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Q1. Why do we slip when we step on a banana peel?

Solution

We slip when we step on a banana peel because the inner side of banana peel being smooth and slippery reduces the friction between the sole of our shoe and the surface of road.

Q2. Will it be possible to cut wood without the force of friction? 

Solution

We are able to cut wood because there is friction between the saw blade and log of wood. If there were no friction between the saw blade and log of wood, then cutting of wood would not be possible.

Q3. Why do gymnasts apply a coarse substance to their hands?

Solution

Gymnasts apply a coarse substance to their hands for a better grip.

Q4. State with reasons whether the following statement is true or false: Friction is a necessary evil

Solution

True, friction is a necessary evil.   Because:(a) Friction wears out the rubbing machine parts.(b) It produces heat which may damage machines.(c) It results in wastage of energy and fuel. Thus even though it has undesirable effects on the mechanism involved, it is necessary for almost all the machines and their mechanisms to have friction.

Q5.  What is ball bearing? What is its use?

Solution

 Ball bearing is a device which consists of a ring of small metal balls. It is designed to make the moving parts of a machine to roll over each other rather than slide.

Q6. Enlist the disadvantages of Friction

Solution

Disadvantages of Friction  Friction produces heat in the moving parts of machines which causes wear and tear of the parts.  A lot of energy is wasted in overcoming friction.  It causes the wear and tear of the soles of the shoes.  It also causes the damage of various parts of machines and equipment.

Q7. Will it be possible to cut wood without the force of friction? Briefly explain.

Solution

We are able to cut wood because there is friction between the saw blade and log of wood. If there were no friction between the saw blade and log of wood, then cutting of wood would not be possible.

Q8. Comment on the special body shapes of most of the fishes.

Solution

Most of the fishes have streamlined body shape which helps them to move through water easily without facing much friction from water.

Q9.  What makes us write on the black board with a chalk?

Solution

 When we write with chalk on the black board, its rough surface rubs off some chalk particles which stick to the blackboard. This happens due to the friction between the surface of the black board and the chalk.

Q10.  A pencil eraser loses tiny pieces of rubber each time you use it. why does this happen?

Solution

When we use a pencil eraser, friction between the eraser and the paper rubs off some rubber particles from the eraser. Thus, the eraser loses tiny pieces of rubber due to friction.

Q11. Why there is much more frictional force on an object when it moves through water than when it moves through air?

Solution

There is much more frictional force on an object when it moves through water than when it moves through air because water is much more viscous or thick than air.

Q12. Is large brake on a bicycle wheel more effective than a small one?

Solution

No, large brake on a bicycle wheel is not more effective than a small one because the force of friction is independent of the surface area in contact.

Q13.  Why do  tyres of vehicles have treads in them?

Solution

 The tyres of vehicles have treads in them to increase the friction between the tyre and the road and to prevent skidding.

Q14. Why is the handle of a cricket bat or a badminton racquet rough?

Solution

Cricket bat or a badminton racquet need to have a firm held grip for optimum usage in their respective games. Thus their handles are made in a manner in which they can be held without slipping. Friction increases with the roughness of the surface and thus to have an increased friction their handles are made rough.

Q15. When a wooden block is moving down an inclined plane what is the direction of the frictional force acting on the block? Explain

Solution

Frictional force is the force which tends to oppose the motion of the body. This force is directed opposite to the direction of the motion. Hence when a wooden block slides down an inclined plane obliquely, the frictional force acting on the body is directed in the exact opposite direction, i.e. obliquely upward in the same plane. 

Q16. Give one example from everyday life where wheels (or rollers) are used to reduce friction.

Solution

Heavy machines can be easily moved from one place to another by placing round logs of wood under them and then pushing with the force of hands. The round logs of wood act as rollers and make it easier to move the heavy machine kept on them.

Q17. Name two common lubricants.

Solution

 Grease and oil are two commonly used lubricants.

Q18. Name two common lubricants.

Solution

Grease and oil are two commonly used lubricants.

Q19. List down the most common methods to reduce friction?

Solution

The most common methods for reducing friction between moving surfaces in contact includes polishing the surfaces, lubricating the surfaces, using ball or roller bearings.However the compatibility between these methods and the mechanism of its application varies.

Q20. State any two methods of increasing friction.

Solution

 Methods of increasing friction:(a) Making the surface rough.(b) Adding weight to the object that is moving.

Q21. Explain why sportsmen use shoes with spikes.

