Follow these instructions to make your own pen lid diver and learn about buoyancy.

You will need

  • 1L or 2L plastic bottle
  • water
  • plastic pen top
  • piece of Blu-Tack or similar
  • hook (optional).

What to do

  1. Pen lid with blutack stuck to the stem of the lid

    Pen lid diver set up

    Pen diver in bottle

    Fill the plastic bottle to the brim with water.
  2. Stick a small piece of Blu-Tack onto one end of the pen top as in the picture.
  3. Put the pen top into the bottle. The Blu-Tack weighs down one end so that the top stays upright with a bubble of air inside.
  4. Seal the bottle tightly and squeeze the sides of the bottle. If you squeeze hard enough, the pen top sinks. Stop squeezing and it rises. Squeeze just the right amount and you can get the top to hang in the middle of the bottle.

What's happening

This is a demonstration of buoyancy. The top floats because it, and the bubble of air trapped in it, displace a greater weight of water than it weighs itself.

When you squeeze the sides of the bottle, you put pressure on the air trapped in the top causing it to shrink in volume. The top and trapped air now displace less water and weigh more than the water.

Try adding a hook to the Blu-Tack and use your top diver (also referred to as a cartesian diver) to lift objects off the bottom of the bottle.

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