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Buoyancy

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Buoyancy

What could be more refreshing than floating in cool water on a hot summer day? Water and other fluids have a special force that allows people and things to float in them. The force is called buoyant force.

What Is Buoyant Force?

Buoyant force is an upward force that fluids exerts on any object that is placed in them. The ability of fluids to exert this force is called buoyancy . What explains buoyant force? A fluid exerts pressure in all directions, but the pressure is greater at greater depth. Therefore, the fluid below an object, where the fluid is deeper, exerts greater pressure on the object than the fluid above it. You can see in the Figure below how this works. Buoyant force explains why the girl pictured above can float in water.  

Diagram illustrating fluid pressure

Q : You’ve probably noticed that some things don’t float in water. For example, if you drop a stone in water, it will sink to the bottom rather than floating. If buoyant force applies to all objects in fluids, why do some objects sink instead of float?

A : The answer has to do with their weight.

Weight and Buoyant Force

Weight is a measure of the force of gravity pulling down on an object, whereas buoyant force pushes up on an object. Which force is greater determines whether an object sinks or floats. Look at the Figure below . On the left, the object’s weight is less than the buoyant force acting on it, so the object floats. On the right, the object’s weight is greater than the buoyant force acting on it, so the object sinks.

Diagram illustrating why some objects float and others sink

Because of buoyant force, objects seem lighter in water. You may have noticed this when you went swimming and could easily pick up a friend or sibling under the water. Some of the person’s weight was countered by the buoyant force of the water.

Density and Buoyant Force

Density, or the amount of mass in a given volume, is also related to the ability of an object to float. That’s because density affects weight. A given volume of a denser substance is heavier than the same volume of a less dense substance. For example, ice is less dense than liquid water. This explains why the giant ice berg in the Figure below is floating in the ocean. You can see other examples of density and buoyant force at this URL:

http://www.youtube.com/watch?v=VDSYXmvjg6M

Floating iceberg

Q : Can you think of more examples of substances that float in a fluid because they are low in density?

A : Oil is less dense than water, so oil from a spill floats on ocean water. Helium is less dense than air, so balloons filled with helium float in air.

Summary

  • Buoyant force is an upward force that fluids exert on any object that is placed in them. Buoyant force occurs because the fluid below an object exerts greater pressure on the object than the fluid above it.
  • If an object’s weight is less than the buoyant force acting on it, then the object floats. If an object’s weight is greater than the buoyant force acting on it, then the object sinks.
  • A given volume of a denser substance is heavier than the same volume of a less dense substance. Therefore, density of an object also affects whether it sinks or floats.

Vocabulary

  • buoyant force : Upward force exerted by a fluid on any object placed in it.
  • buoyancy : Ability of a fluid to exert an upward force on any object placed in the fluid.

Practice

At the following URL, read the short article about submarines and do the animation of a submarine submerging and surfacing. Then answer the questions below.

http://science.howstuffworks.com/transport/engines-equipment/submarine1.htm

  1. What can a submarine do that a ship cannot?
  2. How does a submarine control its buoyancy?

Review

  1. What is buoyant force?
  2. Why does buoyant force occur?
  3. Why is a heavier object more likely than a lighter object to sink instead of float in water?
  4. Assume you have an ice cube and also a small rock that is the same size and shape as the ice cube. Predict what would happen if you placed the ice cube and rock in a glass of water. Explain your prediction.

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