Friction - Class 8 Physics - Extra Questions

Rolling reduces friction. It is always easier to roll than to slide a body over another. Since the rolling friction is smaller than the sliding friction, sliding is replaced in most machines by rolling by means of wheels and ball bearings.

Sprinkling of powder on the carom board reduces friction.
The carrom board may sometimes be very rough. Sprinkling powder on a microscopic level, the powder particles offer rolling friction to the striker because of which the striker can be used for smooth and fast play.

The saw and the wood get hot after cutting the wood this is only because of the friction between the two.

The box which has more mass will have more inertia of rest. Therefore, we need to apply more force to overcome that inertia.

Lubricants basically used to reduce friction. lubricant reduces irregularities between two surfaces so friction reduces.

It is difficult to write on the glass with a pencil because the friction on the glass is very less which creates difficulty in writing. On the other hand, since the paper has more friction it is much easier to write on paper as compared to glass.

Friction is the force that opposes motion between any two surfaces that come into touch with each other. The sliding friction and static frictions are a type of friction.

• Sliding friction: The sliding friction force opposes the movement of two sliding contact surfaces against each other. When the two bodies are moving in the same direction.
  
• Static friction: Friction between two or more solid objects that are not moving in respect to each other is referred to as static friction.
  
  
  Once this motion is started, the irregularities act just as an obstruction against their relative motion, they are not interlocked now.
  Therefore, it is easier to move the object when it is already in motion than to start it from rest.

Lubricant reduces wear and tear of the machine components and increases its life. Contact surfaces in the machine rub each other while running, when we add lubricant, it makes surface smoother, and reduces wear and tear.

Fluid friction is a force that acts between layers of fluid restricting the movement within itself. It also restricts the motion of an object moving through the fluid. The internal resistance to flow of fluid is known as viscosity.

Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction:

In fluid dynamics, drag is a force acting opposite to the relative motion of any object moving with respect to the surrounding fluid. This can exist between two fluid layers or between a fluid and a solid surface.

The force, which opposes the motion of one body over the other in contact with it, is called frictional force 

The eraser gets smaller and smaller as we use it more and more due to frictional force causing wear and tear of the eraser.

Solid - Graphite
Liquid - Oil

The handle of a cricket bat or a badminton racquet is rough, so that while playing, the bat or badminton racquet does not slip away from the hands of the player. Roughness is responsible for the frictional force between handle of the bat or a badminton racquet and'hands, without which gripping is not possible.

 If the seat cover of rickshaw is very smooth, then the friction between our body and the seat is very small. Therefore, when the brakes are applied, we tend to slip.

The sandpaper creates more friction between the metal sheet and it as compared to the plain paper. Therefore, the sandpaper makes the sheet shinier by removing the top layer of the metal sheet.

Friction is friend (Advantages) :
(i) Friction allows us to grip and catch different objects.
(ii) It helps us to walk comfortably on the surface.
(iii) It helps to minimise the speed or to stop the moving objects.
(iv) It helps us to write on paper or blackboard.
(v) The things do not move from their places due to friction.
Friction is foe (Disadvantages) :
(i) Friction causes wear and tear in objects.
(ii) It causes damage the parts of machines. 
(iii) The machines or tools require regular maintenance due to which a lot of money is wasted.
(iv)It reduces the speed of moving objects so more force is required.
(v)It does not allow the free movement of objects.

The body will slow down as a sandy surface produces more frictional force.

A wet surface offers less friction than a dry surface. 

(a) Friction opposes the motion between the surfaces in contact with each other.

$$\textbf{Explanation}$$

The opposing force that is caused due to relative motion between two surfaces in contact is called friction.

Force of friction.

• Friction is caused by the imperfections on the two surfaces in contact interlocking. The interlocking of imperfections on the surfaces of the box and the floor becomes strong when a heavy object is placed on the floor. This is due to the fact that the two surfaces in touch are forced together harder.
• If the two surfaces are forced harder against each other, the force of friction between them will grow. As a result, pushing the heavier box will require more force than pushing the lighter one. As a result, Seema must exert more force than Iqbal.
  

When the road is wet, the car sometimes can skidding due to smoothness of the surface. Thus, the roughness of the dry road can be more than that of wet road. We know that more rough surface will increase the friction force. Thus, wet road will have less friction force than that of dry road. 

As the parts of the bicycle like brakes, chain, wires gets old its wear and tear increases. Also, its parts get rusted due to continuous exposure to air. Due to the increase in wear and tear the friction increases as two parts come in contact with each other. By applying lubricants like oil reduces the wear and tear of parts which reduces friction. Due to a decrease in friction, the noise also reduces considerably as the contact between the parts becomes smooth.

Automobile tyres generally have irregular projections on their 

Causes of Friction:

Friction is a force that resists the relative motion between two objects or materials. The causes of this resistive force are molecular adhesion, surface roughness, and deformations. Adhesion is the molecular force resulting when two materials are brought into close contact with each other

Yes, Theoretically it is possible by making surface smooth. exp:- Ice surface, Highly polished surface. But in real life situation it can't be possible.

