9.6. Fluid Friction

You know that air is very light and thin. Yet it exerts frictional force on objects moving through it. Similarly, water and other liquids exert force of friction when objects move through them. In science, the common name of gases and liquids is fluids. So we can say that fluids exert force of friction on objects in motion through them.

Fluid friction refers to the resistance experienced by an object moving through a fluid, which can be either gas (like air) or liquid (like water). Even though fluids are often light and thin, they still exert a force on objects that pass through them. This force arises because of interactions between the fluid and the surface of the object.

Think about riding a bicycle on a calm day versus pedaling against strong winds. The wind (a fluid) offers resistance to your movement, making it harder to pedal forward. This resistance is a form of fluid friction.

The frictional force exerted by fluids is also called drag. The frictional force on an object in a fluid depends on its speed with respect to the fluid. The frictional force also depends on the shape of the object and the nature of the fluid.

The drag force on an object in a fluid is influenced by two main factors. First, the speed of the object relative to the fluid affects drag; faster objects experience more drag. Second, the shape of the object plays a crucial role in determining how much drag it encounters: streamlined shapes reduce drag, while irregular shapes increase it. Lastly, the type of fluid (thick or thin) also affects the amount of drag experienced.

Consider a football compared to a swimming pool noodle. When you throw the football through the air, it travels faster and experiences less drag because of its streamlined shape. However, if you were to push the swimming pool noodle through the water, it would face more resistance and consequently be harder to move due to its shape.

It is obvious that when objects move through fluids, they have to overcome friction acting on them. In this process they lose energy. Efforts are, therefore, made to minimise friction. So, objects are given special shapes. Where do you think scientists get hints for these special shapes? From nature, of course.

When objects move through fluids, they expend energy to overcome the frictional forces acting against them. To reduce this energy loss, engineers and scientists design objects with shapes that minimize drag. Nature provides inspiration for these designs: many animals have evolved streamlined bodies to move efficiently through air and water.

Think about how fish are shaped: they have sleek, streamlined bodies that allow them to swim efficiently through water. Engineers often mimic this design in submarines and other aquatic vehicles to enhance their performance and minimize energy consumption.

Birds and fishes have to move about in fluids all the time. Their bodies must have evolved to shapes which would make them lose less energy in overcoming friction. Look carefully at the shape of an aeroplane. Do you find any similarity in its shape and that of a bird? In fact, all vehicles are designed to have shapes which reduce fluid friction.

The shapes of airplanes, cars, and boats are often influenced by the forms found in nature, particularly those of birds and fish. These natural shapes are optimized for movement in their respective fluids—air and water. By studying these forms, designers can reduce drag and increase efficiency for human-made vehicles.

Consider how a bird, such as a swallow, is shaped to cut through the air efficiently. Its wings and body are curved and smooth, allowing it to fly with minimal resistance. Designers of airplanes aspire to replicate this efficiency to enhance flight performance, which is why a plane's wings are often similar to bird wings.

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