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(b) decrease friction

A ball bearing is used to decrease friction.

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(a) Smooth

Lubricants reduce friction by making the surface smooth.

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(d) increases; increase

Frictional force increases with increase in roughness.

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(a) limiting friction

Limiting friction is the friction that comes into play when a body is on the verge of moving over the surface of another body.

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(a) dynamic friction

When two bodies slide over each other's surface, dynamic force comes into play.



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We need frictional force to walk on any surface. Because marble itself has a smooth surface, it will offer less friction. The presence of soapy water will further reduce the friction by making the surface smoother. Thus, it will be very difficult to walk on the marble floor when it has soapy water on it.

Page No 187:

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Yes, water offers frictional force to the movement of a boat; however, the friction offered by water is too less compared to that offered by the ground.

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'B' will apply greater force because he is pushing the body having great weight. Because friction varies directly with the weight of a body, the body having greater weight will offer more frictional force. Hence, he will have to apply larger force against this frictional force to push the body.

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No, we cannot have any surface having zero friction. However, we can have a surface having friction near to zero. Friction is due to the presence of irregularity or roughness on a surface. Every surface have some roughness or irregularity. We can reduce the roughness, but we cannot completely eliminate it.

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Frictional force does not depend on the surface area. It depends on the nature of the surface that is in contact with another object's surface.

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In case of rolling motion, the part of the wheel that is in contact with the floor has zero velocity, while the centre of the wheel keeps on moving without being in touch with the ground. When an object slides, its whole surface is continuously in contact with the ground. Therefore, in case of rolling motion, friction is less than sliding. 

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Situations where friction needs to be increased:
(a) Friction is desirable when we move our vehicles on the road. Tyres of vehicles are designed in a way that they can increase the friction, thereby increasing their grip on the road.
(b) Soles of shoes are also designed in a way that they can increase the friction and prevent slipping.

Situations where friction needs to be reduced:
(a) In machines - In machines, a large amount of energy gets wasted in the form of heat due to friction. Thus, machines are regularly oiled or lubricated to ensure the smooth functioning of their parts.
(b) For motion of vehicles - Friction offered by air or water produces hindrance in the movement of ships, submarines and airplanes. Thus, they are given a streamlined shape to reduce this friction and to allow their smooth movement.

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When we apply lubricants between any two surfaces, they fill the minute irregularities between the surfaces and form a thin layer in between those surfaces. This layer offers very little resistance, as a result of which friction gets reduced.

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Yes, air exerts friction on the object moving through it.

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Friction will be higher in the second case when the bodies are rubbed against each other with a greater force. Friction is a self-adjusting force; it increases with the increase in the force being applied.

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In both the cases, he will have to apply the same force. Frictional force is independent of the area of contact. It depends on the weight of the body. In both the cases, the same amount of frictional force will act because of the same weight of the body. Therefore, he will have to make an equal effort to make the body move.

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(a) streamlining

A streamline shape is designed to reduce frictional force.

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1. The friction that exists between a surface sliding over another is called dynamic friction.
2. A frictional force is an example of a contact force.
3. A frictional force always acts to oppose the motion of a body.
4. Limiting friction is independent of the area of contact.
5.  Friction  is a self-adjusting force.

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Column A Column B
1. Frictional force is (d) an example of contact force
2. Force of friction is (a) self-adjusting
3. Rolling friction is (b) less than kinetic friction
4. Limiting friction is independent of (c) area of contact
 

Page No 187:

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1. True

2. True

3. True

4. False
    Static friction is more than rolling friction.

5. True

6. False
    Frictional force depends on the nature of surfaces in contact.



Page No 188:

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It is difficult for a person to move on sand, as sand offers great frictional force. Frictional force varies directly with the weight of the body; therefore, greater weight of the body further increases the friction.

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Rubber offers greater friction than iron. Rubber tyres hold onto the ground better than iron tyres; that is why they are easy to control. Therefore, vehicles with rubber tyres are easier to ply than those with iron tyres.

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A rope made of jute is easier to tie because jute ropes have a rough surface that offers larger frictional force than silk, which has a smooth surface.

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Graphite exists in the form of layers of hexagonal carbon molecules. These layers slide over each other because of less force of friction between them. Diamond, on the other hand, has a different structure; it exists in nature in the form of a crystal unlike graphite. Therefore, graphite can be used as a lubricant but a diamond cannot. 

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Frictional force is an opposing force that comes into play when a body tries to move over the surface of another body.

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Yes, frictional force is a contact force because it acts when two bodies come in contact with each other and try to move over each other's surface.

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Limiting friction is a frictional force that comes into play when a body is on the verge of moving over the surface of another body.

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Rolling friction is the friction that comes into play when a body rolls over the surface of another body.

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Lubricants are used to make a surface smooth so as to reduce friction.

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Streamlining means shaping an object in such a way that it encounters least friction due to air or water.

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Ships and aeroplanes are given streamlined shapes to reduce friction.

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Friction is an opposing force that comes into play when a body tries to move over the surface of another body. Frictional force arises because of the irregularities in the surfaces of interacting objects. When two bodies are in contact, the interlocking of irregularities occurs, which causes friction between them.



Page No 189:

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Yes, friction is a self-adjusting force. It increases proportionally with increase in applied force, but only up to a certain limit.  

