RBSE Solutions for Class 8 Science Chapter 12 Friction

Rajasthan Board RBSE Solutions for Class 8 Science Chapter 12 Friction Textbook Exercise Questions and Answers.

RBSE Class 8 Science Solutions Chapter 12 Friction

RBSE Class 8 Science Friction InText Questions and Answers

Page 152

Question 1. 
Can we reduce friction to zero by polishing surfaces or using large amount of lubricants?
Answer:
No, friction can never be reduced to zero. No surface can be perfectly smoothened by using surface polish or excessive use of lubricants. Some irregularities will always be left in reducing friction.

RBSE Class 8 Science Friction Textbook Questions and Answers    

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) Sprinkling of powder on the carrom board .................. friction.
(e) Sliding friction is .................. than the static friction.
Answer:
(a) motion 
(b) smoothness 
(c) heat 
(d) reduces 
(e) less.

Question 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
Answer:
(c) Static, sliding, rolling 

Question 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.
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.

Question 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?
Answer:
It is difficult to walk on the wet marble floor. The frictional force between the shoes and the wet surface is very small. When we walk over a wet surface, shoes are unable to press the surface properly, hence we are unable to get sufficient reaction force required for forward motion.

Question 6. 
Explain why sportsmen use shoes with spikes.
Answer:
Spikes increase friction. Spikes also provide tight grip with the floor. The possibility of skiding is eliminated. Hence the players prefer spikes.

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:
Seema will experience more friction because static friction due to heavy box is more than the lighter box. The maximum frictional force acting between two surfaces is called limiting friction. When the applied external force exceeds the limiting friction, the body starts moving.

Question 8. 
Explain why the sliding friction is less than the static friction.
Answer:
When a body starts sliding, the contact points of the surfaces do not find sufficient time to interlock the irregularities. Therefore sliding friction is little less than the static friction. Hence it is easier to keep the moving box moving than to start moving a body from rest.

Question 9. 
Give examples to show that friction is both a friend and a foe.
Or
Friction in both enemy and friend in our daily life. Explain.
Answer:
Friction is a friend because:

1. Frictional force helps us walk on the surface of earth. We cannot think of walking in the absence of friction.
2. Friction between the tip of a pen and paper helps us write on the paper. It will be difficult to write on an oily paper or waxed paper due to lack of friction.    
3. The frictional force between wheels of vehicles and the road helps stop vehicles when brakes are applied. In the absence of friction when brakes are applied, the wheels will skid.
4. Conveyor belt and fan belt are able to move on the circumference of machine wheels due to friction.
5. The frictional force between the chalk and the black board helps us write on the black board.
6. Had there been no friction, construction of buildings would not have been possible. 

Friction is a foe because:

1. Due to friction a large amount of energy of a moving body is lost in the form of heat.
2. Frictional force does not permit to increase the speed of buses, trains, aeroplanes, rockets etc. beyond certain limit.
3. The efficiency of machines is lost due to frictional forces.
4. Moving parts of machines are worn out due to frictional forces.
5. Some times various mechanical parts of machines heat up and bum due to excessive friction.

Question 10. 
Explain why objects moving in fluids must have special shapes.
Answer:
Vehicles moving through the fluids are specially designed so as give them a stream lines shape. By doing so the when the vehicles move through the fluids, the motion of fluids (liquid or gas) remains stream line and not turbulent. In this way the speed of vehicles is regularised.