TS 8th Class Physical Science Study Material 2nd Lesson Friction

TS Board Telangana SCERT Class 8 Physics Solutions 2nd Lesson Friction Textbook Questions and Answers.

TS 8th Class Physics 2nd Lesson Questions and Answers – Friction

Reflections on concepts

Question 1.
Explain the types of fractions with suitable examples?
Answer:
I know four kinds of frictions. They are Static Friction: When a force is applied to an object hut it does not cause it to move. Example: pushing on a wall.
Fluid Friction: Occurs when a object moves through a fluid, meaning either a liquid or gas. Example: sky diving, swimming.
Sliding Friction: Occurs when solid surfaces slide over each other.
Example: falling on the pavement.
Rolling Friction: Occurs when an object rolls over another (something with wheels or that is circular like a ball)
Example: riding a motorcycle.

Question 2.
A book placed on the surate of table. The book is pushed in one direction. Draw the forces acting on the book and explain.
Answer:

1. A book is placed on a table.
2. It is slowly pushed.
3. The book gets some speed in the direction of the push.
4. The book slowly decreases between the table finally stops. rushing the book.
5. Here the frictional force between the table and the book opposes the motion of the book.
6. The book finally stops because of this frictional force.
7. The weight of the book balances with normal reaction. Therefore there is no vertical moment for the book.
   W-N=O
8. Along Horizontal direction frictional force opposes the pushing force. The Net resultant force between these to make the book to move.

Here W = Weight of the book
N Normal reaction
F = Pushing force
f = Frictional force
Here W = N
No vertical motion for this book.
The Net force with which book moves is Net Force = F – f

Question 3.
Give few examples for sliding.
Answer:
Examples for sliding friction:
1. Skating on the snow
2. Stop the vehicles by applying brakes because of sliding friction between road and tyres of the vehicles.
3. We are able to write with pencil on a paper or with a chalk on blackboard because of sliding friction.

Question 4.
Explain how does lubrication reduce friction.
Answer:
When oil or grease (lubricant) is applied between the moving parts of a machine, a thin layer is formed between the moving surfaces and hence they do not directly rub against each other. The interlocking of irregularities is avoided to a great extent. Hence friction decreases and movement becomes smooth.

Question 5.
Explain with one example that Frictional force ¡s proportional to the Normal force.
Answer:

1. Connect a brick to Spring balance and measure the frictional force. It is nothing but reading of the Spring balance.
2. Now add another brick to the first one.
3. That means weight of the body is increased.
4. We can observe that the frictional force also increases.
5. Since we know that weight and normal reaction are equal. We can say that Frictional Force ∝ Normal Force.

Application of concepts

Question 1.
Explain why do sportsmen use shoes with spikes.
Answer:
Sportsmen use shoes with spikes. Because spikes provide the shoes better grip on the ground.

Question 2.
Would it be easier or difficult for you to walk on a marble floor with soapy water. Why?
Answer:
It is very difficult to walk on a soapy floor because soapy floor creates very less friction comparing with normal floor. So we can slip on such soapy floors.

Question 3.
What happens if we do not reduce friction in machines?
Answer:

1. In machine parts friction must he less for the free movement of them.
2. In order to reduce friction we need to apply some lubricating oil. Eg: Lubricating oil is applied in the motors, rollers, cycle pedals etc.
3. If we do not reduce the friction the wear and tear of the machine parts comes quickly and finally they will not move because of friction.

Question 4.
What purposes are surved by using bearings in machines? Explain with daily life examples.
Answer:

1. Bearings are used in Fans, Cycle Pedals, Skaters, etc.
2. Since the Bearings are spherical there is only rolling friction which is very less.
3. When we use these bearing the movement of the wheels are very free.
4. When we do not use these bearings it will be very difficult for the wheels to move.
5. Nowadays in sewing machines, pedestal fans, sofa sets also they are attaching wheels at the bottom so that we can move them freely. The wheels at the bottom are fixed with ball bearings.
6. It is always easier to pull the luggage fitted with rollers.

