ingridscience

Fill the bottle with water, to the brim.
Fill the eye dropper half full with water, until it just floats in the top of the bottle.
Top up the bottle with water and cap.

Squeeze the bottle to make the pipette sink.
Release to make it float again.

When the bottle is squeezed, air is compressed in the dropper as water enters it. This makes it more dense, so it no longer floats in the water.
When the bottle is released, the water leaves the pipette again, the pipette contains more air again. The air is therefore less dense again, so the pipette floats.

Human divers use weights to increase their density when diving.
Lifejackets decrease density, to enable a person to float better.
Submarines use tanks of compressed air to control their buoyancy. When ballast tanks are filled with water, the submarine sinks. When the ballast tanks are filled with air from the tanks, the submarine's overall density is decreased so it rises.

This activity is messy, and best done in a tray. Probably not suitable for a whole class, but for breakout groups with an attending adult.

Add a puddle of oil to one glass plate, spread out a little, then sprinkle iron filings on it.
Slowly lower the second glass plate, to avoid air bubbles. Add more oil from the edges with a pipette if necessary.
Move the top glass plate in a circular motion over the lower glass plate, to spread out the iron filings.
Use a magnet over the top glass plate to draw patterns in the iron filings.
Once a design is completed, slide a coloured piece of paper below the lower glass plate, being careful not to get oil on it, then photograph.

Remove the insulation from the last couple of cm of the magnet wire, at both ends, with the sandpaper.
Wrap the magnet wire around and around the pen, to make a coil. Leave 4cm at each end free. Remove from the pen and twist the ends together a few times to secure the coil. The uncoated ends should not be touching.
Tape the coil to the bottom of the cup, with the coil lying flat.
With the headphones removed from the headphone jack, strip the insulation off to reveal the bare wires underneath. Twist any strands of wire together to hold them together. Twist each headphone wire around each of the magnet wire ends, then wrap each in foil to make a good connection.

Start the music on the music player, plug in the headphone jack and turn up the volume.
Hold the cup against your ear with one hand, and hold the magnet against the coil with the other.
You should hear the music, maybe quite quietly. A stronger magnet, or multiple magnets, will make it louder.

The electrical signal through the coil from the music player, turns the coil into an electromagnet. The magnetic field changes with the changing electric signal of the music.
When the magnet is placed against the coil, it alternately attracts and repels the changing electromagnet's field, so pushing then pulling the coil away from it. This pushes the bottom of the cup in and out, which in turn, pushes the air near it back and forth. This is a sound wave is magnified by the cup, so that we can hear it.

Cap each end of a stick with 3 or 4 repelling magnets along the stick.
Students free play with their own stick.

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.

Add a candy and only a drop of water to a well of the paint tray.
Use a stick to turn the candy over and release the colour from its coating. Add one more drop of water if necessary, but keep the colour as concentrated as possible.

Cut a strip of filter paper the correct length to hang into the tub from the skewer with a binder clip. It's end should be about 1.5cm above the bottom of the tub.
Then remove the filter paper and add 1cm water to the tub.

Add a drop of candy dye to one end of the filter paper, at least one cm from the end, using the stick dipped in the candy juice.
Allow it to dry a little, then add another drop, allow to dry, then add another drop.

Attach the binder clip to the end of the filter paper strip away from the dye drop.
Slide the binder clip onto a skewer and lay the skewer over the top of the tub, so that the filter paper just touches the water, but the spot of dye is not immersed in the water.

Allow the chromatogram to run for several minutes, until the colours have moved most of the way up the filter paper.

The colours have different abilities to dissolve in water, and attach to the filter paper surface. All the coloured molecules move with the water, then stick on the paper, then move with the water again, but each different colour sticks to the paper or moves with the water to different extents. Hence the colours move at different rates up the paper and are separated out.

Try with different coloured candies to find out what colours make up each dye.
Compare different candies to see if the same colour is made up of the same component colours.

Make sure the short pencils have good points.
As shown in the photo, lay the long pencil between the two short pencils, with the points of the short pencils above the end of the long pencil. Tape together.
Make a rectangle of cardboard.
Using the points of the short pencils, push holes in the centre of the cardboard rectangle. Use a pin or other sharp object to help if needed.
As shown, bend two opposite corners of the cardboard slightly, in opposite directions, to make crude helicopter blades.

To launch, hold the pencils upright, using the holes in the cardboard, rest the helicopter on the pencil tips.
Spin the launcher rapidly, by spinning the long pencil between two flat hands (look at the position of the hands in the two photos).
The helicopter will lift off the launcher and rise a little before spinning off sideways.

Play with the size of the cardboard, the angle and size of the bends in the cardboard.
How high can it fly?

The forces involved:
As it spins, the bends in the cardboard direct the flow of air downwards. The push of air downwards, results in an equal and opposite reaction (Newton's Third Law) pushing back up against the helicopter. This is the force of "lift", which keeps it up in the air.
Eventually as the spin slows, the lift is not as great, and the force of gravity pulling it to the ground dominates and it falls.

For a simple (but very powerful) shooter:
Cut the neck off the balloon and fit it over the end of the toilet roll, leaving a pocket hanging.
Tape securely all around with duct tape.
To fire:
Drop the ammunition down the tube.
Grab the balloon pocket along with the ammunition.
Pull the balloon back, aim well, and let go.

Coins project fast out of this device, so only use with students that can be monitored closely, and who can obey rules around which way to fire and where to stand.
Foil balls still move pretty fast, but are not as dangerous.

The forces involved:
The balloon stores elastic energy as it is pulled back. As it is released it converts this energy to kinetic energy which pushes the ammunition forward. Air resistance eventually slows the ammo down, and gravity pulls it to the ground.

Convert to a rocket launcher/projectile:
Tape the balloon to a stiffer tube, and secure inside a long tube.
Add fins and a streamlined nose to the long tube.
Place the rocket over another long, stiff tube. Pull back the rocket so that the balloons is stretched. Release.