 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 a two-pulley system (though a bigger difference is noticed if 3. is done now instead). In this case the rope feeds through one upper then one lower pulley, then ties off at the supporting bar - see second photo. The force required to pull a friend up is half that of 1. (because two ropes are now supporting the weight of the swing seat and person).

3. Try pulling a person up using four pulleys (see third photo):
Tie two pulleys to the supporting bar with two short lengths of rope. Feed the long rope through one of these upper pulleys (left pulley in the drawing), then through a lower free pulley, then through the second upper pulley, then through the second free lower pulley, then finally tie the rope off at the supporting bar. Hold the lower pulleys apart by spacing two stopper knots on a short length of rope between the two pulleys. Tie this rope into a loop to hold the seat.
Pull on the free end of the rope to raise your friend upwards. It will be much easier to pull than when using one pulley. This is because there are now four ropes pulling the person up and the force needed on each is 1/4 of the original 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 yourself 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.

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. Also danger of rope burns.

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 1
Gr 2
Gr 3
Gr 4
Gr 5
Teacher: 
Ingrid
Lindsay Izat
Teaching site: 
Carnarvon Elementary
General Gordon Elementary Science Club
ingridscience afterschool
Kerrisdale Elementary
Activity originally developed and delivered: 

Gordon Elementary Science Club