Watch this short video to learn how to make your own straw submarine that can swim in a bottle.

You'll need:

  • Plastic bottle (2 litre bottle)
  • Straw
  • Scissors
  • Sticky tac
  • 2 litres of water

Science behind the experiment

When your straw submarine is in the water it is buoyant, the buoyant force competes again the gravitational pull that is pulling the straw down and the difference between the two forces decides how buoyant the submarine is.

When you squeeze the bottle the air bubble inside the straw compresses (gets smaller) so the volume of the straw decreases and so does the buoyant force. The submarine becomes more dense than the water around it and the gravitational pull will pull it down.

By releasing your hold on the bottle the air molecules in the straw submarine can expand again and the volume increase resulting in a stronger buoyant force.

Key Terms

  • Buoyant force – the upward force on any object in any fluid.
  • Molecules – a molecule is a particle made up of two or more atoms that are chemically bonded together.
  • Dense – a measure of mass per volume.

Activities and Questions

Try and compress the air molecules enough to balance the forces; the straw submarine neither floats nor sinks but remains still in the middle of the bottle.

Does the size of the bottle affect how hard you have to squeeze to make the diver sink?

Does it matter if the bottle is not filled all the way with water?

How does salt change the density of the water?