CLASS VIII SCIENCE

Class- VIII-CBSE-Science Friction

CBSE NCERT Solutions for Class 8 Science Chapter 12

1. Fill in the blanks.

(a) Friction opposes the _____________ between the surfaces in contact with each other.

Solution: relative motion

By definition of friction, friction opposes the relative motion between the surfaces in contact with each other.

(b) Friction depends on the _____________ of surfaces.

Solution: nature

Friction depends on the nature of the surfaces in the contact.

(c) Friction produces __________.

Solution: heat

If surfaces of different bodies in contact are sliding with each other, due to friction

heat will be generated.

(d) The sprinkling of powder on the carrom board ________ friction.

Solution: reduces

Lubricants reduce friction between surfaces.

(e) Sliding friction is ___________ than the static friction.

Solution: less

Sliding friction is less than static friction. It is relatively easy to push an object

which in motion than setting motion of a stationary object.

2. Four children were asked to arrange forces due to rolling, static and sliding friction

in 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

Solution: (C)

Static friction is greater than sliding friction. Due to this reason, it is relatively easy

to push an object which is in motion rather than setting motion of the same object.

Rolling friction is smaller than sliding friction. Due to this reason, it is easy to pull

luggage with wheels than pulling the same luggage without wheels.

3. Alida runs her toy car on a 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

Solution: (A)

4. Suppose your writing desk is tilted a little. A book kept on it starts sliding down.

Show the direction of the frictional force acting on it.

Solution:

5. You spill a bucket of soapy water on a marble floor accidentally. Would it make it

easier or more difficult for you to walk on the floor? Why?

Solution:

While walking, when our feet push the ground backwards, the floor pushes us in

the forward direction. Friction plays the most important role in this process. When

you spill a bucket of soapy water on a marble floor, friction between your feet and

floor decreases and the force responsible for pushing you forward weakens.

Therefore it becomes more difficult for you to walk on the floor.

6. Explain why sportsmen use shoes with spikes.

Solution:

While walking, when our feet push the ground backwards, the floor pushes us in

the forward direction. Friction plays the most important role in this process.

Sportsmen use shoes with spikes to increase the interlocking of the surfaces in

contact, which in turn increases the friction and gives them a better grip.

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?

Solution:

Friction depends on the weight of an object. A heavier object will experience more

friction because the surfaces will be pressed harder, which will result in the greater

interlocking of irregularities of surfaces in contact and thus will require larger force

to move the heavier object.

8. Explain why sliding friction is less than static friction.

Solution:

When the irregularities between two contact surfaces get interlocked, it results in

resistance known as friction. In sliding friction, the time for which the surfaces are

in contact is very small; therefore, the interlocking of irregularities does not create

much resistance. As less force is required to overcome this resistance, sliding

friction is less than static friction

9. Give examples to show that friction is both a friend and a foe.

Solution:

friction as a friend:

(a) Friction helps in walking.

(b) We are able to write with a pen on paper due to friction.

Friction as a foe:

(a) Friction leads to loss of energy as heat

(b) Because of friction, the tires and soles of shoes wear out.

10. Explain why objects moving in fluids must have special shapes.

Solution:

As it happens between the ground and feet, a body moving through fluid also

experiences an opposing force called drag force that depends on the nature of the

surface and the shape of the body. By giving objects special shape, this drag force

can be minimized.

A toy to play with: 

Take an empty box. Take out its tray. Cut a used refill of a ball pen of the same width as the tray as shown. Fix the refill with two pins on the top of the tray as shown in Fig. 12.18. Make two holes on the opposite sides of the tray. Make sure that the holes are large enough to allow a thread to pass through them easily. Take a thread about a metre long and pass it through the holes as shown. Fix beads at the two ends of the thread so that it does not come out. Insert the tray in the outer cover of the matchbox. Suspend the match box by the thread. Leave the thread loose. 

The match box will start falling down due to gravity. Tighten the thread now and observe what happens. 

 Activity 13.5

 Take 6-8 bowls or tumblers. Fill them with water up to different levels, increasing gradually from one end to the other. Now take a pencil and strike the bowls gently. Strike all of them in succession. You will hear pleasant sounds. This is your jaltrang 

 TO DOWNLOAD PPT CLICK HERE

Friction Class 8 Science 

NCERT Textbook Questions

Question 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) The sprinkling of powder on the carrom board _______ friction.
(e) Sliding friction is _______ than the static friction.
Solution:
(a) relative motion
(b) smoothness (or irregularities or nature)
(c) heat
(d) reduces
(e) less

Question 2.
Four children were asked to arrange forces due to rolling, static and sliding frictions in 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
Solution:
(c) static, sliding, rolling.

Question 3.
Alida runs her toy car on a 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.
Answer:
(a) wet marble floor, dry marble floor, newspaper and towel.

Question 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.
Answer:
Frictional force will act upward, i.e., the direction opposite to that of sliding book.

Question 5.
You spill a bucket of soapy water on a marble floor accidentally. Would it make it easier or more difficult for you to walk on the floor? Why?
Answer:
The layer of soap makes the floor smooth due to which the friction is reduced

Question 6.
Explain why sportsmen use shoes with spikes.
Answer:
Sportsmen use shoes with spikes to increase the friction between shoes and the surface.

Question 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?
Answer:
A heavy object produces more friction as it is pressed hard against the opposite surface. So, Seema will have to apply a larger force.

Question 8.
Explain why sliding friction is less than static friction.
Answer:
The sliding friction is less than static friction because the sliding object get less time to interlock into the contact points on the floor.

Question 9.
Give examples to show that friction is both a friend and a foe.
Answer:
Some points are given below which show that friction is both a friend and a foe:

1.    Friction as a friend:

·         It allows us to grip and catch any object.

·         It helps us to walk comfortably on the floor.

·         It helps to minimise the speed or to stop any moving object.

·         It helps us to write.

2.    Friction as a foe:

·         It causes wear and tears in objects.

·         It causes damage to the parts of machines and tools which further require money to get them repaired.

·         It reduces the speed of moving objects, so more force is required.

·         It produces hurdles in moving any object freely.

Question 10.
Explain why objects moving in fluids must have special shapes.
Answer:
The objects moving in fluids must have a special shape to overcome the fluid friction acting on them. Efforts are therefore made to minimise the friction, so objects are given special shape having pointed fronts with little broader middle portion which gets tapered at the back called streamlined.