[responsivevoice voice=”Hindi Female” buttontext=”Listen to This Chapter”]

# Class – 11 Physics, Chapter 8, Friction

##### Frictional force :

It is contact force which resists the relative motion of solid surface, fluid layers and material elements sliding against each other.

Discussion :  In daily life, we observe that with a ball pain, we can not write on mirror whereas we can write with it on papers. It is very difficult to walk on the road covered with ice. The surface of tyre of vehicles are made rough. Rubbing both hands together to create heat. A sled sliding across snow or ice. Skis sliding against snow. A person sliding down a slide is an example of sliding friction. A coaster sliding against a table. A washing machine pushed along a floor.

##### Types of friction :

There are mainly three types of Friction.

1. Static friction
2. kinetic friction
3. rolling friction

Static friction :  Static friction is a force that keeps an object at rest. It must be overcome to start moving the object.

Kinetic Friction : Kinetic friction, also known as sliding friction or moving friction, is the amount of retarding force between two objects that are moving relative to each other.

Rolling Friction : When one body rolls or rotates over the surface of other body, then the friction come into play and this is called as rolling friction.

Angle of friction : The angle made by the resultant of normal reaction force and frictional force with the frictional force is called as angle of friction.

Angle of repose :  Angle made by the inclined surface and horizontal surface.

##### Moving Down Of an Object on an inclined plane :

Let

m = mass of the object

a = acceleration by which the object is moving down from the inclined plane

g = acceleration due to gravity

θ = angle of repose

mg = weight of the object

f = frictional force

R = normal reaction force on the object applied by the surface

Here, mgcosθ and mgsinθ are the rectangular components of mg

cosine component of mg balances the normal reaction whereas the sine component of mg forces the object to move down from the inclined plane with the acceleration ” a ”

mgcosθ = R ————————————- ( 1 )

According to problem

ma = mgsinθ – f ———————————– ( 2 )

we know that

f = μR

μ = coeffcient of kinetic friction

Putting the value of f in equation  ( 2 ) we get

ma = mgcos – μR

By using equation ( 1 ) we can write

ma = mgcosθ – μmgsinθ

##### a = g ( cosθ – μsinθ )

This the acceleration by which the object is moving down from the inclined plane.

##### Moving up of an object on an inclined plane :

Let

m = mass of the object

a = acceleration by which the object is moving up the inclined plane

g = acceleration due to gravity

θ = angle of repose

mg = weight of the object

f = frictional force

R = normal reaction force on the object applied by the surface

Here, mgcosθ and mgsinθ are the rectangular components of mg

cosine component of mg balances the normal reaction whereas the sine component of mg forces the object to move down from the inclined plane with the acceleration ” a ”

mgcosθ = R ————————————- ( 1 )

According to problem

ma – mgsinθ = f ———————————– ( 2 )

we know that

f = μR

μ = coeffcient of kinetic friction

Putting the value of f in equation  ( 2 ) we get

ma – mgcos = μR

By using equation ( 1 ) we can write

ma = mgcosθ + μmgsinθ

### Practice questions

#### ( For 1 marks )

1 ) Why are lubricants are used in machine ?

2 ) It is easier to roll than to pull a barrel along a road ——— Explain.

3 ) Friction is independent of area. But brakes of very small contact area are not used.—— why?

4 ) mention one factor on which the coefficient of friction depends.

5 ) Explain why a horse can not pull a cart and run in empty space ?

6 ) What is friction ?

7 ) Define angle of friction.

8 ) Give an example of use of friction.

9 ) What is the unit of coefficient of limiting friction ?

10 ) What do you mean by angle of repose ?

11 ) Give an example of contact force.

12 ) Define Static friction.

13 ) Define Kinetic friction.

14 ) What is rolling friction ?

15 ) What should be the direction of frictional force ?

16 ) What is the relation between the angle of repose and angle of friction ?

17 ) What is the relation between the normal reaction and frictional force for an object placed on the horizontal surface ?

18 ) What is the relation between coefficient of static and kinetic friction ?

19 ) What is the relation between angle of friction and coefficient of static friction ?

20 ) Why the tyres of a vehicles are made rough ?

#### ( For 2 or 3 marks )

1 ) Define and explain the term ” Frictional force. ”

2 ) There are how many types of frictional force ? Name them and explain each.

3 ) What will be the acceleration of a body of mass m sliding down an inclined plane if angle of repose is θ ?

4 ) Without friction between our feet and ground, it will nit be possible to walk ——– why ?

5 ) Wear and tear of the machinery is due to friction ————– why ?

6 ) Why is it easier to pull a body than to push ?

7 ) Mention two methods of reducing friction.

8 ) mention two methods of increasing friction.

9 ) Why a parachute descends slowly ?

10 ) Why sand is sometimes thrown on the railway tracks in rainy days ?

11 ) ” Friction is a self adjusting force ” ———– Is this statement is correct ? Explain the answer with reason.

12 ) What do you mean by ” dry friction ” and ” liquid friction ” or ” fluid friction ” ?

13 ) Write down the laws of sliding friction.

14 ) What do you mean by limiting friction ?  Give any two laws of limiting friction.

15 ) Derive the relationship between angle of friction and angle of repose.

[/responsivevoice]

### 1 Comment

#### Werner Vandis · June 19, 2019 at 2:41 pm

You completed a number of nice points there. I did a search on the issue and found a good number of people will agree with your blog.