HBSE 8th Class Science Solutions Chapter 12 Friction

Haryana State Board HBSE 8th Class Science Solutions Chapter 12 Friction Textbook Exercise Questions and Answers.

Haryana Board 8th Class Science Solutions Chapter 12 Friction

HBSE 8th Class 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 carom board __________ friction.
(e) Sliding friction is __________ than the static friction.
Answer:
(a) relative motion
(b) nature
(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 arrangement 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 ear 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 is acting opposite to the movement of book i.e. upwards.

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 a soapy floor because layer of soap makes floor smooth. The coating of soap reduces the friction and the foot cannot make a proper grip on the floor and it starts getting to slip on the floor.

Question 6.
Explain why sportsmen use shoes with spikes.
Answer:
Sportsmen use shoes with spike to increase the friction so, that their shoes do not slip while they run or play.

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 frictional force because heavy object will be pressed hard against the opposite surface and produces more friction.

Question 8.
Explain why sliding friction less than static friction.
Answer:
Sliding friction is always less than static friction because two sliding objects find less time to get interlocked against, each others, irregularities of surfaces so they get less friction.

Question 9.
Give examples to show that friction is both a friend and a foe.
Answer:
Friction is a friend because :

• It allows us to walk comfortably on ground.
• It allows us to grip and catch different objects.
• Things don’t move from their places because of friction.
• Speeds of moving objects can be minimized or stopped by friction only.

Friction is foe because :

• It causes wear and tear in objects of our daily use and machines we use.
• Regular maintenance of objects moving against each other (machines and tools) wastes a lot of useful money.
• It makes the movement of heavy objects very difficult.
• It does not allow the regular free movement of substances.

Question 10.
Explain why objects moving in fluids must have special shapes.
Answer:
Objects moving in fluids must have a special shape called steamlined shape. Streamlined shape is that shape which overcomes the friction of fluid. They have pointed fronts with little broader middle portion which gets tapered at the back.

Extended Learning – Activities and Projects

Question 1.
What role does friction play in the sport of your choice? Collect some pictures of that sport in action where friction is either supporting it or opposing it. Display these pictures with proper captions on the bulletin board of your classroom.
Answer:
For self attempt.

Question 2.
Imagine that friction suddenly vanishes. How would life be affected. List ten such situations.
Answer:
For self attempt.

Question 3.
Visit a shop which sells sports shoes. Observe the soles of shoes meant for various sports. Describe your observations.
Answer:
For self attempt.

Question 4.
A toy to play with :
Take an empty match box. Take out its tray. Cut a used refill of a ball pen of the same width as the tray as shown in the figure below. Fix the refill with two pins on the top of the tray as shown in Fig. 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.
Explain your observation. Can you relate it to friction?

Answer:
For self attempt.

HBSE 7th Class Science Friction Important Questions and Answers

Very Short Answer Type Questions

Question 1.
What is friction?
Answer:
Force acting equal and opposite to the relative motion of two objects in contact.

Question 2.
What does the friction depend upon?
Answer:
Nature of object.

Question 3.
Which surface produces more friction?
Answer:
Rough.

Question 4.
Which surface produces less friction?
Answer:
Smooth.

Question 5.
In which direction does friction work?
Answer:
Opposite to the motion.

Question 6.
What causes-friction between two surfaces?
Answer:
Irregularities of two surfaces.

Question 7.
What happens between irregularities of two surfaces in contact which produce friction?
Answer:
Irregularities get interlocked.

Question 8.
Which surface has large number of irregularities to cause friction?
Answer:
Rough surface.

Question 9.
Between flat and sliding surfaces which surface will cause less friction?
Answer:
Sliding surface.

Question 10.
What causes chalk to write on black board?
Answer:
Friction.

Question 11.
What does friction do to soles of our shoes?
Answer:
It causes wear and tear in soles.

Question 12.
What causes a matchstick to catch fire, when rubbed on rough surface?
Answer:
Friction.

