Resources for teachers: PHYSICS

How do submarines work?

Discipline: PHYSICS

Key Stage: 3

Key idea: PARTICLES

TITLE:

How do submarines work?

AIM:

To understand how submarines can be made to dive or surface using compressed gas to adjust their buoyancy.

BACKGROUND

Although submarines have not been widely used in ocean exploration, they demonstrate an interesting application of the ideas we have of buoyancy. Submarines are built with buoyancy tanks which can be filled with adjustable amounts of air or water. The more air, the greater the buoyancy of the submarine.
If the submarine is at the bottom of the sea and needs to come to the surface, compressed air is released from storage tanks into the buoyancy chambers. This forces out the water that was in there and so the weight of the whole submarine becomes less. As it is still displacing the same weight of water because its volume has not changed, the submarine is now more buoyant and will float to the surface.

ACTIVITY

  • Using the rind of a fresh orange or lemon, cut out a model submarine.
  • Make a few small holes down each side using a pencil.
  • Put your submarine into a small bottle and fill this with water.
  • Screw on a plastic cap.
  • By pressing and releasing the cap you can make the submarine rise or fall

QUESTIONS/TASKS

Answer these questions in terms of particles:
1. In the background section, what is meant by ‘compressed air’?
2. Is this a gas, liquid or solid? Sketch a particle model of compressed air.

Tiny air bubbles in the fruit skin cause it to float.
3. What happens to the trapped air when you press on the bottle cap?
4. If the air spaces are compressed and made smaller, what happens to the buoyancy of the submarine?


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February 2007