Fluid Friction

Fluid Friction - Types and Factors of Fluid Friction

Fluid is a substance that can flow and take the shape of its container; either liquid or gas. Friction is a force that withstands the movement of a sliding object.

A fluid flow, e.g. in a tube, not as solid ‘plug’, but with more or less complicated internal motion with continuously changing velocities at all points of reference in the fluid. Resistance to flow stems from attractive forces between the molecules, and therefore work is required if molecules are to be separated, as in a flowing liquid. For many of the molecules in a macroscopic fluid sample, it would be impossible for calculating this work on the molecular level. The friction of any kind, and particularly the internal forces that make a fluid resist motion: the observed phenomenon is reduced to a parameter that we will examine below: viscosity; the resistance to motion by the molecular forces in a fluid is referred to as fluid friction.


Friction is the force that withstands motion between any surfaces that are coming in contact with each other.

The types of friction are: Sliding, Static, Rolling, and Fluid friction.

Sliding, static and rolling friction occurs among solid surfaces. Fluid friction, on the other hand, occurs in liquids and gases.

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

 All the real fluids due to two effects have intrinsic friction:
  • • The attraction between fluid molecules (viscosity), and the transfer of momentum from fluid molecules that bounce off objects (high-speed drag).

  • Did you know?

    Fluid friction is used mostly in the water parks so we can slide smoothly and prompt down giant slides.

    Let us now consider three situations in which fluid-friction is important:

    1. The flow of fluid in a circular pipe,
    2. Object’s slow linear motion through a fluid.
    3. Levitation by an air jet.

    The first two situations are examples of viscous drag, while the third is that of high-speed drag.

    The factors fluid fiction depends on are:

  • 1. Nature of the fluid: The thicker the fluid, the higher the Fluid Friction. For example Honey is thicker than water; hence there is more friction in Honey.

  • 2. The shape of the body: The body of an object which has a streamlined shape, shape which begins and ends in points like the shape of a fish or an airplane, feels comparatively less dragging than other shapes and can cut through the fluid effortlessly. For Example, A car has less drag than a bus because of its streamlined shape.

  • 3. Amount of area faced by the body in the fluid: The area which faces the fluid should be as less as possible and for this, the body should not move vertically but it should move horizontally. This will put more pressure on the fluid and hence the body can cut through the fluid and move easily.

  • 4. The viscosity of the fluid: The viscosity of the fluid is the resistance exerted by the fluid on a body as it tries to locomote. Fluids with more viscosity will offer more drag than fluids with less viscosity.

  • 5. The speed of the body: Decreasing the speed of the body will decrease the drag and vice versa. For example, a vehicle traveling at 100MPH will have more drag than that of a vehicle traveling at 50MPH.

  • 6. The Fluid Friction of liquids increases with increase in pressure. But the Fluid Friction of water decreases (anomalous behavior) whereas the viscosity of these gases remains unchanged.

  • Viscous Drag

    When fluid flows in laminar flow around an obstacle, it exerts a viscous drag on the obstacle. Frictional forces push the fluid backward (against the direction of flow) and the obstacle forward (in the direction of flow). The viscous drag force becomes high linearly with the speed of the fluid. Drag depends on the properties of the fluid and on the shape, size, and speed of the object. This can be expressed by means of the drag equation:

    Drag is the aerodynamic force that holds back an aircraft's motion through the air. It is generated by every part of the airplane, even by the engines!

    It is a mechanical force which is generated by the interaction and contact of a solid body with a fluid (liquid or gas). It is not generated by a force field, in the sense of an electromagnetic field or a gravitational field, where one object can affect another object without being in physical contact. If drag has to be generated, the solid body must be in contact with the fluid. If there is no fluid, there can be no drag, it is generated by the difference in velocity between the solid object and the fluid. A motion must be present between the object and the fluid. If there is no motion, drag cannot be created. It makes no difference if the object moves through a static fluid or the fluid moves past a static solid object.

    Drag is a force, hence a vector quantity having both a magnitude and a direction. It acts in a direction that is against the motion of the aircraft. Lift is perpendicular to the motion. There are various factors which affect the magnitude of the drag. These factors also affect lift but there are some factors that are unique to aircraft drag.

    We can also see drag aerodynamic resistance to the motion of the object through the fluid. This source of drag depends on the shape of the aircraft and is known as form drag. As air flows around a body, the local pressure and velocity are changed. Pressure is a measure of the momentum of the gas molecules and a change in momentum produces a force, hence a varying pressure distribution will create a force on the body. The magnitude of the force can be determined by integrating (or adding up) the local pressure times the surface area around the entire body. The drag is the component of the aerodynamic force that is opposed to the motion; lift is the component which is perpendicular to the motion. Both the drag and lift forces act through the center of pressure of the object.

    The Reynolds Number

    The relative importance of viscous drag and high-speed drag is characterized by the
    (Dimensionless) Reynolds number NR, which is their ratio

    In this, we have identified Δv, the change in velocity relevant to viscosity, with the relative velocity ‘v’ between the object and the fluid. We also replace Δy by L, representing a characteristic length of the object perpendicular to the direction of fluid flow.
    Conditions with Reynolds number less than one are dominated by viscous drag, while for NR> 1 high-speed drag is more important. We see that the Reynolds number increases with velocity, justifying the name ‘high-speed drag’ for fluid friction at large NR.

    Do it yourself!
    You will require:
    Honey - 30 to 40 mL
    A steel ball of around 4 - 5 centimeters in diameter.
    And a ball container is proportionate to the ball.

    Take a steel ball of around 4 to 5 centimeters in diameter and a spherical plastic ball container proportionate to this steel ball (of around 6 to 7 centimeters). Now fill this ball container with around 40 mL of honey and drop the steel ball in it. Now close the ball container and seal it tight.

    This happens due to the viscosity of the honey in the container and the friction and drag created by it.

    You can repeat the same thing with water and then air and slide the ball over a slope and see what happens.

    Share what you find with your friends and show the magic of slow-motion ball you've created!

    Advantages of Fluid Friction:

    1. Flotation of a ship, Hover crafts, hot air balloons etc., which helps in transportation.

    Disadvantages of Fluid Friction:

    1. Fluid Friction decreases the speed of objects moving through the fluids.
    2. It makes speeding up harder.
    3. When objects, move through fluids, they lose some energy in overcoming the fluid friction, this decreases their efficiency.

    1. Friction is a force that acts in the ______ direction of the intended motion of an object.
    i. Same
    ii. Parallel
    iii. Opposite
    iv. All of the Above

    2. Air resistance on a flying airplane is an example of what type of friction?
    i. Static Fiction
    ii. Fluid Fiction
    iii. Rolling Fiction
    iv. None of the Above

    3. In what way does friction help us in our everyday life?
    i. Brakes on a car
    ii. Climbing a hill
    iii. Standing up
    iv. All of the above