Question 5.
Draw a free-body diagram (FBD) to show various forces acting on a body which is sliding on an inclined plane.
Answer:

N – normal force
W – weight of the body
f – friction force

higher Order Thinking Questions

Question 1.
Do you agree with the statement Hfriction is both good and evil?’ Explain with examples.
Answer:
Friction is both good and evil.
I. Friction is good :

• Friction allows us to walk or run without slipping.
• Friction allows vehicles to travel on the road safely.
• To slow down or stop the vehicle we use brakes which use friction between road and tires of vehicles.
• Friction enables us to hold things with our hands.
• Friction helps us to sharpen a knife or other instruments.
• Friction helps us to fix a nail in the call.
• No building can be constructed if there is no friction.

II. Friction as an evil :

• Friction causes wearing and tearing the surfaces which are in contact.
• Friction causes worn-out the tires, which are dangerous because they can slide and skid easily.
• Friction produces heat in machine which can damage some parts of the machine.
• Friction also produces static electricity which is dangerous.
• Friction also causes wasting of energy and fuels.

Question 2.
Reducing friction to the lowest possible level in machine tools solves the problem of energy crisis and conserves biodiversity. How do you support the statement? Explain.
Answer:
If friction is more in machine tools, more fuel is consumed. This led to the wastage of fuel and causes energy crisis. If energy crisis formed, biodiversity in nature was damaged. So, reducing friction to the lowest possible level in machine tools solves the problem of energy crisis and saves the biodiversity.

Question 3.
Explain why sliding friction ¡s less than static friction.
Answer:
The interlocking of irregularities in the two surfaces which in contact causes friction force. When the object starts sliding the contact points on its surface do not get enough time to lock into the contact points on the floor (or other objects). So sliding friction is less than static friction.

Question 4.
Scientists designed the shape of aeroplane using the technique of reducing fluid friction by observing birds’ shape. How would you appreciate it?
Answer:

1. It is obvious that when objects move through fluids, the have to overcome friction acting on them.
2. In order to minimize the friction objects are given special shapes.
3. Scientists after observing the birds and fishes they thought of designing the aeroplanes in these shapes.
4. Birds and fishes have to move about in fluids all the time.
5. Their bodies must have evolved to shapes which would make them not to lose much energy in overcoming the friction.
6. The aeroplanes are made in this ship because they experience more drag and move freely.
7. They consume less fuel to fly in air, so as birds use less energy in overcoming the fluid frictional force.

Multiple Choice Questions

Question 1.
Which material do gymnasts apply on their hands to increase friction for better grip [ ]
a) Oils
b) Soap
c) Coarse substance
d) water
Answer:
c) Coarse substance

Question 2.
The relation between the change in the length of spring and the applied force is [ ]
a) Inversely proportional
b) Equal
c) Directly proportional
d) Never depends on the force
Answer:
c) Directly proportional

Question 3.
The bodies of birds and fishes must have evolved to shapes which would make them not to loose much energy in overcoming [ ]
a) Rolling friction
b) Static friction
c) Sliding friction
d) Fluid friction
Answer:
c) Sliding friction

Question 4.
Which of the following friction has self adjusting force [ ]
a) Sliding friction
b) Static friction
c) Rolling friction
d) Fluid friction
Answer:
b) Static friction

Suggested Experiments

Question 1.
Conduct an experiment to understand the nature of fiction and the concept of Static friction.
Answer:
Nature of friction and concept of static friction:
The faite which opposes the relative motion altivo surfaces of bodies in contact is called frictional force.
Static friction is the friction which comes into play when surfaces of the objects are at rest relative to each other.

