Before the class
The teacher should understand the set up of the different pulley systems (image at https://en.wikipedia.org/wiki/Block_and_tackle#/media/File:Tackles.png):
A single fixed pulley has one pulley at the top with a string over it.
A gun tackle has a single fixed pulley at the top and a single pulley at the bottom (which will move). The string is tied off at the top, wraps around the bottom pulley then around the top pulley. Result is that two strings are pulling up on the bottom pulley (and the weight attached to it).
A luff tackle has a fixed double pulley at the top and a single pulley at the bottom (which is moveable). The string is tied off at the bottom pulley, wraps around the top pulley then the bottom pulley then the top pulley again (in the second groove). Result is that three strings are pulling up on the bottom pulley (and the weight attached to it).
A double tackle has two double pulleys, top and bottom. The string is tied off at the top, wraps around the bottom, then top, them bottom, then top pulley (using a new groove each time). Result is that four strings are pulling up on the bottom pulley (and the weight attached to it).
After my experimenting, I recommend using a single fixed pulley, a gun and a double tackle so that the change in forces is clear.
I made this activity a demonstration for primaries, with one of each pulley system that the class looked at together.
I recommend only asking grade 7s and dextrous 6s to set up their own systems (give them a system to copy). For intermediates, I made a set of 9 pulley systems (three of each kind), and placed them around the classroom for intermediates to try in turn.
Optional: no-pulley system
Simply pass the string over the bar, attach a cup to each end, and see how many counters it takes to lift another cup of counters.
Because of the friction between the bar and the string, it will take more counters in the top cup to lift the bottom cup.
One of the functions of a pulley is to provide a low friction system, with a wheel that turns.
Single fixed pulley
Hang one pulley from the bar, pass the string through it, then attach cups at each end, so that one cup is on the floor and the other is next to the pulley. Coil up extra string and add it into the clip that is holding the cup. Add (maybe 8) counters to the bottom cup, and then slowly add counters to the top cup until it moves the bottom cup upwards. [There should be about the same number of counters in each, maybe a couple more in the top cup.]
A single pulley simply changes the direction of a force. Show images of flag poles and window blinds where fixed pulleys change the direction of a force.
Composite pulley systems (with fixed and moveable pulleys)
Set up the other systems with one cup hanging from the bottom pulley. Attach it with a mini binder clip (take the arm off a mini binder clip, pass it though the bottom eye of the pulley, then reattach arm). And another cup to the free end of the string, pulling the string through until the bottom cup is on the floor and the top cup is next to the top pulley. Add (maybe 8) counters to the bottom cup, then slowly add counters to the top cup. Count how many counters are needed in the top cup (the "effort") to pull the bottom cup (the "load") upwards, for each system.
See attached worksheets for recording this data. (An added complexity is that the top cup effort is pulling up the weight of the pulley as well as the counters in the bottom cup. If the pulleys are light, this weight can be discounted, but if they are as heavy as several counters, the results need to take this into account.)
Transcribe all the data to the board for discussion. Students will (hopefully) see that the gun, luff and double tackle require relatively fewer counters in the top cup to raise the bottom cup, than the single fixed pulley. If the data is clean, the tackles with the greater number of strings pulling upwards will require the fewest number of counters.
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.
A greater number of wraps of string provide a mechanical advantage: a greater load can be lifted with the same effort but more string is pulled through.
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.