Q 1. What is the force of friction?
Ans. The force acting on a moving object equal and opposite to the direction of motion is called force of friction
Q 2 – What is the cause of friction?
Ans. The irregularities on the two surfaces in contact is the cause of friction.
Q 3 – What is the force responsible for wearing out of bicycles tyres?
Ans. Frictional force
Q 4 – Is the friction same for all objects?
Ans. No
Q 5 – Why are wheels of vehicles made circular?
Ans. Rolling friction is less than sliding friction.
Q 6 – hat is the direction of force of friction acting on a moving object?
Ans. Opposite to the direction of moving objects.
Q 7 – Why do pieces of luggage fitted with rollers?
Ans. To reduce friction and to pull them easily.
Q 8 – Why does a matchstick catch fire when rubbed on the rough surface of the box?
Ans. Due to friction between the stick and rough surface a matchstick catches fire.
Q 9 – What is the name of special shape given to objects moving in fluids?
Ans. Streamlined
Q 10 – Give one example where ball bearings are used.
Ans. Ceiling fan
Q 11 – Does friction depend on the nature of objects?
Ans. Yes
Q 12 – What is the use of lubricants?
Ans. They are used to reduce friction.
Q 13 – Which surface does produce more friction—smooth or rough surface?
Ans. Rough surface
Q 14 – Why is fine powder sprinkled on the carrom board?
Ans. In order to reduce friction powder is sprinkled on carrom board.
Q 15 – Why is it difficult to walk on the wet marble floor?
Ans. The wet marble floor has less friction. So it is difficult to move on it.
Q 16 – Why does the jar of a mixer become hot when it is run for few minutes?
Ans. It becomes hot due to friction.
Q 17 – Name the types of friction.
Ans.
- Static friction
- Sliding friction
- Rolling friction
- Fluid friction
Q 18 – Which force helps us to write on the paper or blackboard?
Ans. Frictional force
Q 19 – If we push the book on the table, it stops after sometime. Why?
Ans. When we push the book on a table, a force is applied on it in the opposite direction of the motion called force of friction. This force opposes the movement and the book stops.
Q 20 – Why are the worn out tyres discarded?
Ans. The worn out tyres are discarded because their tread are worn off and limiting their effectiveness of providing required amount of friction.
Q 21 – Why is it easier for the fish to swim in water?
Ans. Fish can easily swim in water because it has a streamlined body which produces less friction. So it easily cuts through water and moves in it very easily.
Q 22 – Give some examples of smooth and rough surfaces.
Ans. Smooth surfaces: Surface of the mirror, surface of wet soaps, glazed tiles, surface of an oily pot, etc. Rough surfaces: Surface of plyboard, unpainted wall, brick, tyres, etc.
Q 23 – What is a spring balance?
Ans. A spring balance is a device which measures the force acting on an object. It contains a coiled spring. When a force is applied on it, the spring gets stretched. The pointer moves on the scale showing the magnitude of the force.
Q 24 – What is a measure of static friction?
Ans. The force required to overcome friction at an instant an object starts moving from rest is a measure of static friction.
Q 25 – How does the friction depend on the nature of the surface?
Ans. Nature of the surface is one of the factors affecting friction. When we move on any surface, we apply a force to overcome the interlocking of the surfaces. Rough surfaces have a larger number of irregularities and greater frictional force.
Q 26 – What is a measure of sliding friction?
Ans. The force required to keep the object moving with the same speed is a measure of sliding friction.
Q 27 – Why is it easier to move the box already in motion than to get it started?
or
The sliding friction is slightly smaller than the static friction. Explain why?
Ans. When the box starts sliding, the contact points on its surface, do not get enough time to lock into the contact points on the floor. So, the sliding friction is slightly smaller than the static friction. Hence, it is somewhat easier to move the box already in motion than to get it started.
Q 28 – Why are lubricants applied between the moving part of a machine?
Ans. When lubricants are applied between the moving part of a machine, a thin layer is formed there and moving surfaces do not directly rub against each other. Interlocking of irregularities is avoided to a great extent and the movement becomes smooth.
Q 29 – In some cases, we deliberately increase friction. Explain why?
Ans. We need to increase friction in some cases like grooving in the sole of shoes, the treaded tyres of cars, trucks or other vehicles, using brake pads in the brake system of bicycles and automobiles. If we do not do so, we may slip on the floor or roads, vehicles may slip on the roads and they will not stop, our bicycles and automobiles will not stop, etc.
Q 30 – Explain increasing and decreasing friction with suitable examples.
Ans. In our daily life, in many cases we increase or decrease friction for our own convenience. Some examples
are given below:
Increasing friction:
- Soles of our shoes are grooved to provide the shoes better grip on the floor so that we can move safely. Similarly tyres of vehicles are treaded to increase friction to provide better grip.
- We increase friction by using brake pads in the brake system of bicycles and automobiles.
- Gymnasts apply some coarse substance on their hands to increase friction for better grip.
- Kabaddi players rub their hands with soil for a better grip of their opponents.
- Roads are made rough to prevent from slipping.
- Handle of cricket and tennis are made of rough materials to get better grip.
Decreasing friction: In some situations, however, friction is undesirable and we would want to minimise it. For example:
- A few drops of oil are poured on the hinges of a door, the door moves smoothly.