Question 13.
What does friction produce?
Answer:
Heat.

Question 14.
What are things like oils, creams and grease called?
Answer:
Lubricants.

Question 15.
How are the shoes made to counter the friction and make better grip?
Answer:
They have grooves on their soles.

Question 16.
How are athletes and other sports shoes are designed to reduce friction?
Answer:
There soles have nails fixed on them.

Question 17.
What is done to the tyres of the vehicles to reduce the friction?
Answer:
They are treaded.

Question 18.
What is used in cycle brakes to increase friction?
Answer:
Brake pads.

Question 19.
What does the kabaddi players use to increase friction of their hands to grip their opponents?
Answer:
They rub their hands with soil.

Question 20.
What do we use on carrom board to reduce friction?
Answer:
Talcum powder.

Question 21.
What do we use on the hinges of the door to make door move smoothly?
Answer:
Machine oil/ Oil.

Question 22.
What is avoided between two surfaces to make movement smooth?
Answer:
Interlocking of irregularities.

Question 23.
What is used to reduce friction in machines, where lubrication are not advisable?
Answer:
Oil cushion.

Question 24.
Can we eliminate friction completely?
Answer:
No.

Question 25.
What is used to make heavy luggages move easily?
Answer:
Wheels are attached to their base.

Question 26.
Which friction is applicable when wheels are used to carry heavy weights?
Answer:
Rolling friction.

Question 27.
Among Static, Sliding and Rolling friction which is the smallest?
Answer:
Rolling friction.

Question 28.
How sliding is replaced by rolling in machines?
Answer:
By using ball bearings.

Question 29.
Name any machine, where ball bearing is used to reduce friction.
Answer:
Ceiling fan.

Question 30.
What is frictional force exerted by liquids called?
Answer:
Drag.

Question 31.
What does frictional force on an object in a fluid depend on?
Answer:
On its speed with respect to the fluid and its shape.

Question 32.
What is the special shape given to objects moving in fluid called?
Answer:
Streamlined shape.

Question 33.
Give three examples of streamlined objects.
Answer:
Ships, boats, aeroplanes.

Question 34.
How is the shape of the body of a fish which helps it in moving in water?
Answer:
It has streamlined shape.

Question 35.
When does rolling friction come in play?
Answer:
When a body rolls on other body.

Short Answer Type Questions

Question 1.
What is friction?
Answer:
Friction is a force which comes in play when two -bodies move on each other. Friction works in opposite direction to the direction of the movement of objects.

Question 2.
What causes friction?
Answer:
Friction occurs when two bodies move on each other. Each surface has some irregularities on it. When two such objects move on each other their irregularities get interlocked and friction arises.

Question 3.
How does the nature of surface effect friction?
Answer:
Nature of the surface plays major role in production of friction. The rough surface produces more friction than the smooth surface. The rough surface has more irregularities on it than the smooth surface, so more force is required to overcome the friction caused by irregularities.

Question 4.
What is a spring balance? How does it work?
Answer:
Spring balance is a device which is used to measure the force acting on a body. It consists of a coiled spring, which is attached to pointer and scale. When some object is hung with it the spring stretches and pointer points to the graduated scale to give the measurement of the force.

Question 5.
Which friction is less static friction or sliding friction? Why?
Answer:
Sliding friction is lesser than static friction, because in sliding friction the relative motion between two surfaces is less opposed by friction as the irregularities of the two surfaces in contact do not get enough time to interlock with each other which makes the motion easy.

Question 6.
What would happen, if the floor we walk on, does not produce any friction?
Answer:
If there is no friction on the floor, we would not be able to stand or walk on the floor, because it is the friction which unables us to stand or walk on the surface. The grooves of our feet or shoes get locked into the irregularities of the floor and makes us stand. If friction is not there, then we would simply slip on the floor.

