DAV Solutions for Class 8 Science Chapter 5 Friction

DAV Solutions Class 8 Science Chapter 5

DAV Solutions for Class 8 Science Chapter 5 Friction of The Living World Book is one of the most crucial resources for the students of 8th Standard. The solutions provided here aim to help students understand the chapter in an interesting way. These Chapter 5 Friction Solutions are created by subject experts, according to the latest DAV syllabus (2021-22). This solution consists of answers to the questions given in the DAV Class 8 Science textbook.

Students are advised to practice the solutions regularly to prepare effectively for their examination. Along with DAV Solutions students are also advised to read and solve previous year question papers and sample papers. Solving the previous year questions will help you to get acquainted with the latest exam pattern as well as the marking scheme.

DAV Class 8 Science Chapter 5 Question Answer

DAV Solutions for Class 8 Science Chapter 5 Friction is given below. Here DAV Class 8 Science Chapter 5 Question Answer is provided with great explanation. This chapter deals with friction. In this chapter, you will study about cause of friction, factors affecting friction, methods of increasing friction, methods of reducing friction, etc.

Highlights

• Fill in the blanks.
• True or False.
• Tick the correct option.
• Answer the following questions in brief.
• Answer the following questions.

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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.

Answer: (1) opposes (2) greater (3) increase (4) decrease (5) speed (6) streamlined

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 (2) False (3) True (4) False (5) True (6) False (7) True

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-

Answer: option D

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-

Answer: static friction

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 –

Answer: rolling, sliding, static

4. The energy, ‘used up’ in overcoming friction, gets converted mainly into-

Answer: heat energy

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-

Answer: sprinkle talcum powder on the wooden surface.

6. A block is sliding on a horizontal surface. The force of friction between the two can be increased by-

Answer: by putting a second identical block on top of the given block.

D. Answer the following questions in brief.

1. Define the terms (a) static friction (b) sliding friction.

Answer: (a) Static Friction- The Frictional force which is self-adjusting force that exists between a stationary object and the surface on which it has been kept is called static friction.

(b) Sliding friction-The force of friction that acts between an object and the surface when the object is sliding over that surface is called Sliding friction.

2. State the meanings of the terms (a) rolling friction (b) fluid friction.

Answer: (a) Rolling friction-The force of friction that comes into play when an object rolls over the surface of another object is called Rolling friction.

(b) Fluid Friction – The Frictional force exerted by fluids (liquids and gases) on the object moving through them is called Fluid friction.

3. State the likely cause of friction.

Answer: The likely cause of friction are as follows:

• Interlocking of irregularities/ roughness between the two surfaces in contact with each other.
• The force pressing the two surfaces together.

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: 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)?

Answer: It is because the force applied is less than that of the limiting friction as in figure (a) and figure (b).

(c) Mark the direction of frictional force, and mention the kind of frictional force that comes into play, in each case.

Answer: The direction of the frictional force is always opposite to the motion of box in given figure.

The kind of frictional force that comes into play, in

• Case a-Static Friction
• Case b-Static Friction
• Case c-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

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 Fluids experiences the ‘fluid friction on it, which can be minimised by giving a special shape.

This is called Streamlined shape that is given to object moving through Fluids.

E. Answer the following questions.

1. Define Force of friction. List the factors affecting the force of friction. Explain with examples.

Answer: The opposing force which comes into play Whenever an object moves or tends to move over the surface of another object, acting parallel to the surface of contact is called force of friction.

Following factors affects force of friction

• Interlocking of irregularities of two surfaces in contact.
• Force pressing the two surfaces together.

Example– A box placed on smooth surface and less mass will be easy to move but a box placed on a rough surface with greater mass will experience more friction.

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 move for longest distance it is because,

• glass surface is smooth
• less interlocking
• less friction

3. Explain why sliding friction is slightly less than static friction.

Answer: As we know that, the friction is caused by the interlocking of irregularities or roughness in the two surfaces in contact. So, in the case of static friction the interlocking between the two objects is very strong, but in case of the sliding friction there is a less time of getting interlocking of the roughness of the surface with each other. So, the sliding friction is less than the static friction.

4. Give reasons for the following-

(a) We tend to slip when we step on a banana peel.

Answer: As we stepped on a banana peel, then there is no grip between our feet and the banana peel, and due to very less friction we get accidentally 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 heavier one, because friction depends on the force pressing the two surfaces together. The heavy box will exert more force as in comparison to the lighter box. So, the friction will be more in the case of the heavy box. Hence, the lighter box can be easily pushed as in comparison to the heavy box on the same floor.

(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 a cart is greater than the force needed to keep it moving with a uniform speed, because the force needed to start a cart is applied against the Static friction caused by the stronger interlocking of the irregularities or roughness of the surface of wheel and floor. Once, the motion starts the contact points on one surface do not get enough time to lock into the contact point of another surface.

(d) Sportsmen use shoes fitted with spikes.

Answer: Sports man use shoes with Spikes in order to avoid getting slip in the field. The Spikes in the shoes increases the friction, so the sportsman can run on the ground easily.

(e) Ball-bearings are used in machines.

Answer: Ball bearings are used in machine to reduce friction and to support the load.

(f) Tyres of the vehicles need to be changed regularly.

Answer: Due to the ‘wear and tear’ in the tyres by regular use, the friction between road and the tyre reduces. The chances of the tyres to slip on road becomes more. So, the tyres of the vehicles need to be changed regularly to increase the friction between road and the wheel.

(g) Machines parts are frequently oiled or greased.

Answer: Machines parts are frequently oiled or greased to reduce the friction between parts of machine, as well as to increase the efficiency of the machine.

(h) Sport cars, aeroplanes and boats are designed to have a streamlined shape.

Answer: Sport Cars, aeroplanes and boats are designed to have a streamlined shape to overcome the fluid friction.

(i) Metal chains are wrapped on tyres when they run on icy roads.

Answer: The metal chains are wrapped on tyres for increasing the friction by increasing grip, that help the car to stop after applying the brakes, when they run on the Icy Road.

(j) Rollers are used for transporting luggage.

Answer: It is always easier to roll the body than to slide it over a surface, because the rolling friction is less than that of the sliding friction. So, it is convenient to pull the luggage fitted with rollers.

5. State and explain, with examples, how friction can help the cause of motion in certain situations.

Answer: Some of the examples are as follows-

a) When we walk, we push the ground under our feet in the backward direction, the friction then provides forward reaction and it is the main cause of the motion.

b) Vehicles run on the road due to the friction between tyres and the road.

c) Friction also helps in writing by moving a pen on a paper.