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What is Buoyancy?

Physics | 7-14 yrs | Interactive, Learning Pod

What is Buoyancy?

Boats, canoes, ships, kayaks, and rafts all have one thing in common- they are used for transport on seas and rivers, because they float.
It’s easy to figure out that if they are made out of wood they will float, because wood floats in water. However, when you see a huge cargo ship or a cruise-liner that looks like a floating building on the water, you can’t help but wonder why this massive chunk of metal doesn’t sink.

The Archimedes principle

A long time ago in ancient Greece, a mathematician named Archimedes was taking a bath. When he got in, a certain amount of water got displaced and overflowed over the rim of the tub.

Archimedes figured out that if the weight of the object being placed in the water is less that the weight of the water displaced, the object will float. This is known as buoyancy or the Archimedes principle.

How Ships float?

A ship made out of metal is able to remain lighter than the amount of water it displaces, because it is not a complete solid. The very bottom of the ship, called the hull, is hollow and therefore adds support to the ship without adding any mass.

When a ship is fully loaded, there is a maximum amount of weight it can carry before the weight of the ship increases past the amount of water it displaces.

If a ship has excess water during a storm, it is simply pumped back out into the ocean and it’s once more safe from sinking.

How Life Jackets work?

Life jackets use buoyancy too. Since your body is mostly water, holding on to a light life jacket is enough to keep you afloat in the deep.

The material inside a life jacket traps air when the jacket is covered in water. This trapped air weighs much less than the water it displaces. So, the life jacket floats and its buoyancy is strong enough to support your weight too.

Things to do
The next time you are in a swimming pool, try floating. Lie flat on your back and try to keep afloat. You will notice that you are more buoyant when you inhale, than when you exhale. Can you figure out why?

For more such interesting Physics videos and articles visit our Physics for Kids category.


Comments23 reactions

  1. Hi Sadiya,
    Definition of Buoyancy: Buoyancy is a force on an object making that object rise or move upward. Buoyancy is made by the difference in pressure put on the object by the Fluid or air that the object is in.

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