What is Needed
What to Do
Place the condiment packet in the two liter bottle, and fill it up, all the way to the top. Screw the top of the bottle on, hard. If the packet floats, you are probably OK. If it sinks, you are going to need to try a different packet. Ketchup packets seem to work well for us, so you may need to try something else depending on what part of the country you live in - your local air pressure makes a difference!
Next, squeeze the bottle. If you squeeze it hard enough, the packet should sink. If you can't make it sink (mayonnaise packets seem to float no matter how hard you squeeze), you will need to try another packet.
Once you get just the right packet, it will float when you aren't squeezing the bottle, and sink when you do. You can make it go up and down with very little effort.
What is Happening?
There is a small air bubble inside the packet. When the packet is inside the bottle and you are not squeezing, the bubble is large enough that it will make the packet float very nicely. However, when you squeeze the bottle, you increase the pressure inside the bottle. This will compress the air bubble, which will increase the density of the packet. The packet will now sink.
This is a classic experiment called the Cartesian diver, updated for a modern flair in the spirit of the Little Shop of Physics.
Submarines use a similar principle to control their buoyancy, as do some fish.
Other Things to Try
With our packet, the buoyancy is also affected by temperature. We are a travelling program, so our experiments get left out in the van overnight frequently. When the bottle gets too cold, the packet will not float any more. Can you see why this might happen?
Currently, we are working on making a Galilean thermometer by putting a bunch of different packets in a bottle, each of which has a specific temperature at which it makes a change between floating and sinking.
The packet also floats or sinks depending on air pressure - which varies with altitude. We are at 5000 ft, and the packet works like a charm. The packet sinks like a stone in Lamar, which is at an altitude of 4000 ft; it's very hard to make it sink at all in Steamboat Springs, at an altitude of 7000 ft. The packet could in principle be used as a barometer....
If you find anything cool, or want to share what packets work better than others, drop us a line on our query page.