1. Friction-
ü Friction is a force that comes into play when two surfaces come in contact with each other.
ü The Frictional force acts in the opposite direction of the motion of the body.
ü The force of Friction opposes the relative motion between the surfaces of two moving objects.
2. Factors That Can Affect Friction-
ü Roughness Of a Surface- If we move an object with has an irregular or rough surface on another surface which is also irregular, the force of Friction will be high in this case and the movement of the object would be restricted. If the surfaces of either the objects or are smooth, the force of Friction would be less and the object would move easily over the surface.
ü If Two Surfaces Are Pressed Over One Another- The force of Friction increases between two surfaces if they are pressed hard and hence the movement of the object becomes restricted. Although, if there is no pressure the object can easily move.
3. Types Of Friction-
ü Static Friction- The Frictional force that comes into play until an object starts moving is called static Friction. An object has to overcome the static Friction force in order to start its movement.
ü Sliding Friction- Sliding Friction comes into play whenever an object moves along the surface of another object. Such a movement is called ‘slide’. Hence, sliding Friction is the force that opposes the movement or slide of an object.
ü Rolling Friction- When an object is rolling on a surface the force of Friction which acts upon it is called rolling Friction.
ü Fluid Friction- When an object moves in a fluid, the fluid exerts a fluid Friction upon the object. It is also called air Friction (when the medium of travel is air) and viscous Friction (when the medium of travel is water).
Rolling Friction < Sliding Friction < Static Friction
Q. How circular wheels reduce Friction?
Ans.- We are aware that when an object rolls over a surface rolling Friction is produced which resists its motion on the surface. Although, its magnitude is lesser than the static and sliding Friction. Wheels are circulating objects that turn around a center point. The wheels allow an object to roll over a surface rather than allowing them to slide over the surface. This results in less Friction and hence easy movement of the object. Rolling Friction is also used in the parts of machines that are moving continuously. In this way, the sliding Friction is removed with the help of rolling Friction and therefore the adverse effects of Friction such as the production of heat or wear and tear of the mechanical parts are also reduced. For say, ball bearings are being used in fans and bicycles to reduce Friction.
4. Applications Of Frictional Force-
ü A moving object would never be able to come to the state of rest without the Frictional force.
ü It allows us to walk on the earth surface.
ü It allows us to write with a pen on a surface or a paper.
ü It would not be possible to drive any automobiles on the road without the Friction force.
ü It would not be possible to construct any buildings without the Frictional force.
ü It allows us to fix a nail in the wall.
5. Demerits Of Frictional Force-
ü Frictional force results in wear and tear of objects such as the moving parts of a machine, the tyres of a vehicle, sole of the shoes etc.
ü It also results in the production of heat. In the case of machines, the production of heat leads to wastage of energy.
ü It leads to a decrease in the speed of a moving object or some time stops it.
ü It can lead to noise pollution in certain cases. For say, aircrafts produce loud sound due to the resistance of the air.
6. Methods To Reduce Or Increase Friction-
ü Increasing Friction-
n Sometimes we need to increase Friction so as to avoid slipping of objects.
n For example, the sole of the shoes is grooved so that the Friction between our feet and ground increases and we can walk safely.
n The tyres are treaded so that they can have a better grip over the ground and allow the smooth movement of the vehicles.
n Brake pads are used in bikes to stop them suddenly from moving by increasing the amount of Friction.
n Kabaddi players rub their hands with soil which helps in increasing the Friction between the hands and allows them to have an easy grip of the opponent.
n Gymnasts also apply a coarse substance so that they can have a better grip due to increased Friction in their hands.
ü Reducing Friction-
n Sometimes in order to have a smooth movement of an object we need to reduce Friction.
n For example, the powder is sprinkled over the carrom board to decrease the Friction between the board’s surface and the striker. In this way, the surface of the carrom board becomes smooth.
n Grease is used in bicycles and other motors or different parts of a machine to reduce Friction and increase their efficiency.
n Oil is applied on the hinges of the door so that they can move easily.
Note: Applying substances like oil, grease or powder allows in smooth movement as they block the irregularities of a surface. The substances that can reduce the amount of Friction between different objects are called lubricants. We can never reduce Friction to a level of zero. There are always some irregularities present on the surface.
7. Fluid Friction-
ü Air, water and other liquids collectively are called fluids.
ü They also exert a Frictional force on the object that travels through them.
ü Such a Friction is called fluid friction or drag.
ü The amount of Frictional force by a fluid on an object depends upon the following-
n The speed of the object with respect to the fluid.
n The nature of the fluid, its viscosity or its tendency to resist the flow.
n The shape of the object.
