Friction is an important chapter in Class 8 Science book. In this article we are going to discuss all the question and Answers of chapter 5 of DAV Class 8 Science book. We have verified the answers thrice and all the answers have been written by the subject matter experts.
Friction Question Answers DAV Class 8
A. Fill in the blanks
1. Friction always ___________ any relative motion between the two surfaces that are in contact with each other.
2. Static friction is always ___________ than sliding friction.
3. Sportsmen use shoes, fitted with spikes, to ___________ friction between their shoes and the ground.
4. Sprinkling of talcum powder, on the carrom-board, helps to ___________ friction.
5. Frictional force, on an object moving in a fluid, depends on its ___________.
6. The shape of an aeroplane is ___________ to reduce, the effects of friction, due to air.
Ans 1. opposes
2. greater
3. increase
4. reduce
5. Speed
6. streamlined
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B. State True or False for the following statements.
1. When a body slides over smooth and wet surfaces, the amount of interlocking of irregularities, of the two surfaces in contact, increases.
2. If a car moves eastwards, the force of friction acts southwards.
3. Friction depends on how hard the two surfaces press against each other.
4. The sole of shoes and the tyres of the vehicles are ‘treaded’ to decrease the effects of friction.
5. Lubricants can be liquids, semi-liquids or solids.
6. Friction can be increased by using ball bearings between the moving parts of machines.
7. Fluid friction can be minimised by giving suitable shapes to the objects moving through the fluid.
Answer: 1. False ( reason: it decreases because wet or smooth surface will have very less friction. You must have experienced that tyres of cycle or scooty tends to slip on rainy days when the roads are wet)
2. False
3. True
4. False ( Reason: They are treaded so as to create the effect of friction)
(5) True
(6) False
(7) True
DAV Class 8 Science Chapter 5 Solutions (Q/A)
C. Tick the correct option
1. Suppose your writing desk is tilted to a position where a book kept on it just starts sliding down. The figure, showing the correct direction of frictional force acting on it, is-
Ans. Option D will be the correct answer.
2. Two boys are applying oppositely directed, and equal in magnitude, forces on a box as shown in the figure. In such a case, the force of friction, that would exist between the lower end of the box and the ground, would be called-
Ans 2. Such a force between the lower end of the box and the ground will be known as Static Friction.
Also Check: DAV Class 8 Science Chapter 3 Solutions
3. Four students were asked to arrange the (frictional) forces, due to rolling, static and sliding friction, in an increasing order. Their arrangements are listed below. The correct arrangement is –
Ans 3. RollingFriction< Sliding Friction< Static Friction
4. The energy, ‘used up’ in overcoming friction, gets converted mainly into-
Ans 4. The energy gets converted into the head. This can be observed by the example of tyres of your bike or a scooty. Go for a long ride on your bike someday and then touch the tyres of the bike when you return back. You will experience that the tyres are a bit hot.
5. Rahul rolls a ball on a wooden surface. The ball covers a certain distance before coming to rest. To make the same ball cover a (much) longer distance, before coming to rest, Rahul should-
Ans 5. If Rahul wants the ball to cover a larger distance he should sprinkle some talcum powder on the wooden box.
6. A block is sliding on a horizontal surface. The force of friction between the two can be increased by-
Ans 6. The force of friction can be increased if we add another box of the same dimensions on the top of the first box. It will create more pressure thus more friction will be generated.
D. Answer the following questions in brief.
1. Define the terms (a) static friction (b) sliding friction.
Ans 1.
a) Static Friction: It is a kind of frictional force that is present between an object at the stationary position and the surface on which the object is kept.
b) Sliding Friction
This force of friction is present between the objects and surface when the objects slide over the surface on which they are kept.
2. State the meanings of the terms (a) rolling friction (b) fluid friction.
Answer:
(a) Rolling friction-The force of friction which present or generated when an object rolls over the surface of another object is termed as Rolling friction.
(b) Fluid Friction – When we put an object in a fluid, the fluid exerts a force on them. Such a force is called as fluid friction.
3. State the likely causes of friction
The two causes of friction are:-
The pressure on the objects when they are kept on a surface. The objects experience friction against the surface.
Interlocking between two surface or an object or surface also produces frictional force.
4. A box is resting on the floor. To move it, a (variable) force is applied as shown in the diagram below.
(a) Give the (likely) value of the force of limiting friction.
Answer: The value will be more than 2f and less than 3f.
(b) Why does the box stay at rest in the cases corresponding to Fig (a) and Fig (b)?
The box stays at rest because sufficient force has not been applied. The force is less than the limiting friction.
(c) Mark the direction of frictional force, and mention the kind of frictional force that comes into play, in each case.
The direction of the frictional force will be opposite to the motion of the box in the given figure.