To increase friction : - Friction force can be increased by making the surface rough. For example, tyres of cars are made rough to increase the friction.

A friction force exist only when, there is tendancy of relative motion between objects in contect. In a given situation a block is simply kept on the table and the table is in rest so there is no tandancy of relative motion between book and table. hence no friction act. 

We push or pull a heavy object on a surface against the friction between object and the surface. The force of friction increases as the normal reaction of the surface on the object increases (Limiting i.e. maximum static friction, kinetic friction and rolling friction all are directly proportional to the normal reaction of the surface on the object). If surface is a level ground, this normal reaction in magnitude is equal to the weight of the object.

If a heavy object is pushed on a level road, the vertical component of the pushing force acts on the body in vertically downward direction and thus increases the effective weight of the object and therefore increases force of friction between object and the ground.

If the heavy object is pulled on a level ground, the vertical component of the pulling force acts on the object in vertically upward direction and thus reduces the effective weight of the object and therefore decreases friction between object and the ground.

We see that effective weight of the object and as a consequence friction between object and the ground in case of pulling are lower than those in case of pushing. So, it is easier to pull a heavy object than to push it on a level ground.

$${{F}_{r}}=frction\,force=\mu N$$

In pulling normal reaction in smaller, therefore friction is smaller and it easy to pull.

If we stop paddling our bicycle, it gradually slows down and ultimately it stops after travelling a certain distance. This is due to frictional force between bicycle and ground.

Answer: Wooden surface

Explanation:

• We know, higher the roughness of surfaces in contact with each other, greater is the friction between bodies. Force of friction will be less between smooth surfaces.

• Since, a wooden surface has more roughness compared to a glass surface, the body will experience greater frictional force on the wooden surface.

The examples to illustrate that friction is a necessary evil are:
1. If friction were absent, we would not be able to walk.
2. Friction is necessary to burn a matchstick.
3. It is due to friction that we can write on a board by a chalk.

When the hands are oily, then the oil acts as a lubricant and reduces the friction.
As the friction force is less, it is difficult to get a grip of the inkpot and it becomes difficult to open it.

• After rain, the road becomes wet, due to which the friction between the legs of man and the road is reduced.

• Due to this reduction in frictional force ,the foot of man and the road don’t get proper grip and the man slips sometimes.This is why it is difficult to walk on a wet road.

Solution:

Factors affecting force of friction

Two factors which directly affect the force of friction are as follows:

i. Roughness of the surfaces in contact with each other .

Higher the roughness of surfaces in contact with each other, greater is the friction between  bodies. Force of friction will be less between smooth surfaces.

ii. Weight of the sliding/ rolling body: Greater the weight of the moving body on the surface, more is the force of friction on the body by the surface.

In order to prevent their hands from slipping over their opponent's body. The powder increases the friction.

Smoother the surface, lesser is the frictional force offered by it.

Disadvantages of friction:
1. Friction produces heat which damages the moving parts of a machine.
2. Friction produces wear and tear on the contacting surfaces. This reduces the life of machine parts, tyres and shoe soles.
3. A lot of energy is wasted due to friction to overcome it before moving.,

Oiling or lubricating is a method to reduce friction.

Characteristics of force of friction are:
1. Friction opposes the direction of motion.
2. It depends on the nature of the surface that are in contact. Rough surfaces offer greater degree of friction as compared to smooth surface. More the roughness of a surface, greater is the friction.
3. Friction depends upon the weight of the object - greater is the weight of the object, greater is frictional force it faces.
4. Friction is independent of area of surface in contact, when the weight remains unchanged.

Ball bearings used to reduce friction between the pedals and the crank.

Ways by which friction can be increased are:
1. Tyres of all vehicles have grooved designs or treads to make their surface rough.
The roughness of a surface increases friction and prevents vehicles from slipping or skidding on a wet or muddy road.
2. Spikes in the shoe soles used by players and athletes increase friction and they get a firm grip on the group. The soles of footwear are also provided with grooved designs to increase friction and avoid slipping.
3. The handles of motorbikes are covered with spiked rubber to provide a good grip and have better control in holding the handles.

Absence of reaction force is the cause for this.

Measures to reduce friction:
1. Grease, oil, graphite etc. are used as lubricants in machines to reduce friction.
2. Ball bearings and roller bearing are used in vehicles and machines to reduce friction.
3. Oil is used to reduce friction in engines.
4. Powder is sprinkled on the board while playing carrom.
5. Vehicles are shaped keeping aerodynamics in mind so that friction with air can be reduced.

The lubricants spread over the irregularities on the surface that makes the contact. So, the contact between the lubricant and the moving objects reduces the friction. 

The property by virtue of which an opposing force is created between two bodies in contact, which opposes their relative motion is called friction.