Buoyant Force: Definition, Equation & Examples

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  • 0:01 How a Heavy Object Floats
  • 0:32 What is the Buoyant Force?
  • 1:21 Types of Buoyancy
  • 4:05 Buoyancy Equation & Example
  • 5:35 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov
This lesson defines the buoyant force, and it explains where buoyant force comes from and where it exists. It describes the different kinds of buoyancies while giving you a formula and an example of this force.

How a Heavy Object Floats

Let's step on a big cruise ship. What do you notice underneath your feet? Heavy solid metal, right? You know that if you were to take a small solitary piece of heavy solid metal and place it on the water, it would sink. So how in the world has the cruise ship you're standing on stayed afloat?

Well, it's all thanks to the concept of buoyancy. Let's define buoyancy, give you the magical equation for it, and work through an example together.

What Is the Buoyant Force?

Buoyancy is, basically, the ability of something to float or rise in a fluid. Simple as that. More scientifically, buoyancy refers to a force that arises from the pressure exerted on an object by a fluid (a liquid or a gas). Since it's a force, we call it the buoyant force.

You can think of the buoyant force as an upward thrust, a force that moves a body in the upward direction. Consequently, it helps counter the downward pull of gravity upon the same object. The buoyant force exists regardless of whether the object is floating or is submerged in the fluid, and the magnitude of the buoyant force is equal to the weight of fluid that's displaced by the object in question.

Types of Buoyancy

If something is positively buoyant, the object floats. That's like our cruise ship. What does this mean? It means that this entire object is less dense than the fluid in which it is located. Now, clearly we know that the metal that makes up our ship is denser than water, and so we'd think it would sink in water. So, maybe you're confused by what I mean when I say that the object is less dense than the fluid in which it is located.

In the case of our cruise ship, the entire ship needs to have a greater ratio of empty space to mass than the water it is sitting on. It is less dense because it has relatively more empty space than mass - that's it. In the case of positive buoyancy, an object's buoyant force is greater than the gravitational force, and so the object floats. And if you were to submerge the object, it would float to the surface. This is like placing a wine cork into water.

A neutrally buoyant object has a buoyant force that's equal to the gravitational force, so the object doesn't sink, nor does it float. That means it stays where it's placed vertically in a fluid medium. Submarines and divers use this concept to stay in the same vertical position within water.

Finally, an object is said to be negatively buoyant if the buoyant force is less than the gravitational force. This means the object will continue to sink, in a downward direction obviously, until it hits a solid surface like the ocean floor.

I need to emphasize the fact that just because a negatively buoyant object within a fluid like water has settled at the bottom, that doesn't mean it has lost the buoyant force. The buoyant force is simply less than the gravitational force. An obvious example of this would be throwing a pebble into a pond. It would sink because it is negatively buoyant, but it still has a buoyant force acting upon it, nonetheless.

Now, you might be wondering why the buoyant force exists in the first place. Why is it an upward force? Remember, fluid exerts pressure upon objects, and the pressure exerted on an object by fluid increases as the depth increases. Ergo, the pressure on the bottom of the object is always larger than on the top of the object. Thus, there is a net upward force on the object within the fluid.

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