Eg:

1. Push a heavy box which is kept on a floor with small force.
2. The box does not move because frictional force is equal to the applied force.
3. Even if applied force is increased the box still does not move, because the frictional force also increases which balances with applied force. So we can conclude that static frictional force is self-adjusting force.
4. After crossing the limit of static friction if the applied force is increased the box starts to move. Here the applied force crossed the limit of static friction.

Question 2.
Conduct an experiment to find out the effect of roughness on frictional force.
Answer:

1. A surface is said to be smooth surface if there are no irregularities.
2. A surface is said to be rough surface if there are some irregularities.
3. Lake an inclined smooth plane
4. Allow a pencil or pen or dIl other rolling body to roll down on it.
5. Find the distance travelled  the pencil on it
6. Repeat the same experiment on rough inclined plane
7. once again find the distance travelled by the object on rough inclined plane
8. It is observed that the distance travelled by the object on smooth inclined plane is more than on rough inclined plane
9. so we can conclude that the irregularities on a surface increases the frictional force which opposes the motion of the body.

Suggested Projects

Question 1.
Collect information about various new techniques being adopted by human beings to reduce energy losses due to friction. Prepare a report on it.
Answer:
Methods of Reducing Friction: there are a number of methods to reduce friction in which some are discussed here.
Use of Lubricants : (Cycle and door): The parts of machines which are moving oven one another must be properly lubricated by using oils and lubricants of suitable viscosity.

Use of Ball Bearings: In machines where possible, sliding friction can he replaced by rolling friction by using ball hearings.
Design Modification: Friction can he reduced by changing the design of fast-moving objects. The front of vehicles and airplanes made oblong to minimize friction.

Question 2.
Collect information about the substances using to reduce the friction in different machines and prepare a report on it.
Answer:
Methods of Minimising Friction:
Polishing: Friction in machines decreases by means of polishing the machine parts moderately.
Using Ball bearings: By using Ball bearings in rolling parts of the machines the frictional force decreases.
Using Lubricating Oils: Using Lubricating Oils like ‘2T’ oil, machine oil, etc. we can reduce the frictional force in machine parts. The machine parts will more freely.
Aerodynamic Model: Automobiles are manufactured in aerodynamic shape to reduce the friction due to fluid.

TS 8th Class Physics 2nd Lesson Friction Intext Questions

Think and discuss

Question 1.
Does friction oppose motion or relative motion of surfaces in contact? Discuss. (Tnt. P. No. 22) (Conceptual understanding)
Answer:
Relative motion

Question 2.
What observations and experiments can you cite to show the existence of friction? (Field investigation/ Experiments/Daily life applications)
Answer:
The following observations and experiments show the existence of friction.

• Fan comes to rest after switched off because of friction.
• Heat is produced, when two hands rubbed vigorou%Iv, because of friction.
• Marble from a inclined plane comes to stop after some distance on the horizontal floor because of friction.
• Wearing and tearing of machinery parts and shoes was because of friction.

Question 3.
When do we speak of sliding friction? (Text. P. No. 23) (Making hypothesis)
Answer:
Sliding friction comes into p1a when an object is sliding over another.

Question 4.
Does friction act on a table resting on the floor? (Text. P. No. 25) (Making hypothesis/ Field investigation)
Answer:
No. friction is not acting on a table standing on a floor. When a force is applied on table to move it then friction comes into play. The force applied is balanced by the force of friction.

Question 5.
If normal force is doubled, what happens to friction? Discuss. (laking hypothesis/ Conceptual understanding)
Answer:
Frictional force is proportional to the normal force. i.e., Friction u Normal force. So, if normal force is doubled, then friction ∝ also doubled.

Question 6.
Your friend says “it depends on area of contact’. How can you correct your friend through some experiments? (Communication through drawing, model making)
Answer:
I will correct my friend through the following experiment.

• Tie a string around a brick. Put the brick in horizontal position.
• Pull the brick by using a spring balance.
• Note down the reading of spring balance when brick just begins to move.

