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Cartesian Diver
You will need:
- Version 1: a 1 or 2 litre plastic drink bottle and some condiment sachets (e.g. tomato ketchup)
- Version 2: a 1 or 2 litre plastic drink bottle and a pipette
- Version 3: a 1 or 2 litre plastic drink bottle and a ball point pen lid, weighted with paper clips or blue tack
What to do:
Fill the bottle to the brim with water. Select a sachet that just floats/ suck water into the pipette so that it just floats/ weight the pen lid so that it just floats (open end down). At this point, the weight of the sachet/ pipette/ lid is balanced by the weight of the water it is displacing (Archimedes principle). Put the float in the bottle and close the bottle tightly. What happens when you squeeze the bottle?
The air in the float compresses more easily than the water, and so when the pressure in the bottle increases, the air is compressed. At this point, the float weighs the same, but the volume of water it is displacing decreases (version 1) or the amount of water in the float increases (versions 2&3) so that its weight increases but the volume of water it displaces remains the same. The net effect is the same: the float sinks.
Possible extension: turn it into a competition by adding a hook to the bottom of your float. Place something heavy with an upward hook at the bottom of the bottle. Can you anchor the float?