by Ron Kurtus
When a fluid is in contact with a solid object or another fluid, the force resisting motion between them is called fluid friction.
A fluid is defined as a substance that has no fixed shape and yields easily to external pressure. Liquids and gases are considered fluids.
(See Characteristics of Fluids for more information.)
Situations where fluid friction may occur include a fluid moving over an solid object, a solid object moving over a fluid, lubrication of two solid objects by a fluid layer, a fluid moving over a different fluid, and internal fluid friction.
Questions you may have include:
- What is fluid motion against a solid object?
- What is lubrication?
- What is fluid-fluid motion?
This lesson will answer those questions. Useful tool: Units Conversion
Fluid motion against solid
A fluid can move over a solid object, the object can move over a fluid, or the solid object can move within the fluid.
Fluid moving over solid object
One form of fluid friction is when a fluid is moving over a the surface of a solid object.
For example, water moving along the ground is held back slightly due to fluid friction. This resistance is also seen with water running through a hose.
Fluid moving over a solid surface exhibits fluid friction
The pressure from the fluid times a unit area equals the normal force between the fluid and the solid.
Solid object moving over a fluid
When an object is moving along the surface of a fluid, there is a friction resistance holding back the motion. Most objects will sink somewhat in the fluid, such that the object is plowing through the fluid.
This can be seen when a boat skims across the water. There is both fluid friction on the bottom of the boat and water resistance as it plows through the water.
Object moving over a fluid held back by fluid friction
Object moving inside fluid
When a solid object is moving inside a fluid—such as water or air—there is a fluid resistance as the object tries to plow through the fluid. This resistance is a combination of resistance due to the shape of the object and surface fluid friction.
Object moving inside a fluid
Examples include air resistance on an airplane and water resistance on a submarine. There is also some water resistance effect on a boat moving along the surface of some body of water.
When a layer of fluid is between two solid objects, the effect of lubrication takes place. Lubrication can greatly reduce the sliding friction between the objects. The lubricating fluid can vary between a thin coat of water to a layer of grease.
Layer of fluid lubricates motion of objects
A fluid can move over a different fluid or can have internal motion within a given fluid.
Fluid moving over a different fluid
An example of a fluid moving over a different fluid is running water over a layer of grease or oil. The friction between the two materials holds back the motion. The weight of the top fluid makes up the normal force between the materials.
Internal fluid motion
There can be internal friction between layers in a fluid. One example is in the flow of different temperature areas in the ocean. This topic is usually considered under the subject of fluid dynamics and not in friction.
Internal friction can occur in a fluid
Examples of fluid friction include a fluid moving over an solid object, a solid object moving over a fluid, lubrication of two solid objects by a fluid layer, and internal fluid friction.
Be fluid in your ability to change
Resources and references
Friction Resources - Extensive list
Friction Concepts - HyperPhysics
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