• Now turn the brick upright (less area) so that the contact area with the floor becomes small.
• Do the same experiment and measure the friction using spring balance reading.

• The frictional force is same in both cases and irrespective of area of contact of the bodies.
• So the frictional force is independent of the area of contact.

Question 7.
“Fiction is independent of weight, but depends on normal force between surfaces of contact where friction exists”. Do you agree with this statement? Discuss. (Conceptual understanding)
Answer:
Yes. As we increase the weight or applied a force pressing vertically, normal force increases. As normal force increases, frictional force also increases.

Question 8.
What important role does the friction play in the life of human beings and animals? (Text. P. No. 26) (Daily life applications/Field investigations)
Answer:

• Friction helps for human beings and animals for safe walking on the floor or ground.
• Friction is helpful to hold objects.
• Friction is helpful for animals for climbing trees and rocks.

In this way friction has played important role in the life of human beings and animals for getting food, walking and also for survival.

Question 9.
Why Is friction important for transport? (Daily life applications)
Answer:
Friction is important for transport because vehicle moves safely on roads due to friction. Tires of vehicles grooved. Grooves increase friction. This helps easy rolling of vehicles on the road. Boats and ships are also able to move through water because of friction. From these we conclude that friction is important for transport.

Question 10.
Can we reduce friction to zero? Explain. (Text. P. No. 28) (Making hypothesis /Conceptual understanding)
Answer:
Friction can never be entirely eliminated, No surface is perfectly smooth. Some irregularities are always there.

Question 11.
What purposes are served by bearings in machines? Explain with daily life situations. (Intonation skills/Conceptual understanding)
Answer:
To reduce friction between the rotating shafts of machine tools, we use ball bearings. In our daily life we use the fan to give air and cycle to go anywhere. Ball bearings will be used in both of them.

TS 8th Class Physics 2nd Lesson Friction Activities

Activity – 1:
Identifying forces acting on a body and effect of frictional force: (Text. P. No. 19)

Question 1.
How do you Identify forces acting on a body and effect of frictional force? (Field investigation/Experimentation)
Answer:
i) Gently push a book on a horizontal floor. Then the book acquires a certain speed in the direction of push. However the speed of the book gradually decreases and after some time it get stops.

ii) Two forces act on the book in the vertical direction. They are
a) Weight of the book (W) or gravitational force acting vertically down.
b) Normal force (N) or reaction force applied by the table.

The force applied by the table on the book is called frictional force.

Questions based on the above Activity-1

a. Why does the book stop after covering some distance?
Answer:
The frictional force between book and horizontal Floor opposes the relative motion of the book. So the book stops after covering some distance.

b. Why does the speed of the book change gradually?
Answer:
Due to the action of frictional force opposite to the motion of the book, the speed of the book changes gradually.

c. Is the book moving with uniform speed?
Answer:
No, book is not moving with uniform speed. The speed of the book decreases in the horizontal direction that means book travels with deceleration.

d. How many forces are acting on the book when it is pushed on the floor?
Answer:

• Frictional force applied by the floor in the horizontal direction.
• The force used to move the book.

Activity – 2 :
Observing the variation of friction: (Text. P. No. 22)

Question 2.
How do you conclude that friction Is a self-adjusting force? (Experiment/Making hypothesis)
Answer:

Push a heavy box which is kept on a floor F with less force to move horizontally the applied force on the box. Now gradually increase the applied force, the box still does not move, because the frictional force also increases accordingly and balances the increased applied force. From this experiment, we conclude that static friction is a self-adjusting force.

