What Makes a Ship Float?

According to Newton’s law of gravity, every object exerts a pull on the things around it. Since the earth is millions of times bigger than the things around it, it pulls everything toward itself. If this were not so, everything on earth, including people, would go floating off into space.

Why then, when a ship is on the water, doesn’t the earth’s pull make it sink? After all, its frame, machinery, cargo, crew, and passengers are heavy, weighing thousands and thousands of tons.

The reason is that all boats and ships are designed with hollow areas that are filled with air. And while a ship does push some of the water aside as it floats, the large quantities of air inside it make it weigh less than the water it pushed aside. The water below it, however, weighs a great deal more and pushes the ship up, or makes it buoyant.

You can see this buoyant force when you put a blown-up beach ball in the water and the water seems to be pushing up on the ball.

What determines whether an object will sink or float is its density, how much air is inside it. A large piece of wood will float because air fills some of its spaces, but a small stone will sink because it has greater density, with no air in it.

The world’s largest ship of any kind is a French oil tanker, the Pierre Guillaumat, which weighs 554,000 tons!


  1. Carps Lionel says

    The weight of the water dislocated by the ships hull is the force that pushes the ship upwards. I found this out from a teacher during my Canal holidays UK.

  2. Thaislinhares@yahoo. says

    In that case is not the air, is not the hollow areas! Is a force made by the surface of water pushed. As found by Arquimedes, the greek sage. Infortunatly my english is not good enough to explain better…

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