Ships float because water pushes upward on them with a buoyant force. This upward force comes from pressure that is greater at the bottom of the hull than near the surface. A ship sinks lower into the water until it displaces enough water to balance its own weight.
This idea is called Archimedes' principle, and it explains how huge steel ships can float.
Key Facts
- Archimedes' principle: buoyant force equals the weight of the fluid displaced.
- F_b = rho_fluid g V_displaced
- A floating ship has F_b = W_ship.
- Weight is W = mg.
- An object floats if its average density is less than the density of the fluid.
- A submarine changes its buoyancy by taking water into or pushing water out of ballast tanks.
Vocabulary
- Buoyant force
- The upward force a fluid exerts on an object placed in it.
- Displacement
- The volume of fluid pushed aside by an object in the fluid.
- Archimedes' principle
- The rule that the buoyant force on an object equals the weight of the fluid it displaces.
- Waterline
- The level where the surface of the water meets the side of a floating ship.
- Ballast tank
- A tank in a submarine that can be filled with water or air to control sinking and rising.
Common Mistakes to Avoid
- Thinking steel cannot float because steel is dense. A steel ship floats because its hollow shape makes its average density lower than water.
- Confusing mass with weight. Mass is the amount of matter, while weight is the gravitational force W = mg that buoyancy must balance.
- Using the total size of the ship instead of the submerged volume in F_b = rho_fluid g V_displaced. Only the volume below the waterline displaces water and creates buoyant force.
- Assuming a floating ship has no gravity acting on it. Gravity still pulls downward, but the upward buoyant force is equal in size when the ship is floating at rest.
Practice Questions
- 1 A ship displaces 2.0 x 10^6 kg of seawater. What is the weight of that displaced water, and what is the buoyant force on the ship? Use g = 9.8 m/s^2.
- 2 A small boat has a weight of 4900 N and floats in freshwater with density 1000 kg/m^3. What volume of water must it displace? Use g = 9.8 m/s^2.
- 3 A submarine is floating at the surface and then fills its ballast tanks with seawater. Explain how this changes its average density, buoyant force compared with weight, and motion.