Solution

Sportsmen use shoes with spikes so as to have a better grip while running. This is because the force of friction between the shoes and the ground increases with the help of spikes.

Q22.  Why is it difficult to light a matchstick by striking it on a smooth surface?

Solution

It is difficult to light a matchstick by striking it on a smooth surface because enough friction is not provided by a smooth surface to produce sufficient heat.

Q23. List any 2 advantages of friction.

Solution

Advantages of friction:(i) It enables us to walk on road.(ii) It enables us to write on paper.

Q24. Is friction a self adjusting force?

Solution

No, all types of friction are not self adjusting. Only static friction is self adjusting.

Q25. How does a matchstick light up? Explain.

Solution

When we rub a matchstick against the rough surface of a matchbox, then friction between the head of matchstick and rough side of matchbox produces heat. This heat burns the chemicals present on the head of matchstick due to which the matchstick lights up.

Q26. Our palms become hot when rubbed against each other. Why?

Solution

It is because friction takes place between the surfaces of our palms which produces heat.

Q27. Why winter tires usually have deeper treads than regular car tires?

Solution

Winter tires have deeper treads than regular car tires. This makes their surface rougher and help them get a good grip on snow and ice covered roads.

Q28. Why does a violinist put rosin on his bow?

Solution

A violinist puts rosin on his bow to increase friction between the bow and the violin strings, thereby producing sound.

Q29. Is frictional force a contact or non contact force. Explain

Solution

Frictional force is a contact force because it comes into play only when two surfaces are in physical contact with each other.

Q30. A pencil eraser loses tiny pieces of rubber each time you use it. Why does this happen?

Solution

When we use a pencil eraser, friction between the eraser and the paper rubs off some rubber particles from the eraser. Thus, the eraser loses tiny pieces of rubber due to friction.

Q31. What is skidding and how is it related with friction?

Solution

Skidding occurs when the friction available is less than that of friction required to hold moving bodies in contact.It is sliding of a body for eg a vehicle directed sideways or obliquely on a slippery ground.Skidding also occurs as a result of stopping or turning too quickly on surfaces having low friction.

Q32. What is the importance of grooves on tyres?

Solution

Grooves are designed cuts or depression usually present on rubber tyres. If the grooves on the tyres are not present or get weared off then the friction between the wheels and the contact surface decreases. This causes the tyres to easily slip or skid, thus making the vehicle accident prone. That is why it is important for the vehicles having rubber tyres to have proper grooves in them which maintains required amount of friction between the tyres and contact surface. 

Q33.  Why aeroplanes and ships are designed with special shaped bodies? What is such a shape called?

Solution

The aeroplanes and ships have streamlined shaped bodies to reduce drag. The streamlined shaped body experiences minimum resistance when travelling through water or air. It helps to reduce friction.

Q34. Why is wood polishing done with the help of a sand paper but not with an ordinary paper?

Solution

Wood polishing includes the additional chips and attached wood flakes on the woods to be removed to get perfect finishing. In order to trim those wooden chips it is required to treat it with roughness. Sand paper is preferred over ordinary paper because it provides adequate friction to the wood for proper finishing whereas friction provided by ordinary paper is too less to have an impact on the wooden surface.

Q35. Differentiate between sliding and rolling friction.

Solution

The frictional force that comes into action when an object slides over another surface is called sliding friction. The frictional force that comes into action when a roller rolls over a surface is called rolling friction.

Q36. What are the factors on which friction depends?

Solution

Friction depends on two factors:(i) Nature of the surfaces in contact.(ii) How hard the surfaces press together.

Q37. What is threading? Why is it done on screws?

Solution

Threading is a process in which the outer body of screws is cut in a sophisticated manner. Most common example of threading is spiral cut. The outer body of screws are cut spirally which acts as a guiding pathway for the screws to penetrate wherever required. Threading also plays an important role in maintaining the friction between the applied screw and the mechanism to which it is attached

Q38. Arrange the following scenarios in the decreasing order of the friction experienced : Ice skating Sea-surfing Swimming Sand walking

Solution

Sand walking

Swimming

Sea-surfing

Ice skating

Q39. How can we say that friction is an evil?

Solution

Friction is an evil because:(i) It produces heat which may damage machines.(ii) It results in wastage of energy and fuel.

Q40. Explain why sliding friction is less than static friction.

Solution

Friction is because of irregularities of two surfaces in contact. In sliding friction, time given for the interlocking the irregularities of the two surfaces is very small that is why sliding friction is very less than static friction.