Activity- 3 :
Effect of roughness on frictional force: (Text. P.No. 23)

Question 3.
How do you prove that ‘Roughness effects the frictional force’? (Experiment/Field investigation)
Answer:
Effect of roughness on frictional force:

1. Set up one inclined plane on the horizontal floor.
2. Use a wooden board as inclined plane. Put a mark at any point A’ on the inclined plane.
3. Now lei a marble moves down from this point. Note the distance covered by the marble from the bottom of the inclined plane to point where it comes to stop.
4. Now spread a cloth over the floor. Make sure that there are no wrinkles in the cloth.
5. Do the same with marble and note down the distance. The distance is less when cloth is spread over the floor when compared the distance on the floor without cloth.
6. We can conclude that smoothness/roughness of the surfaces affect the distance travelled by marble.
7. The force of friction is greater for rough surfaces i.e. frictional force increases with roughness of the surfaces.

Questions based on the above Activity-3

a. What are your observations from this experiment?
Answer:
Frictional force increase with the increase in roughness of the surface.

b. In which case the distance is covered maximum?
Answer:
The distance covered by ball when there is no cloth on the floor is maximum.

c. In which case the distance is covered minimum?
Answer:
The distance covered by ball when there is cloth on the floor is minimum.

d. Why is the distance covered by the pencil cell different on different surfaces? Discuss the result.
Answer:
In first case due to lack of cloth on the floor and less frictional force ball covers more distance. In the second case the ball covers less distance when there is a cloth on the floor. Because frictional force is less.

Activity -4: Effect of area of contact in frictional force: (Text. P. No. 24)

Question 4.
How do you prove that the frictional force Is independent of the area of contact? (Expectation/ Field investigations)
Answer:

• Tie a string around a brick. Put the brick in horizontal position.
• Pull the brick by using a spring balance
• Note down the reading of spring balance when brick just begins to move.

• Now turn the brick upright (less area) so that the contact area with the floor becomes small.
• Do the same experiment and measure the friction using spring balance reading.

• The frictional force is came in both cases and irrespective of area of contact of the bodies.
• So the frictional force is independent of the area of contact.

Questions based on the above Activity-4

a. How many forces are acting on the brick in horizontal direction?
Answer:
There are two forces acting on the brick in horizontal direction one is frictional force and the other is applied force.

b. How does the frictional force vary with the change In the area of contact?
Answer:
Frictional force does not depend on the area of contact. So there is no change in frictional force with the change in contact forces of the bodies.

Activity -5: Effect of normal force in friction: (Text. P. No. 24)

Question 5.
Describe an experiment to know the effect of normal force on friction. (Experimentation)
Answer:

1. Tie a string around a brick. Pull the brick by using spring balance.
2. Note down the reading of spring balance when the brick just begins to move.
3. Scale of the spring balance gives directly the applied force in newtons.
4. Now put another brick over the first brick tied to the spring balance.
5. Measure the force of friction by the method described above.
6. From the above activity, we can understand that when we add a brick to the existing brick, it’s normal (once increases and hence frictional force is also increased.
7. So, frictional force is directly proportional to the normal force.

Activity -6
Friction produces heat: (Text. P. No. 25)

Question 6.
Describe some examples where heat ¡s produced due to friction. (Conceptual understanding/Application in daily life)
Answer:
Friction is the force which opposes the relative motion between the two surfaces and can also produce heat.
When we rub our hands together vigorously for few minutes, we feel hotness. When we strike a match stick against rough surface, it catches fire.

Activity -7:

Question 7.
How to reduce friction increasing and decreasing friction: (Text. P. No. 26)
Answer:

1. Take a spoon and how its head in the (broader portion) left hand and hold the mid portion of the spoon by the right hand and pull it towards the other end of the spoon.
2. Now you are noticing here friction increases gradually.
3. Now dip your right-hand fingers in water do it again as said above.
4. Now you are noticing here friction decreases gradually.
5. Thus friction can be advantageous in some cases and disadvantageous in other cases.

Questions based on the above Activity -7

a) Have you ever thought why the sole of your shoes is grooved as shown in figure?
Answer:
The sole of our shoes is grooved because it is done to provide better grip on the floor so that we can move safely.

b) Why do you need to change the lyres when its threading is worn out?
Answer:
We need to change the fires when its threading is worn out because the grip confliction between the tires and road in order to increase efficiency.

