Why Do Ships Float?

What You'll Learn

• Things float when they push aside an amount of water equal to their weight
• A ship is heavy, but it's also mostly hollow, so it pushes aside a LOT of water
• The water pushes back up with a force called buoyancy
• If buoyancy is stronger than gravity, the object floats
• Shape matters — a big hollow ship displaces more water than a small solid chunk of metal

Words to Know

Buoyancy

The upward push of water that helps things float

"Buoyancy pushed the ship up."

Displace

To push something out of the way

"The ship displaced a lot of water."

Hollow

Empty inside

"The ship's hull is mostly hollow."

Density

How much stuff is packed into a space

"A rock is denser than a sponge."

Check What You Learned

Fun Facts

• Archimedes' principle: an object floats if it displaces a weight of fluid equal to its own weight

  Source: Encyclopaedia Britannica

• Buoyant force is the upward force exerted by a fluid that opposes the weight of an immersed object

  Source: NASA

• Steel is denser than water (~8 times), but a steel ship floats because of its hollow shape

  Source: HowStuffWorks

• The hull of a ship displaces a large volume of water, creating buoyant force greater than the ship's weight

  Source: US Naval Academy

• Objects with density less than water float; greater than water sink — but shape affects total displacement

  Source: Physics textbooks

• Aircraft carriers can weigh over 100,000 tons and still float

  Source: US Navy