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Fluid Friction : Friction Exerted By Liquids And Gases
Before we describe fluid friction, we should know the meaning of the term ‘fluid’. Those substances which are able to flow easily are called fluids. Fluids have no fixed shape. Liquids and gases are fluids (because they can flow easily). The most common liquid around us is water, so water is a fluid. The most common gas (or mixture of gases) around us is air, so air is also a fluid. Thus, water and air are the most common fluids. There is friction whenever an object moves through a fluid. It is called fluid friction.
Air is very light and thin, yet it exerts a frictional force on objects moving through it (which opposes their motion). When an object moves through the air, it pushes the air out of the way and air pushes back on the object. This push of air on the moving object creates friction which tends to slow down the moving object. Thus, air exerts frictional force on cars, buses, aeroplanes, rockets, and birds, etc., moving through it. Similarly, water (and other liquids) exert force of friction on objects which move through them and oppose their motion. When an object moves through water, it pushes the water out of the way and the water pushes back on the object. This push of water on the moving object creates friction which tends to slow down the moving object. Thus, water exerts frictional force on objects like boats, speedboats, ships, submarines and fish, etc., which move through it.
From the above discussion we conclude that air and water exert force of friction on objects moving through them. Since air and water are fluids, so in general we can say that: Fluids exert force of friction on objects moving through them. The frictional force exerted by a fluid (air or water) is called drag (or drag force). Thus, drag is a kind of frictional force exerted by a fluid (like air or water) which opposes the motion of an object through that fluid. Drag force acts in a direction opposite to the direction of motion of the object. So, drag slows down the object moving through fluids and makes speeding up harder. Typical examples of drag forces are the air resistance force experienced by a car or an aeroplane when they move at high speeds, and the water resistance force experienced by a speedboat moving rapidly in the sea.
The magnitude of frictional force (or drag) exerted by a fluid on an object moving through it depends on four factors :
- speed of the object,
- shape of the object,
- size of the object, and
- nature of the fluid (or viscosity of the fluid).
Higher the speed of an object moving through a fluid, greater will be the frictional force (or drag) acting on it. For example, an aeroplane flying at a higher speed of 1000 km/h will face a greater frictional force (or drag) of air than another similar aeroplane which is flying at a lower speed of say, 600 km/h.
The objects having streamlined shapes face much less frictional force (or drag) when moving through a fluid than the objects which do not have streamlined shapes. For example, a car has a streamlined shape (like a wedge) due to which it faces much less frictional force of air (or air drag) while running at high speed. On the other hand, a bus does not have a streamlined shape so it encounters a much greater frictional force (or drag) from air while running at the same speed.
Larger the size of an object moving through a fluid, greater will be the frictional force (or drag) acting on it. For example, a big aeroplane flying at a particular speed will face more frictional force of air (or drag) than a small aeroplane flying at the same speed.
Higher the viscosity (or thickness) of fluid, greater will be the frictional force (or drag) acting on an object moving through it. For example, water is much more viscous (or thick) than air, so there will be much more frictional force (or drag) on an object when it moves through water than when it moves through air.
Disadvantages of Fluid Friction
The main disadvantages of fluid friction are as follows :
- Fluid friction reduces the speed of objects moving through the fluids (by opposing their motion). It makes speeding up harder.
- When objects move through fluids (air or water), they lose some of their energy in overcoming the fluid friction. This decreases their efficiency.
For example, when a car is running on the road, then some of the energy (or petrol) of the car is used up or lost in overcoming the friction of air which opposes its motion. Similarly, when a speedboat rushes through water, then some of its energy (or diesel) is used up or lost in overcoming the friction of water. So, in order to improve speed and to reduce the loss of energy (or fuels), efforts are made to reduce or minimise the fluid friction (or drag).
Before we discuss how fluid friction can be reduced we should know the meaning of the term ‘streamlined’ or ‘streamlined shape’. A ‘body shape’ which offers very little resistance to the flow of air or water around it, is called streamlined (or streamlined shape). A streamlined shape is like a thin wedge (or thin triangular object) lying on its base and sloping upwards gradually.
Method of Reducing Fluid Friction
The fluid friction (or drag) can be reduced or minimised by giving special shape called ‘streamlined shape’ to the objects which move through fluids (like air or water). When an object having a streamlined body shape moves very fast, then the fluid (air or water) can flow past the moving object smoothly, reducing the fluid friction (or drag). For example, cars are built with streamlined body shape to reduce air resistance (or drag) caused by air (see Figure).
A car with streamlined shape moves through the air easily (without facing much air resistance) and hence consumes less petrol than another car of same size running at the same speed that has a shape which gives it more air resistance (or drag). More streamlined the shape of a car, less petrol it will consume.
An aeroplane has a streamlined shape to reduce air friction (air resistance or drag) that it encounters when flying at high speed through the sky. The shape of an aeroplane is similar to that of a bird in flight (see Figure). Both, the aeroplane and the bird have a streamlined body in the middle, two thin wings (one on each side of body), and a tail. The streamlined shape of an aeroplane has been built by scientists
and engineers whereas the streamlined shape of a bird has evolved in nature. In fact, the scientists and engineers got the idea for making streamlined shapes of various moving bodies from the living things in nature (such as birds and fish). The rockets are also built with streamlined shapes so that they encounter the minimum air resistance (or drag) due to air when they fly off at extremely high speeds. From the above discussion we conclude that cars, aeroplanes and rockets are streamlined to reduce friction with the air (air resistance or drag).
Most of the fish have streamlined body shape which helps them to move through water easily without facing much friction (or drag) from water. For example, dolphins are streamlined by nature to reduce friction with water in which they move. The streamlined shape of dolphin helps it to move easily through water.
The objects such as boats, speedboats, ships and submarines which move in water are also built with streamlined body shapes to reduce the frictional force of water (or drag) and make them move easily through water (see Figure). Thus, boats, speedboats, ships and submarines are streamlined to reduce frictional force (or drag) due to water.