Activity- 8:
Effect of rollers on friction: (Text. P. No. 27)

Question 8.
Explain the effect of rollers on friction.
Answer:

• Pull a suitcase without rollers and pull a suitcase which has rollers.
• Try to push a book lying on the table. Now place Pushing a book on pencil the same book on two or three pencils. Push the book again.
• it is always easier to roll a body than to slide it over a surface.
• So rollers are used to reduce frictional force.

Activity -9:
Understanding the principle of ball bearings (Text. P. No. 28)

Question 9.
Explain the principle of ball bearings with an activity. (Field investigation)
Answer:

• Take two lids and rotate them by putting one on the top of the other.
• Now place two or three marbles on one lid and place the other lid on the top of the marbles.

 

• Now try to rotate the top lid. Comparing in the first case it is easy to rotate the lid in the second case.
• From the above experiment (activity) the principle of ball bearing is to reduce friction between the rotating shafts of machine tools, we use hail bearings. Ball bearings convert the sliding friction into rolling friction.

Activity – 10 :
Observing the fluid friction: (Text. P. No. 28)

Question 10.
How do you say that fluids exert force of friction on objects in motion through them? (Experinensation/ Field investigation)
Answer:

• Take a glass of water and stir it with a spoon.
• Water whirls around in glass stop stirring and observe.
• The whirling speed of water gradually decreases and after some time, the water stops whirling and comes to stable state.
• Because of the frictional forces between the liquid layers or between the liquid surfaces that is in contact with glass surface are responsible for stopping the rotation of the water.
• From the above experiment, we conclude that water and other fluids exerts force of friction on objects move through them.

Questions based on the above Activity – 10

a. Which force is responsible for stopping the rotation of the water?
Answer:
Due ro frictional force between the layers of surface of liquid and surface of glass water comes to rest.

Activity – 11 :
identifying factors influencing the fluid friction: (Text. P. No. 29)

Question 11.
Conduct an activity to show the factors influencing the fluid friction.
Answer:

• Take a tub of water. Trw to move your hand in water in the direction of fingers stretched.
• Now try to move your hand in the direction perpendicular to the open face of the hand.
• In second case (perpendicular to the open face) you feel more frictional force. (drag)
• From this we conclude that the frictional force on an object in a fluid depends on its speed with respect to fluid and on the shape of the object and nature of the fluid.

Lab Activity

Question 1.
Explain an activity to understand the nature of frictIon and the concept of static friction. (Text. P. No. 20) (Experimentation)
Answer:
Aim: Understanding nature of friction and the concept of static friction.
Material required: Toy trolley, small wooden block, inextensible string, weight hanger, weights pulley, and table.

Procedure: Take a small toy trolley and keep a small wooden block on it as shown in fig. Tie an inextensible string to the trolley and pass it over a pulley. And other end of the string is fixed to weight hanger. Take a small weight and keep it on weight hanger and observe the changes in motions of block and trolley.

1. Take a small toy trolley and keep a small wooden block on it as shown in the figure.
2. Tie an inextensible string to the trolley and pass it over a pulley.
3. Other end of the string is fixed to weight hanger. Take a small weight and kept it on weight hanger and observe the changes in The trolley accelerating left motions of block and trolley.
4. You may notice that the trolley with the block moves towards left with an acceleration.
5. The block is at rest with respect to the surface of the trolley, but it is in motion with respect to the surface of the table.
6. If you increase the weight on the hanger, the surface of the trolley tries to keep the block at rest with respect to its floor.
7. If we increase trolleys acceleration gradually, at certain limiting acceleration or limiting weight. the block comes into motion in the reverse direction.
8. Now there exists relative motion between the surface of the trolley and the block.

Friction: The force which opposes the relative motion of two surfaces of the bodies which are in contact is called frictional force.