Extreme Science: Inertia Sticks

Cartesian Diver

What you will need:

  1. Empty one liter pop bottle
  2. Several Ketchup packets from your favorite fast food restaurant
  3. Water
  4. Adult supervision


1) The first thing you will want to do is to test your ketchup packets to find one or more that will float. You can do this in a drinking glass or even in the kitchen sink. You will notice that most of the packets will sink to the bottom. Do not use these “sinkers” for this experiment.

2) Once you find a packet that floats easily, carefully fold it so that you can fit it inside of your one liter pop bottle.

3) Now fill your bottle with water. Make sure that you fill it up all the way to the top. Carefully secure the lid onto the bottle nice and tight so that it doesn’t leak.

4) Hold your bottle in front of you. You should notice that the ketchup packet is floating at the top of the bottle.

5) Slowly squeezer the bottle. This should make your ketchup packet dive down to the bottom of your bottle. As you release the pressure, the ketchup packet will now float back up to the top.

What is going on?

The reason that some of the ketchup packets will float is because during the manufacturing process, a small air bubble will sometimes get trapped inside the package with your ketchup. If this air pocket is large enough, it will cause the packet to be less dense and float. Since gas molecules are a lot farther apart than liquid molecules, when you squeeze the bottle, the air molecules will move closer together creating a smaller air bubble. “Density” is Mass divided by Volume. When you squeeze the bottle and make the air bubble smaller, you are actually decreasing the volume (how much space the air bubble takes up) which then increases the density and causes the packet to sink. You didn’t realize that you were doing math when you squeezed that bottle did you? By releasing the pressure, the air bubble returns to its’ original size (increasing the Volume) and decreasing the density of the packet causing it to rise back to the top.

Try this:

Try using different types of condiments such as mustard or  hot sauce. Can you get the same results from different condiment packets? What would happen if you didn’t fill your bottle all the way full of water? Would this experiment still work? If so, try decreasing the water level to find out how much air you can have in the top of the bottle and still get it to work.

Radical Rick

Extreme Science