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Buoyant Force

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Lesson Focus

Buoyant Force

Physical Science

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In this experiment, you will observe the factors that affect buoyant force.

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Now You Know

After completing this tutorial, you will be able to complete the following:

  • Explain that the buoyant force on an object is equal to the weight of the liquid it displaces.
  • Explain that for submerged objects of the same volume, the bouyant force does not depend on the mass of the object or the volume of liquid in the container.
  • Explain that the buoyancy of an object changes in liquids of different densities.

Everything You'll Have Covered

When an object is placed in a fluid, the fluid exerts an upward force due to the pressure difference of the fluid between the top and bottom of an object. This force is known as buoyant force. Because the pressure at the bottom of an object is always greater than the pressure at the top, every submerged object feels an upward buoyant force.

Buoyancy is what enables objects to float or appear lighter in water and is present whether an object floats or sinks. According to Archimedes' Principle, the buoyant force on any submerged object is equal to the weight of the fluid the object displaces. If the fluid density is greater than the average density of the object, the object floats. If the fluid density is less than the average density of the object, the object sinks. It is important to note that an object will sink only enough to displace its own weight.

Objects of equal volumes, regardless of their masses, feel equal buoyant forces when submerged in the same type of fluid. If an object is submerged, it displaces its total volume of fluid. The buoyant force on each object is the same because of identical pressure environments and equal water displacement. However, the behavior of two objects of different density but the same volume would certainly be different upon release from rest in the water. For example, if you have a ball of cork and a ball of lead that are equal in volume, they will have different masses. Because they are equal in volume, they will experience equal buoyant forces since the weight of the displaced fluid is directly proportional to the volume of the displaced fluid. When released from rest in the water, the cork will bob up and the lead will sink. The difference in behavior is a result of the weight of the object compared to the buoyant force.

Tutorial Details

Approximate Time 20 Minutes
Pre-requisite Concepts Students should be familiar with buoyancy, density, weight, and force.
Course Physical Science
Type of Tutorial Experiment
Key Vocabulary Archimedes, buoyancy, buoyant