Question 7.
What helps us write with a pen or pencil?
Answer:
It is the friction between the hand and pen or pencil which causes us write with pen. The grooves on our fingers make a grip on the pen’s irregularities and we are able to write.

Question 8.
Why is it difficult to move an object from its static position?
Answer:
When an object is in its static position, it has better hold of the surface on which it is placed. The irregularities of the surface are interlocked properly and more force is needed to overcome the friction offered by interlocking surfaces. So it is difficult to move the object from its position of rest.

Question 9.
Why do we rub our hands in winter when we feel cold?
Answer:
We rub our hands to make them warm, when we feel cold. Rubbing hands against each other cause friction between them. Friction produces heat, this heat makes us warm. We can feel the warmth by touching our hands on our face.

Question 10.
Why do we need to decrease friction?
Answer:
Friction is an evil which causes a lot of wear and tear in objects coming under its effects. It causes a lot of wear in machine parts which rub against each other, it erodes the surfaces and destroy their symmetries. So, we lubricate surfaces to decrease the friction.

Question 11.
How can we increase friction?
Answer:
We need to increase friction of surfaces to control the movement. Friction can be increased by increasing the irregularities of the surfaces in contact. We use spikes in the shoes of the athletes so that they can lock better in the irregularities of the ground to make better grip.

Question 12.
How can we decrease friction?
Answer:
Friction can be decreased by rolling, sliding and lubricating the surfaces in contact. We use ball bearings, wheels and lubricants like oils, grease, cream etc. to decrease the friction. Rolling, sliding and lubricating decrease the irregularities of the surfaces and make them smooth.

Question 13.
What is drag?
Answer:
Drag is the frictional force exerted by fluids. Liquids and gases are collectively called fluids. So the friction caused due to movement of objects in liquids and in air medium is called a drag.

Long Answer Type Questions

Question 1.
What is friction? How do friction arise? Explain.
Answer:
Equal and opposite force exerted by relative motion between two surfaces is called friction. Friction is always exerted in opposite direction to the force exerted. Friction acts between two surfaces. It is caused due to the movement of these two surfaces in contact. Every surface has some irregularities on it. When two surfaces come in contact, these irregularities get locked with each other. When objects are moved, the friction arises in the opposite direction of the movement.

Question 2.
What factors effect the force of friction? Explain.
Answer:
Force of friction depends upon the nature of the surfaces in contact and the force with which these two surfaces are pressed against each other. Nature of the surface on which an object moves, effects the force of friction. Rough surfaces produce more friction, while smooth surfaces produce less friction. Rough surfaces have more irregularities on them so they produce more friction. Smooth surfaces allow easy movement as they have less irregularities and the interlocking of irregularities with other surface is less as compared to the rough surfaces, so they produce less friction.

Secondly, force of friction depends upon how hardly two surfaces are pressed against each other. If two surfaces are pressed hard against each other, then the friction produced will be more.

Question 3.
How many types of frictions do you know about? Explain.
Answer:
Friction is of different types : Static friction, sliding friction and rolling friction.
Static friction is one which is produced between two surfaces in contact with each at the position of rest. This friction is more powerful than others. It is most difficult to overcome the static friction at this positions, the irregularities are most effectively pressed against each other. This type of friction needs a lot of energy to overcome.

Sliding friction: arises when two objects in contact are in motion. It is lesser than static friction. This friction is lesser because the irregularities of both surfaces do not get enough time to get locked with each other as an object is already in motion.

Rolling friction: comes in effect when two bodies are rolling on each other. Rolling movement of both objects don’t allow the locking of irregularities, thus it is the smallest friction of all they three types. Drag is the frictional force which is exerted by the fluids on the bodies moving in it. This friction needs a lot of energy to overcome so, bodies of such objects which move in fluids are streamlined to counter the friction offered by fluids.

