# What is Fluid Friction? - Definition, Equation & Example

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• 0:03 What Is a Fluid?
• 0:58 What Is Fluid Friction?
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Lesson Transcript
Instructor: Stephanie Kubik

Stephanie has worked as a university teaching assistant and has a master's degree in mechanical engineering.

Fluid friction affects your everyday life from trying to squeeze honey into your tea to dictating how far you can drive on a tank of gas. Learn the difference between internal and external friction and what role each plays when working with a fluid.

## What Is a fluid?

Have you ever wondered why it's easier to squeeze a tube of toothpaste than it is to squeeze honey into your tea? Have you ever put your hand out the window while driving and felt the wind resistance pushing back on your hand? Both relate to fluid friction.

Let's first get a grasp on fluids. A fluid, in contrast to a solid, is a material with no fixed shape that constantly deforms when acted upon by an outside force. You probably think of things like water and coffee as fluids; however, gases, such as air, and substances like motor oil are also fluids. The property that causes them to seem very different from each other is viscosity. Viscosity is the measure of a fluid's resistance to flow because of its internal friction. This is why honey is much harder to squeeze out of the bottle than ketchup. It also describes why it is more difficult to move in a swimming pool than it is on dry land.

## What Is Fluid Friction?

Fluid friction is the force that resists motion either within the fluid itself or of another medium moving through the fluid. There is internal friction, which is a result of the interactions between molecules of the fluid, and there is external friction, which refers to how a fluid interacts with other matter.

### Internal Friction

Let's first take a look at internal fluid friction. To the naked eye, a fluid is a continuous medium. If you look at a fluid under a high powered microscope, however, you'd see that fluids are actually made up of molecules separated by a considerable amount of empty space. To deform a fluid (e.g., squeeze honey through a small hole), the molecules need to move relative to each other by squeezing past or displacing one another. This is internal friction and is what prevents a fluid from flowing. The more internal friction, the harder it is to get the molecules to move and the harder it is to force the fluid to deform. But internal friction isn't always a negative consequence. For instance, without internal friction, you would not be able to drink through a straw because the fluid would not be cohesive enough to enable it to be transported in such a manner.

The most common illustration of internal friction is that for Couette Flow. This is when a viscous fluid is sandwiched between two flat plates. The bottom plate is stationary, while the top plate moves.

#### Equation

The force needed to overcome fluid friction and move the top plate at a velocity vo is:

The fluid closest to the plate moves as fast as the plate assuming a no-slip condition at the plate's surface. The fluid farther away from the moving plate has a lower velocity because of the fluid friction.

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