The tendency of an object to rise up to the upper levels of the fluid or to float on the fluid surface is known as floatation. Sinking is just the opposite process of floatation which means the tendency of an object to go deep down to the lower levels of the fluid.

The phenomenon of floatation totally depends on the density of the object placed in the fluid and the density of the fluid in which it is placed. A ship can only float or sail on the sea if its density is lesser than that of the seawater.

Law of Floatation

The fluid in which a body floats should relocate or displace the fluid of its own weight to float; this is known as the law of flotation.

For example: If a brick of wood weighing 300kg (3000N) floats in water, then this means that it displaces 300kg (3000N) of water; however, in other fluids of different properties, if the same wood brick is placed, then it will only float if it is able to displace the same 300kg of fluid.

Conditions for an Object to Float

  • The fluid in which the object has to float should have a density more than that of the average density of the floating object.
  • The total weight of the object must be equal to the up-thrust force of the fluid on the object.
  • To displace a huge amount of fluid, the volume of the object submerged must be huge enough.

Relationship between Real Weight of a Floating Body and Upthrust

As we know,

Real weight = apparent loss in weight + apparent weight

Where apparent loss in weight is equal to the weight of the fluid displaced, which is the upthrust


Real weight = upthrust + apparent weight

But, the apparent weight of an object becomes zero when it floats.

Hence, apparent weight = 0


Real weight = upthrust + 0

Therefore, real weight = upthrust.

The relationship between real weight and the upthrust of a floating object shows that the real weight is equal to the upthrust, and the floating body displaces its own weight of the fluid in which it floats because the upthrust is equal to the weight of the fluid displaced. Therefore, to make an object float, it is important that its real weight must be equal to upthrust.

Daily Life Applications of the Law of Floatation

  • In waterways transportation

The law of floatation is applicable to all the containers that travel by waterways, consisting of ships, boats and submarines.

  • In transportation by airways

Hot air balloons and airships are the most widely used air transport mediums that use the law of floatation.

  • In decoration

Balloons filled with lighter gases used for decorative purposes also work on the law of floatation.

  • In the measurement of specific gravity of liquids

To measure the specific gravity of liquids, an instrument is used, which is named hydrometer. It employs the law of floatation.

Mechanical Properties of Fluids

Principle for flotation of ships

The ship can be assumed to be like a floating bowl of iron in water, but the bowl does not sink even though iron is denser than water. Because of its hollow shape, it contains air, which causes its average density to be lesser than that of water.

If you fill sand into the bowl, it will keep floating, but in a surplus mass of sand, the bowl will sink, which is called overloading in ships.

Even though the ships are made of metals that are denser than water, like, aluminium alloy or steel, they manage to float in water because of their hollow structure. This hollow structure filled with air results in a lesser average density as compared to the average density of water even when it is full with cargo.

The ship travels in diverse densities of water like seawater, hot water, cold water or freshwater. Therefore, the ship can displace more or less water according to the change in waterways in which it floats, which means that the ship may lose or gain thrust. The ship sinks if the up thrust becomes less than its own weight.

Submarine sink and float

A ship that can travel deep underwater as well as on the surface too is known as a submarine. Using the law of floatation, the submarine owes its ability to dive and come up to the surface of the water. It holds weight tanks on either side, which are occupied with water when it has to dive into the waters and are emptied when it has to come up. The tanks are filled and emptied using the special pumps.

How does a hot air balloon float?

A hot air balloon comprises three parts, the burner, the balloon, and the basket. Propane gas is used by the burner to heat up the air in the balloon, and as soon as the air in the balloon gets hot, it enlarges or expands, which results in the rise of a balloon because its density becomes lower than the surrounding air.

To get the balloon down, the pilot can open a parachute valve at the top of the balloon, which decreases the temperature and density inside the balloon because it causes cold air to enter the balloon. Also, by burning less fuel, the pilot can let the air inside the balloon cool itself.

How does an Airship Float?

The airship floats in the sky using the helium and hydrogen gases because these gases are lighter, and they help the airship to rise up; by means of a propeller attached to the ship, maneuverings are done. The engine is used to propel the ship, and the lighter gas gives it a lift.