Question 4.
What is fluid friction and on what factors does fluid friction depend?
Answer:
Friction exerted by fluids is called fluid friction. Bodies moving in air and liquids bear friction caused by air and liquid. Air and liquids are collectively called fluids. Fluid friction is also called Drag.Drag depends upon the speed of the object with respect to fluid, nature of the fluid and the shape of the object. Slow moving objects face more friction and heavy fluids offer more friction. Objects with pointed front, face less friction. All objects moving in fluids have streamlined shape so as to minimise the friction exerted by fluids.

Streamlined objects have narrow front, broader middle portion and which tapers at the back. All living organisms moving in air and water have streamlined shapes. Birds, fish etc. have streamlined body. So, Aeroplanes, boats, ships etc are designed according to birds and fish, so that they have to face less friction and has to spend less energy to overcome the friction.

Question 5.
prove with the help of an experiment that smoothness of surface reduces friction.
Answer:
Take a pencil or pen. Now put it on a table. Slightly push the pencil and see how much distance it covers, mark the point where the pencil stops after covering the distance. Now take some.cream or oil. Make a small coating of oil on the table or spread polythene on the table if possible. Now again take the pencil and put it at the same initial point. Now push the pencil slightly and let it stop on its own as done earlier. Now mark the point where the pencil stops. See the difference in the distance. In second attempt the pencil has covered more distance. This means it had to face less friction in second attempt. It proves that smoothness of the surface reduces friction.

Question 6.
Why and how do we increase friction?
Answer:
We increase friction deliberately to control the movements and motion of the objects. We also increase the friction for better grip and hold. Friction can be increased by making surfaces coarse and pressing two surfaces harder. Coarse surface has more irregularities to grip irregularities of other surface better, thus making the things come to a halt and give a better grip if object is stationary. We use brake pads in vehicles. When brake pads are pressed by pressing the lever, it gets pressed against the tyres and arrest the movement of the tyres and with this the vehicle comes to stop.

Pens are provided with ridges at the point where we hold them for writing, so that the friction between fingers and pen becomes more and a better grip can be made to write fluently. Pens without ridges get slipped out from our hands easily.

Question 7.
Why and how do we decrease friction?
Answer:
We decrease friction to save the objects from wear and tear. Friction causes wear and tear on the surfaces of the objects. It causes damage to the objects specially machines and machine parts. So, we apply a film of oil or grease between the surfaces in contact, so that it produces less friction and it causes no damage to the parts. Oils, grease etc. are called lubricants, which are used to make surfaces smooth and minimise friction. In most of the cases rolling of objects is used to reduce friction. Ball bearings are used in cycles, vehicles and machines because it reduces friction. Wheels are used to move heavy objects because wheels roll on the ground thus reducing friction and this makes the dragging of heavy objects easy. Tyres carry heavy and loaded trucks on them, as they produce less friction and need lesser energy to move them.

Friction Class 8 HBSE Notes

1. Friction is the force equal and opposite to the relative motion between two surfaces in contact.

2. Force of friction acts on both the surfaces in contact.

3. There are many factors which affect friction :

• It depends on the nature of surface.
• It depends on smoothness of surface.
• It depends on the hardness with which two surfaces in contact are pressed against each other.

4. Friction has no relation with area exposed to force of friction.

5. Friction is static, when it opposes the force applied on the body.

6. When two surfaces slide on each other, sliding friction comes into play.

7. Sliding friction is less than the static friction.

8. Friction is an evil which causes a lot of wear and tear on the surfaces of the objects due to opposite forces in action.

9. Wear and tear is more in case of static friction and less in case of sliding friction.

10. Friction is an evil but a necessary evil. No body would have been able to walk on the floor, if friction was not in play :

11. When two surfaces roll on each other the rolling friction is applied, for example in case of wheels under attache-cases. Rolling reduces the friction and movement becomes easy and smooth.

12. Fluids also exert force of friction and reduce the speed. To avoid friction due to fluid, bodies of objects moving in fluids have pointed and streamlined fronts and bodies.

13. Aeroplanes and ships etc. have streamlined bodies to overcome the friction caused by air and liquids.