- A bicycle and motor mechanic uses grease between the parts of these machines to reduce friction.
- By polishing surfaces, sprinkling powder on carrom board, we reduce friction.
- Wheel and ball bearings are used in machines and vehicles to reduce fric
Q 31 – What do you mean by fluid friction? How can fluid friction be reduced?
Ans. The gases and liquids are called fluid. All of them exert force of friction on solid bodies moving in either of them. The force exerted by fluid is called drag. It depends on speed of the object with respect to fluid.
It also depends on the shape of the object and the nature of the fluid.
It is obvious that when objects move through fluids, they have to overcome friction acting on them. In this process, they lose energy. Efforts are, therefore, made to reduce friction. So, the objects are given special shapes. Birds and fishes have to move about in fluids all the time. Their bodies must have evolved to shapes which would make them lose less energy in overcoming friction. Such shapes are called streamlined. This is why shapes of aeroplanes, ships and all vehicles are designed to have shapes which minimise fluid friction.
Q 32 – Cartilage is present in joints of our body, which helps in their smooth movement. If cartilage wears off, how would this affect the movement of joints?
Ans. Wearing off cartilage will make its surface rough and increase the friction. As a result the movement of joints will become difficult which may lead to joint pains.
Q 33 – On Sunday Razak invited everybody to play carrom in his house. While playing, striker was not moving properly from one place to another. Everybody thought it is their bad luck. But Razia, one of the player, quickly got up and sprinkled some talcum powder on the board.
- Why Razia sprinkled some talcum powder?
- Can you give some other example which serve the same purpose?
- What value of Razia is shown here?
Ans,
- Razia sprinkled some talcum powder to make the surface of carrom board smooth and hence to reduce friction.
- Friction can be reduced by using lubricants, ball bearings and painting the rough surface.
- Razia is an intelligent girl with scientific aptitude.
Q 34 – Ram and Shyam while returning from school saw a labour pushing hard to move a heavy box. Ram immediately ran to him and started helping him in pushing the box whereas Shyam brought two iron bars and asked the labour to put them below the box and then to push the box.
- What are the factors that affect friction?
- Who will help the labour more-Ram or Shyam? How?
- What values of Ram and Shyam are shown here?
Ans.
- Nature of surfaces in contact, weight of the object, force of adhesion and deformation.
- Shyam’s suggestion of keeping iron bar below the box will help the labour more. Putting the iron bar below the box will make it to roll over the bar. As rolling friction is less than sliding friction, it will help in moving the box with much easier.
- Ram and Shyam are helping in nature. Shyam is intelligent with scientific aptitude.
Q 35 – Imagine that friction suddenly vanishes. How would life be affected? List ten such situations.
Ans. If friction suddenly vanishes then:
- No moving object will come to rest.
- We will not be able to stand on a floor.
- Buildings will not remain intact at their places.
- We will not be able to make paintings and drawings.
- Nails and screw will not work.
- Vehicles will not stop by applying brakes.
- We will not be able to write on paper or blackboard.
- Vehicles on the road may slip.
- We will not be able to walk on the road.
- We will not be able to hold things.
Q 36 – Visit a shop which sells sports shoes. Observe the soles of shoes meant for various sports. Describe your observations.
Ans.
Visit a sports shop and observe different soles there. You will find that different sports shoes have different soles. For example,
- Running shoes: Such shoes are equipped with soles that provide cushion, stability, flexibility and traction.
- Cycling shoes: They have stiffer soles for efficient energy transfer when pedalling.
- Field sports: Soccer, baseball and football shoes are spiked or created with varying formations.
- Court sports: Tennis, volleyball and badminton shoes require durable and supportive roles with plenty of traction for making grip on the court. They provide maximum sock absorption.
Q 37 – 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?
Ans. Force of friction arises because of interlocking of irregularities on the two surfaces in contact. When a heavy object is placed on the floor, the interlocking of irregularities on the surfaces of box and floor become strong. This is because the two surfaces in contact are pressed harder. Hence, more force is required to overcome the interlocking. Thus, to push the heavier box, Seema has to apply a greater force than Iqbal.
Q 38 – Which is easier to hold in hand: a kulhar (earthen pot) or a glass tumbler?
Why?
Ans. It is easier to hold a kulhar (earthen pot) in our hand than a glass tumbler. This is because due to rough surface of kulhar, the friction between kulhar and our hand is much more which makes it easier to hold it. On the other hand, due to the smooth surface of glass tumbler, the friction between glass tumbler and hand is much less which makes it comparatively difficult to hold it.
Q 39 – Give examples to show that friction is both a friend and a foe.
Ans. Advantages of friction:
- Friction enables us to walk.
- Friction enables us to write with pen or pencil
Disadvantages of friction:
- Friction wears away the soles of our shoes.
- The tyres of vehicles wear out gradually due to friction.
Q 40 – 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?
Ans. We are able to walk because of the friction present between our feet and the ground. In order to walk, we push the ground in the backward direction with our feet. The force of friction pushes it in the forward direction and allows us to walk. The force of friction between the ground and feet decreases when there is soapy water spilled on the floor. Hence, it becomes difficult to walk on the soapy floor.