Floating and sinking jellyfish

Let's take a look!

What type of experiment is this?

Experimental procedure and explanation:

• Materials required: dropper, large nut (M6), small nut (M5), zip tie, plastic hemisphere, plastic bag, rubber balloon, thick rubber band, wide-mouth plastic bottle
• First, make a normal Cartesian diver.
• Make a hole at the top of a plastic hemisphere (can be a capsule toy container) to release the air.
• The Cartesian diver is glued to this. Glue the legs made by cutting the plastic bag to this to complete.
• Float this on the water and adjust the amount of air in the dropper such that the tip of the head floats out slightly.
• Fill the wide-mouth plastic bottle with water and add the jellyfish.
• Cut off the mouth of the rubber balloon, put it over the mouth of the wide-mouth plastic bottle, and secure it with a thick rubber band. Work in a bucket filled with water to prevent air from entering.
• Press the rubber membrane of the mouth with a ping-pong ball. Then, the jellyfish will sink, and if you do not press as hard, it will float up (if you do not have a ping-pong ball, then you can press with the palm of your hand).
• Pressing the mouth increases the pressure of the water and air inside the plastic bottle. The air is compressed and the volume decreases, so the buoyancy is reduced and the object sinks. Conversely, if you do not press as hard, the air expands, the buoyancy increases, and the object floats.

[Keywords]	Pressure, buoyancy
[Related items]	Floating and sinking squid, Cartesian diver
[Reference]	“The Wonders of Flow,” Japan Society of Mechanical Engineering, Kodansha Blue Backs, p. 42–47.

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