The kind of frictional forces that comes into play is:-
- Case A: It will be a Static Friction
- Case B: It is a case of Static Friction
- Case C: This clearly the case of Sliding Friction
5. Name the kind of frictional friction, that comes into play, when a book, kept on a collection of cylindrical pencils, is moved by pushing it.
Answer: Rolling friction is acting here. The book will move slowly towards the direction of force.
6. Why are objects given special shapes when they are moving through fluids? Write the name given to these special shapes?
Answer: Objects moving in the liquid or fluids experience fluid friction on them, this force can be reduced by giving a special shape to the objects.
This is called a Streamlined shape that is given to an object moving through fluids. An example is: Aeroplane bodies are streamlined because they have to move cutting the pressure of air.
DAV Class 8 Science Chapter 5 Solutions
E. Answer the following questions:
1. Define Force of friction. List the factors affecting the force of friction. Explain with examples.
Answer: Force of friction is a force generated when two objects rub against each other or an object moves against a surface.
Factors affecting the force of friction
- Interlocking activity between 2 surfaces in contact with each other.
- Pressure created when an object is kept over a surface.
Example– This can be explained by the example: It is easy to move a bag on a marble surface but to move it on a rough surface of the road you need to put more force. The bag can also experience wear and tear if slid for a long time.
2. A ball is allowed to roll down an inclined wooden plane from a given height. At the foot of the inclined plane, it moves on a horizontal surface, differently ‘covered’, one by one, as shown in the following figures.
In which case, is the ball likely to move the longest distance? Give reason for your answer.
Answer: In case ‘C’ (glass sheet), the ball will cover the longest distance, let’s have a glance at the reasons:-
- the glass surface is smooth
- less interlocking present in the glass surface
- very less force of friction
3. Explain why sliding friction is slightly less than static friction.
Answer: We all have learnt that the force of friction is produced by the interlocking between the two surfaces in contact. So, in the case of static friction, the interlocking between the two objects is very high as compared to the case of sliding friction. The object and the surface do not have much time to create an interlocking effect between each other as the object is sliding against the surface.
4. Give reasons for the following-
(a) We tend to slip when we step on a banana peel.
Answer: We tend to slip when we step on the banana peel because there is a lack of friction due to the smooth surface of the banana peel. So, we slip.
(b) It is easier to push a lighter box than a similar heavy box on the same floor.
Answer: It is easier to push a lighter box than a heavier one because the force of friction also depends on the force acting between the two surfaces in contact with each other. The heavier box will produce more force in comparison to the box having a lighter weight. We will have to put more effort into moving the heavier box as it exerts more friction.
(c) The force, needed to start a cart, is (somewhat) greater than the force needed to keep it moving with a uniform speed.
Answer: The force needed to start the cart is higher because it is experiencing strong static friction exerted due to the interlocking between the wheels of the cart and the surface. To overcome this strong static friction more force needs to be applied.
The force of friction reduces when the cart comes in motion because there is no time for creating the interlocking between the wheels of the cart and the surface.
(d) Sportsmen use shoes fitted with spikes.
Answer: Sportsmen uses shoes fitted with spikes because the shoes creates higher friction with the surface and reduces the chances of slipping. This can not be achieved by the normal pair of shoes.
(e) Ball-bearings are used in machines.
Answer: Ball bearings are used in machines to lessen the force of friction and to balance the overall load.
(f) Tyres of the vehicles need to be changed regularly.
Answer: Tyres of vehicles may get worn out due to repeated action of frictional force on them when they rub against the surface of the road.
(g) Machines parts are frequently oiled or greased.
Answer: Oil and grease reduce the friction between parts of machines. If we oil them regularly they will function smoothly.
(h) Sports cars, aeroplanes and boats are designed to have a streamlined shape.
Answer: The aeroplanes and boats have to move cutting the force of air and liquid respectively. They both experience fluid friction. To overcome this fluid friction, their bodies are streamlined.
(i) Metal chains are wrapped on tyres when they run on icy roads.
Answer: Metal chains are wrapped on the tyres to increase the friction because the icy roads are quite slippery and can cause an accident if the vehicle slips.
(j) Rollers are used for transporting luggage.
Answer: Rolling friction is less than sliding friction. And rolling the objects is easy as compared to sliding them that’s why rollers are used for transporting the luggage.
5. State and explain, with examples, how friction can help the cause of motion in certain situations.
Answer: Let us understand this fact from the examples given below:
- We are able to walk just because of friction.
- We are able to write on paper because of the frictional force. Try writing on a polythene or extreme smooth surface. It will become difficult to write.
- Vehicles run on the roads just because of friction. Without friction, the movement of vehicles would not have been possible. You must have experienced that the vehicles slip on rainy days when the roads are slippery. This is because we experience a lack of friction on the roads.
So, these were DAV Class 8 Science Chapter Number 5 Solutions. We have given solutions to all the questions and answers of the Friction chapter with explanations. If you have any queries, feel free to ask me through the comments section below. Stay connected to CareerAdvice4u.
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