ingridscience

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 fixed and moveable pulleys.
A flag pole uses a single fixed pulley. Cranes use many wraps of cable to provide a large mechanical advantage and allowing very large loads to be lifted, as well as fixed pulleys to change the direction of the force. Boat rigging allows one person to pull in sails that would be too hard without the aid of moveable pulleys. Crevasse rescue systems use a pulley set-up which includes fixed pulleys to change the direction of the force, moveable pulleys to trade force for distance and hitches which clamp ropes.

We use water to turn wheels to make electricity (hydroelectricity), catch fish (fish wheel) or grind flour.

Pour water over a wheel to show how it turns.

Optionally lift an object from the floor to the table top, using the force of falling water on the water wheel.

Before the class, prepare discs from the pie plates for the students, as cut edges are sharp:
Carefully cut out the flat part of the pie plate and discard the sides.
Punch a hole exactly in the centre with a pencil and enlarge slightly.

Students make cuts from the edge of the pie pan 2/3 of the way to the centre, in four or six places (to start, try modifications later). Fold over the sides of each section to make a paddle wheel shape.

Wrap a small piece of foam around the wooden rod and tape it in place. Push the rod through the hole in the pie plate, enlarging the hole until the plate fits snugly on the foam around the rod. The plate should not turn easily at all without the rod (the axle) turning.

Tape U-shaped pieces of foam to the centre of each of the long sides of the tray.
Lay the wheel and axle over the foam supports on the tray. Test that it can turn without hitting the bottom of the tray. If necessary, fold over the outside edges of the pie plate.

If objects are to be lifted with the falling water, make sure one end of the rod protrudes over the edge of the tray more, then tape the string to this end of the rod. Cut the string off where it meets the floor, and tie on a small weight e.g. the scissors.

Pour water from the 2L bottle onto the wheel. The weight of the water hitting the paddle blades generates a force which makes the blades move and the wheel turn.
Once the water runs out, pour the tray of water back into the bottle (using the funnel) for reuse.

If a weight is being lifted, the turning wheel turns the rod, which winds the string and pulls up the weight.
Challenge student to control how they pour the water to make the weight raise slowly or faster.

In a traditional water mill, a water wheel turns grindstones to make flour:
https://www.youtube.com/watch?v=1L5Pt4BLeos

A fish wheel turns in the moving water of a river and has attached netting to catch salmon:
Image here: https://gallery.janeandjohn.org/index.php/haines/FishWheel and in here: https://salmonlife.org/archived/stories/copper-river/
https://www.youtube.com/watch?v=-l81d3R1-OY
https://www.youtube.com/watch?v=uPoKHOMlhZ0
Fish wheels are used to catch salmon during a run, either for food or for tagging (to track salmon populations).

Hydroelectric power is made from a water wheel (called a turbine):
image: https://www.energy.gov/sites/default/files/styles/full_article_width/pu…
Video: https://www.youtube.com/watch?v=OC8Lbyeyh-E

Crumple the paper into a ball.
Place it in the mouth of the bottle.
Blow into the mouth of the bottle.
What happens?

The paper will usually shoot out of the bottle towards you!
If it does not, check that the paper ball does not take up more than half of the diameter of the bottle opening.

Why?
When you blow into the bottle, you add air to it, which increases the pressure inside.
The higher pressure air in the bottle will move towards the lower pressure air outside the bottle.
In flowing out of the bottle it pushes the paper out.

Please note that in a class of students it is likely that one of them is at least partially colourblind (1 in 12 males are colourblind). As this is an activity distinguishing colours, these students will not be able to tell some colours apart and perceive some colours differently, although the activity will be no less interesting for them. The common red/green colour blindness means reds and greens (or colours containing reds and greens such as browns) look similar. More information at colourblindawareness.org and colorblindguide.com/post/the-advantage-of-being-colorblind.

Turn out the room lights.
View the cloth under different light colours and see how the cloth colours change.
Colours will look their normal colour, a different colour or even black.

The colours appear in the cloth because of the colours (wavelengths) of light they reflect.
If white light hits them (as we usually have with the sun or room lights), the cloth will absorb parts of the white colour spectrum and reflect others - the reflected colours are the ones that we see.
However if only part of the white light spectrum hits the cloth, it is only able to reflect that part of the spectrum. If the cloth is white it will reflect all the colours hitting it, so will look the same as the light colour. If the cloth is another colour, it will still absorb some of the wavelengths, reflect some, and maybe appear a different colour.
For example, a red piece of cloth will reflect red light and absorb all other colours. So if a red light hits it it will appear red, but if a blue light hits it it will appear dark because it absorbs the blue light and reflects no light.
In the photos above, the colours that are fuchsia-coloured in white light (first photo) appear blue in blue light (second photo) and brown in green light (third photo). The pink reflects mostly reds and blues. With blue light hitting it, it can only reflect blue so appears blue. With green light hitting it (a mixture of yellow and cyan), it does not reflect many wavelengths, so appears a darker. Check the last light colour mixing image to see how light colours mix.

Explain that the ball is a model of the earth. Check that students know where the equator, poles, and their own city are.
Turn the room lights off.
Hold the flashlight level with the centre of the ball, from a couple of metres, so that a circle of light falls on the equator. Ask a student to draw around the circle. This models the sun's rays reaching the equator.
Keep the flashlight at the same distance but move it up so that it now shines on a more northern region of the "earth". Ask a student to draw around the patch of light on the ball now - it should be an elipse. This is how the light falls on the more northern regions.
Turn the room lights on and compare the outlines of the light patches. Discuss the intensity of light in each of the regions - it must be more intense at the equator as it covers a smaller area.

Explain that in the same way, the equator of the earth receives much more intense sunlight than the more northern and southern latitudes.

Relating to how weather starts, the tropics are warmed up more by the sun, and the warmer land and ocean there means the air above it is heated up more.

The sun's angle on Earth helps determine why we get different biomes;
Biome map: https://askabiologist.asu.edu/sites/default/files/resources/articles/bi… (from this article - https://askabiologist.asu.edu/explore/biomes) or https://cdn.britannica.com/38/102938-050-6B5388D9/distribution-biomes.j… for terrestrial (Earth, not water) biomes.