The following are the two disadvantages of friction:
1. A large amount of energy gets wasted in the form of heat because of friction.
2. Friction can be a hindrance to the motion of an object.

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Sewing machine is often oiled to reduce the friction between its moving parts and to ensure that they could function smoothly. This also minimises the wear and tear of the machine.

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Painters use sand paper to polish doors so that the surface of the doors becomes smooth and shiny.

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Frictional force is a contact force. It comes into play because of the irregularities in the surfaces of interacting objects. More the surface area of one object in contact with the other, more is the friction between them. Further, the surface area of one object in touch with the other depends on the shape of the interacting objects. Thus, we can say that friction also depends on the shape of objects. For example, a round or spherical object such as a ball bearing creates lesser friction than a flat object. 

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The soles of older shoes are smoother and more prone to slipping than the newer ones. The force of friction acting between the soles of shoes and the ground smoothens the soles, thus causing the wear and tear of the shoes. Similarly, tyres of a vehicle become worn out after some time because of the friction between the tyres and the road.

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Air cushion prevents the moving objects from coming in contact with the solid surface. Thus, it helps in reducing frictional force to a great extent.

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Irregularities in a surface are caused by tiny hill- and valley-like structures on the surface of an object. When two objects interact with each other, these structures interlock and resist any relative movement among the interacting objects, thus producing friction. Thus, we can say that frictional force increases with increase in irregularities (roughness) of the surface. Irregularities of the surfaces give rise to frictional force.

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Frictional force increases with increase in the roughness of a surface. When we look at a rough surface through a microscope, we see that the rough surface has structures like tiny hills and valleys. Friction is caused by the interlocking of these irregularities in the surfaces of two interacting bodies. For a smoother or relative motion, these imperfections must be overcome. Roughness of the surface increases when these imperfections increase. Thus, friction increases with increase in roughness.

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Frictional force is required to move around. If there is no friction, we cannot step forward. Friction is also required for the movement of a vehicle on the ground. Frictional force is used in the brakes of a vehicle. A moving body cannot be brought to rest if there is no friction. It would be impossible to write with a pen or pencil on a paper without friction. Friction holds the screw in the wood.

Friction is also responsible for the wastage of energy, as it resists the movement of the parts of a machine. It generates heat in a machine and is also responsible for the wear and tear of machine parts. Natural disasters such as forest fires are a result of friction between the trees.

Despite so many disadvantages, friction is necessary in everyday life. Thus, we can say that it is a necessary evil.               

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Types of Friction

1. Static Friction - Static friction is the friction that comes into play between two surfaces in contact, but there is no relative motion between the objects. In this case, frictional force is more than the applied force.
2. Limiting Friction - Limiting friction is the frictional force that comes into play when a body is just on the verge of moving over the surface of another body.
3. Sliding or Dynamical Friction - Sliding friction is the friction that resists the movement of a body while it is sliding over the surface of another body.
4. Rolling friction - Rolling friction is the friction that comes into play when a body rolls over the surface of another body. Rolling friction is always less than sliding friction.

Page No 189:

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Friction is an opposing force that comes into play when a body tries to move over the surface of another body.

The following are the three methods by which friction can be minimised:
1. By using lubricants - Lubricants make surfaces smooth by filling the unevenness of two surfaces and by forming a very thin layer between the surfaces, as a result of which friction gets reduced.
2. By using ball bearings - Ball bearings reduce friction by making the body roll. Thus, it offers rolling friction, which is less than sliding friction.
3. By polishing - Polishing reduces the roughness by making the surface even.

Page No 189:

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Limiting friction is an opposing force that comes into play when a body is just on the verge of moving over the surface of another body.

The two uses of friction are:
1. Friction is required for writing with a pen or pencil on a piece of paper.
2. Friction is also required for lighting a matchstick.

The two abuses of friction are:
1. Friction causes loss of energy in the form of heat in machines and is also responsible for the wear and tear of machine parts.
2. Friction is a hindrance to motion of an object.

Frictional force is necessary for the movement of objects. If there is no friction, we cannot step forward. Friction is required for the motion of a bicycle on the ground. Frictional force is also required in the breaks of the bicycle. A moving object cannot be stopped in the absence of friction. It would be impossible to write with a pen or pencil on a piece of paper without friction. Friction holds a screw in the wood.
Friction also leads to the wastage of energy. It opposes the movement of machine parts and causes their wear and tear.   
Although the resistance to movement created by friction causes some problem, without friction, our life would be inconvenient. Hence, we can say that friction is a necessary evil.

Page No 189:

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Friction can be classified into the following types:

1. Static Friction - Static friction is the friction that comes into play between two surfaces in contact when there is no relative motion between them. In this situation, frictional force is more than the applied force.

2. Limiting Friction - Limiting friction is the frictional force that comes into play when one body is just on the verge of moving over the surface of another.

3. Sliding or Dynamic Friction - Sliding friction is the friction that comes into play when a body actually moves over the surface of another body.

4. Rolling Friction - Rolling friction is the friction that comes into play when a body rolls over the surface of another body. Rolling friction is always less than sliding friction.

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Friction is affected by the following factors:

1. Nature of the surface - Friction increases with the increase in the roughness of a surface. Rough surfaces have more irregularities, as a result of which they offer more friction.
2. Weight of the body - Friction depends on the weight of the body. Greater the weight of the body, more will be the frictional force.



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