8. Fluid Friction- Internal And External Friction-
ü Fluids comprise of internal Friction that occurs between the molecules of a liquid as they are also in motion. The viscosity of a liquid, therefore, depends upon the internal Friction.
ü An external Friction in fluids is the force exerted by the fluid on the object that flows through it. As an object travels in a fluid it loses its energy in order to overcome the fluid Friction. Hence, things that can easily flow in fluids have special shapes.
ü For example, ships and boats have a particular shape that allows them to resist fluid Friction.
ü Birds and airplanes have a specific shape that allows them to fly in the air.
ü Fishes also have a specific shape such as a tail and fins that allow them to swim continuously in water and even prevent the loss of energy due to fluid Friction.
ü Not only this, all the vehicles are designed in a way that they can resist the fluid Friction because air is present everywhere.
NCERT PRACTICE QUESTIONS
1. Fill in the blanks.
(a) Friction opposes the _____________ between the surfaces in contact with each other.
(b) Friction depends on the _____________ of surfaces.
(c) Friction produces __________.
(d) Sprinkling of powder on the carom board ________ friction.
(e) Sliding friction is ___________ than the static friction.
Ans.- (a) motion (b) nature (c) heat. (d) reduces (e) lesser
2. Four children were asked to arrange forces due to rolling, static and sliding frictions in a decreasing order. Their arrangements are given below. Choose the correct arrangement.
(a) rolling, static, sliding (b) rolling, sliding, static (c) static, sliding, rolling (d) sliding, static, rolling
Ans.- (c) static, sliding, rolling
3. Alida runs her toy car on dry marble floor, wet marble floor, newspaper and towel spread on the floor. The force of friction acting on the car on different surfaces in increasing order will be
(a) wet marble floor, dry marble floor, newspaper and towel.
(b) newspaper, towel, dry marble floor, wet marble floor.
(c) towel, newspaper, dry marble floor, wet marble floor.
(d) wet marble floor, dry marble floor, towel, newspaper
Ans.- (a) wet marble floor, dry marble floor, newspaper and towel.
4. Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of frictional force acting on it.
Ans.- When book slides down on the desk, a frictional force acts between the book and the surface of the desk. The direction of the friction force on the book is opposite to the direction of its motion and acts in an upward direction. It is shown in the diagram below.
5. You spill a bucket of soapy water on a marble floor accidently. Would it make it easier or more difficult for you to walk on the floor? Why?
Ans.- It is possible to walk on the floor because of the friction present between our feet and the ground. For walking, we push the ground in a backward direction with our feet. The force of friction pushes it in the forward direction and allows us to walk. The force of friction decreases between the ground and the feet when there is soapy water split on the floor. Hence, it becomes difficult to walk on the soapy floor.
6. Explain why sportsmen use shoes with spikes.
Ans.- Sportsmen use shoes with spikes because of the better grip given by spikes while running. This is because the force of friction between the shoes and the ground increases with the help of spikes.
7. Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Who will have to apply a larger force and why?
Ans.- Due to the interlocking of the irregularities on the two surfaces in contact, the force of friction arises. On the floor when a heavy object is placed, the interlocking of irregularities on the surface of the box and floor become strong. This is because the two surfaces in contact are pressed harder. Hence, more force is required to overcome the interlocking. Thus, to push the heavier box, Seema has to apply greater force than Iqbal.
8. Explain why sliding friction is less than static friction
Ans.- When irregularities present in the surfaces of two objects in contact get interlocked with each other, friction come into play. The time given in sliding for interlocking is very small. Thus, interlocking is not strong. Therefore, less force is required to overcome this interlocking. Due to this reason, sliding friction is less than static friction.
9. Give examples to show that friction is both a friend and a foe.
Ans.- Advantages of the friction- a) Due to friction, we are able to walk. b) We are able to write because of the friction between the tip of the pen and paper.
Disadvantages of friction- a) Because of friction, the tires and soles of shoes wear out. b) Friction produces heat between different parts of the machines. This can damage the machines.
10. Explain why objects moving in fluids must have special shapes.
Ans.- When a body moves through a fluid, it experiences an opposing force which tries to oppose its motion through the fluid. This opposing force is known as the drag force. This frictional force depends on the shape of the body. By giving the objects a special shape, the force of friction acting on it can be minimized. Hence, it becomes easier for a body to move through the fluid.
Junior Science Topic - Friction. Chapter notes, video lectures, power point presentation and NCERT exercise solution. (Class 8, CBSE syllabus, chapter 12)
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