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Introduction to Forces

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Lesson Focus

Buoyancy and Archimedes’ Principle

Physics

In this experiment, students will observe the factors that affect buoyancy as described by Archimedes’ Principle.

Now You Know

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

• After completing this Activity Object, students will be able to:
• Describe Archimedes’ Principle as: the buoyant force of an object is equal to the weight of the displaced liquid.
• Explain that when an object sinks, the liquid that is displaced is less than the weight of the object.
• Explain that when an object floats, the liquid that is displaced equals the weight of the object.

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 the 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. It 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. It is important to note that an object will sink only enough to displace its own weight. If the fluid density is less than the average density of the object, the object sinks.

Objects of equal volumes, regardless of their masses, feel equal buoyant forces when completely 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. The behavior of two objects of different density but the same volume would 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, as 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 weights of the objects compared to the buoyant force.

Tutorial Details

 Approximate Time 25 Minutes Pre-requisite Concepts buoyancy, density, weight, force Course Physics Type of Tutorial Experiment Key Vocabulary Archimedes’ principle, buoyancy, buoyant