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Activity

Desktop pulleys to measure forces

Summary: 
Use a single fixed pulley, then more complex pulley systems, to lift counters and measure forces in pulley systems.
Science content (2016 curriculum): 
Physics: Motion and Forces, Newton’s Laws, Gravity (K, 2, 6)
Physics: Simple and complex Machines (5)
Materials: 

For simple string over a bar:

  • stand, or plastic rod between two tables - must be made of slick material
  • string length that loops over the stand and reaches the floor on one side
  • optional: S-hooks for string ends
  • small pots for marbles and for pouring water from e.g. dollar store plastic shot glasses
  • masking tape
  • marbles
  • tray and/or towel to catch water spills

For fixed pulley:

  • wooden bar with a hook to straddle desks
  • masking tape
  • string (blue in photo), long enough to loop from the floor, through the hook and back to the floor (mine was 1 metre)
  • single pulley (plastic)
  • 4 mini binder clips
  • mini carabiners that fit through pulley hooks
  • heavy glass counters
  • two little pots with handles e.g. plastic shot glasses with masking tape handles
  • baggies to contain kits
  • worksheet (attached)

Add to fixed pulley materials for moveable pulley activity:

  • additional string for gun tackle (we used 1.45m (green) but measure for your desk height)
  • additional single and double pulley for each group
  • additional string for luff tackle (we used 2m (red) but measure for your desk height)
Procedure: 

Simple string over a bar
Bridge a rod between two tables, then tape down to hold in place. Hang the string over the rod. Cut the string so that one end just reaches the floor and the other end just hangs over the rod.
Give each student group a handful of marbles or glass counters, and optional water (with a towel laid under the materials).

Challenge students to raise a marble in one pot, only by adding marbles (or water) to the other pot. Although this task is relatively easy, students will be absorbed with this activity.

Discuss how the system changes the direction of the force: when the weight moves one side down, an equal and opposite force pulls the weight on the other side up.

Allow students to experiment with number of marbles/counters. Given the chance they will will naturally experiment with these and other variables to explore the characteristics and uses of this simple system that models the action of a pulley.
If students need focused challenges to guide their experimentation here are some ideas:
How many marbles are needed to raise another number of marbles?
How can you balance the pots half way up from the floor?
What can you change to make the marble move upwards slower or faster?

Discussion
Gather as a group for reporting of what students found. Use their discoveries to guide concluding remarks about the forces, balanced and unbalanced.
This system changes the direction of a force - when one weight moves down, it moves the other weight up.
By increasing the weight on one side the speed that this side moves down is increased, therefore the speed that the other side moves up is also increased.
When the weight on each side is equal, the forces are balanced and the system does not move.

This set up is similar to a pulley - a rope over a bar. It is just missing a grooved wheel which makes the rope move more smoothly.
See next activities using pulleys.

Fixed pulley
Students will compare a system with and without a pulley.
Straddle a wooden bar across two desks and tape down so that it does not slide around. Show students how to loop a string (length that is double the height of the bar) over the hook and attach little pots at each end of the string with mini binder clips (drawing A on the attached worksheet).
Ask students to add counters to the bottom cup, then see how many counters in the top cup it takes to lift the bottom cup, and record their results on the worksheet. Then do the same experiment, but with the string looped through a pulley hung from the hook (drawing B).

Once they have compared with and without a pulley, they can try dragging small objects across their desk with the pulley system.

Discuss results:
Students should find that it takes more counters to lift the bottom cup with just the hook, rather than with the pulley.
The force of friction makes it harder for the string to slide over the hook. Hence one of the functions of a pulley is to provide a low friction system, with a spinning wheel.
Also point out that as one cup goes down the other cup goes up, so a fixed pulley changes the direction of a force. Through pulling objects across their desks they will have created other situations where the direction of the force is changed.
Show images of flag poles and window blinds where fixed pulleys change the direction of a force.

Moveable pulleys
Show students how to set up a more complex pulley system with the fixed pulley they have already set up as well as a moveable pulley. Assemble the gun tackle and the luff tackle and leave set up for students to copy one of them (see photos). Relate to the drawings on page 2 of the worksheet. Note that this is a tricky experiment to set up and students will need some help and time to assemble their chosen pulley system.
Once the system is assembled, place counters in the bottom cup (called the “load”) and see how many counters in the top cup (called the “effort”) are required to lift them.
Students can record their date in the table on page 2 of the worksheet. Note that the weight of the pulley is equivalent to two counters and is added in to the total load.

Alternatively, and more simply, students can note whether more or fewer counters are needed compared to a single fixed pulley

Transcribe all the groups’ data to the board for discussion, along with the data for the single pulley. Students will see that both the gun tackle and the luff tackle require relatively fewer counters in the top cup to raise the bottom cup, than the single fixed pulley.

The explanation is because a greater number of wraps of string provide a mechanical advantage: more string is pulled through but a greater load can be lifted with the same effort.

Show students images of moveable pulleys. Cranes use many wraps of cable to provide a large mechanical advantage and allowing very large loads to be lifted. Boat rigging allows one person to pull in sails that would be too hard without the aid of pulleys.

Getting more complicated:
The ratio of the load/effort can be calculated to see how it changes with more wraps of the string: the ratio is greater with more wraps of the string i.e. less load is required as the number of wraps goes up (from single pulley, to gun tackle to luff tackle). In a perfect system the ratio would be the same as the number of string lengths i.e. ratio of 1 for single pulley, 2 for gun tackle and 3 for luff tackle.

Attached documents: 
Grades taught: 
Gr K
Gr 2
Gr 3
Gr 4
Gr 5
Teacher: 
Brenda Koch
Fiona Laporte
Ingrid
Sheela Kishi
Todd Millway
Teaching site: 
MacKenzie Elementary
Xpey' Elementary School (formerly MacDonald Elementary)
Activity originally developed and delivered: 

Aboriginal Focus School with the Vancouver School Board's Scientist in Residence Program for the Simple string over a bar activity.
MacKenzie Elementary with the Scientist in Residence Program for the Pulley activities.