Application of Archimedes Principle

Plimsoll Line

The Plimsoll line is an imaginary line marking the level at which a ship or boat floats in the water.
It indicates how much load is allowed at different types of water.


  1. Air ship is filled with helium gas.
  2. Helium gas has density lower than the surrounding air, hence an upthrust which higher than the weight of the airship can be produced and cause the airship float in the air.

Hot Air Balloon

  1. Hot air in the balloon has lower density than the surrounding air.
  2. As a result, when the buoyant force produced is higher than the weight of the balloon, the balloon will start rising up.
  3. The altitude of the balloon can be controlled by varying the temperature of the air in the balloon.


  1. Hydrometer is used to measure relative density of liquids.
  2. How deep the hydrometer sink into the liquid is affected by the density of the liquid.
  3. The lower the density of the liquid, the deeper the hydrometer will sink.
  4. This is used as the indicator of relative density of a liquid.


A submarine use ballask tank to control its movement up and down.
To get submerge, water is pumped into the ballast tank to increase the weight of the submarine.
To surface, the water is pumped out to reduce the weight of the submarine.

Q & A

Q: The diagram shows a picture of a hydrometer. What is the function of the lead shot at the bottom of the hydrometer?


To lower down the centre of gravity of the hydrometer. The hydrometer will topple if the centre gravity of the hydrometer is above the surface of the liquid.