Activity: Cartesian diver

This week, a classic science toy that illustrates one way that fish control their depth.

You will need

• A plastic soft drink bottle and lid
• Plasticine or blu-tak
• Water
• Cup
• A pen lid. A transparent one works best.
• Optional: A plastic container (an empty ice-cream container works well)

What to do

1. Fill the cup with water. We will use this to test your diver.
2. The pen lid will have a hole in it where the pen goes. Stick some plasticine around the hole, so that if you place it in water it will float with the hole pointing down and the tip just barely above the surface.
   • Use the water in the cup to test if it is floating correctly.
   • If your pen lid has a hole in the tip, block it off with plasticine.
   • Don't block the hole in the bottom of the pen lid.
3. Fill the bottle with water, right up to the top.
4. Place the diver in the bottle.
5. Screw the lid on the bottle.

To use your diver

• Squeeze the bottle and the diver will sink.
• Release the bottle and the diver will float.
• Can you squeeze just enough that the diver floats halfway down the bottle?

Note:

• You may find the diver sinks as you screw the lid on. Just remove a little bit of the plasticine from the diver.
• If the bottle is shaken or turned upside-down, the bubble can escape the diver. You will need to take the diver out, shake the water out of it and return it to the bottle. You can pour the water into the ice-cream container then pour it back into the bottle.

What's happening?

There are two forces acting on the diver:

• Gravity is pulling it down.
• Buoyancy is pushing it up. When you drop something into a liquid, it displaces some of the liquid (pushes it out of the way). An object placed in a liquid feels an upward force equal to the weight of the water it is displacing. This force is called buoyancy.

The buoyancy force on your diver depends on the volume of the pen lid and the volume of the air bubble inside it. Normally, the force of buoyancy is greater than the force of gravity, so the diver floats.

Water doesn't squash very well, but air does. When you squeeze on the bottle, it squashes the bubble inside the pen lid (if you used a clear pen lid, you can see it). With a squashed bubble, the volume of air in the diver is less, so the buoyant force on the diver is less. If you squeeze the bottle enough, the buoyant force becomes less than the force of gravity, and the diver is pulled down.

Applications

• Many fish have an internal organ called a swim bladder. The swim bladder is like a small balloon that the fish can fill with oxygen from its gills. By changing the size of the swim bladder, the fish can control their buoyancy, so they can go up or down easily. Sharks have no swim bladder, so they have to keep swimming constantly or they will sink.
• SCUBA divers wear vests called Buoyancy Compensators (BCs). By changing how much air is in their BCs, scuba divers can adjust their buoyancy.

A SCUBA diver wearing a Buoyancy Compensator.