Fluid Friction

Question 1 What are fluids. Give example?

Question 2 What is fluid friction?

Question 3 What is the special name of frictional force exerted by fluids?

Question 4 What is a streamlined shape?

Question 5 Explain why a speedboat has a streamlined shape?

Question 6 Why are cars, aeroplanes and rockets streamlines?

Question 7 Give two examples of drag force?

Question 8 How can you reduce the drag on something moving through the air?

Question 9 On what factors does drag force depends?

Question 10 State few disadvantages of fluid friction?

Those substances which are able to flow easily are called Fluids. Water and air are the most common fluids.

Fluid Friction

There is a friction whenever an object moves through a fluid. It is called fluid friction.

Air is very light and thin, yet it exerts frictional force on objects moving through it. 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 fiction which tends to slow down the moving object. Air exerts frictional force on cars, buses, aeroplanes, rockets and birds moving through it. Water exerts force of friction on object which move through them and opposite 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 fiction which tends to slow down the moving object. Water exerts frictional force on objects like boats, speed boats, ships, submarines and fishes which Move through it.

The frictional force exerted by a fluid is called drag.

Drag is a kind of frictional force exerted by fluid which opposes the motion of an object through that fluid.

Drag force acts in direction opposite to the direction of motion of the object.

 

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

(1) speed of the object

(2) shape of the object

(3) size of the object

(4) nature of the fluid or viscosity of the fluid

(1) Higher the speed of an object moving to a fluid, greater will be the frictional force acting on it .

For example An aeroplane flying at the highest speed of 1000 km/hr will face a greater fictional force or drag of air than another similar aeroplane which is flying at a speed of 600 km/hr.

(2) The objects having streamlined shape face much less frictional force or drag when moving through a fluid than the objects which do not have streamlined shape.

For example A car has a streamlined shape due to which it faces much less frictional force of air while running at high speed. A bus does not have a streamlined shape so it encounters a much greater frictional force from air while running at the same speed.

(3) 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 than a small aeroplane flying at the same speed.

(4) 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 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:

(1) It reduces the speed of objects moving through the fluids. It makes speeding up harder.

(2) When objects move through fluids, they lose some of their energy in overcoming the fluid friction. This decreases their efficiency.

When a car is running on the road, then some of the energy of the car is used up or lost in overcoming this friction of air which opposes its motion. When a speed boat rushes through water, then some of its energy is used up or lost in overcoming the friction of water.

A body shape which offers very little resistance to the flow of air or water around it is called streamlined shape.

A streamlined shape is like thin wedge or triangular objects lying on its base and sloping upwards gradually.

Methods of reducing Fluid Friction

The fluid friction or drag can be reduced are 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 can flow past the moving objects smoothly, reducing the fluid friction or drag.

For example:

(1) Cars are built with streamlined body shape to reduce air resistance or drag caused by a air.A car with streamlined shape moves through the air easily and hence consume 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.

(2) An aeroplane has a streamlined shape to reduce air friction that it encounters when flying at high speed through the sky.

(3) The rockets are also build with streamlined shape so that they encounter the minimum air resistance or drag due to air when they fly off at extremely high speed.

(4) Most of the fishes have streamlined body shape which help them to move through water easily without facing much friction from water.

(5) Dolphins are streamlined by nature to reduce friction with water in which they move.

(6) The objects such as boats, speed boats, ships and submarines which move in water are also build with a streamlined body shapes to reduce the frictional force of water and make the move easily through water.