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.