ANGLE OF FRICTION


When one body moves over the surface of the other, the surface opposes the relative motion of the moving object. The opposing force is called frictional force in angle of friction.


The nature of two surfaces and the force with which the surfaces are pressed together.


The frictional force is not the same for all the surfaces. It depends on the roughness or smoothness of surfaces in contact. When the contact surfaces are smooth then the frictional force is very less. As the roughness increases, the frictional force is also increases.

FRICTIONAL FORCE DEPENDS ON THE FORCE WITH WHICH TWO SURFACES ARE PRESSED TOGETHER: As the force is caused by interlocking of irregularities on the two surfaces in contact. So, even those surfaces which appear very smooth, have a large number of minute irregularities on them.  When two surfaces of objects are pressed together, then  the force will increase. Because pressing together of two surfaces with greater force will increase the interlocking of irregularities.

TYPES OF FRICTION: There are three types of force

  • Static : This friction is produced between the two surfaces in contact with each other at the position of inactivity. It is not easy to overcome. This friction is more powerful than any other friction.
  • Sliding : During sliding of a body over a surface the resistance to the motion is called sliding friction. This frictional force is needed to maintain the same speed .
  • Rolling friction: The frictional force is active when a body rolls over the surface of another body. This friction is less than the sliding friction.

There are advantages and disadvantages , that is why it is a necessary evil.


  • We walk easily on the ground due to  this.
  • It enables us to fix the nails into the wall.
  • We can light a matchstick due to  this.
  • The brakes slow down or stop moving bicycle due to this. 
  • We can write with our pen due to  this. 
  • It enables us to climb a tree 
  • Due to this a belt drives machines in the factories
  • If there is no friction between tyres of vehicles and road, then the wheels would spin at the same place and the vehicles do not move 


Wears away of soles of our shoes take place due to the force

Wears of steps of staircases in foot over-bridges take place due to friction 

  • Heat produces due to  sliding force. 
  • The efficiency of machines gets reduced due to this force.


The soles of shoes are made up of grooves to increase the force: A long and shallow cut or depression in the surface of hard material is called  groove. In the soles of our shoes, sometimes we see that some grooves are made, these grooves increase the friction with the ground and shoes get better grip even on a slippery surface.

The tyres of vehicles are made up of treads to increase force: A series of pattern made into the surface of tyre is called tread. When a ground is wet, then the friction between ground and tyres of vehicle reduces. So, the tyres are designed with treads which provide a better grip with the ground.


With the help of polishing the surface: We sprinkle fine powder on the carom board and then we polish its surface to make it smooth so that the striker slides easily on the surface.

By applying lubricants like grease and oil to the rubbing surface: The substances which reduce friction are called lubricants.  In between the moving part of a machine if oil grease or graphite is given, a thin layer is formed there and moving surfaces can easily move without rubbing each other. Interlocking of irregularities is avoided to a great extent, thus reducing friction and movement becomes easier.

By attaching wheels:  In our daily routine, we usually see  some objects like attaches and other bags fitted with rollers. Because the sliding friction is converted into rolling friction which gives less resistance to the motion of luggage.


  • Qs1 Name the force helps to write by chalk on blackboard.

 Ans1 Frictional force helps to write by chalk on black board.

  • Qs2 It is difficult to move on a wet marble floor. Why?.

 Ans2 The frictional force is very less between our foot and wet marble floor. 

  • Qs3  List the parameters on which frictional force depends.

 Ans 3  Frictional force depends upon smoothness or roughness of surfaces and their mass. 

  • Qs 4 Name the type of surfaces that has more irregularities smooth or rough.

Ans4  Rough surface has more irregularities.

  • Qs5 A jar of a mixture becomes hot when it is run for few minutes, comment. 

Ans5  The jar of mixture becomes hot when it is run for few minutes due to friction.

  • Qs6 Will frictional force come into play, when a rain drop rolls down a glass window pane? 

Ans6  Yes, friction comes into play when two surfaces are in contact e.g. glass and water in this case. 

  • Qs7 Explain fluid friction in brief.

Ans7 When an object moves in a fluid, then the resistance to the movement of object exerted by fluid is called fluid friction.

  • Qs8 Which is the special shape of the objects moving in fluids?

 Ans8 Streamlined shape or spindle like shape is the special shape given to the objects moving in fluids.

  • Qs9  Liquids and gases are called fluid. Why? .

Ans9  Liquids and gases can flow and they do not have definite shape. So, they  are called as fluids.

  • Qs10  Fishes move easily in water. Why? 

 Ans10 Fishes have streamlined shape, so, they can move easily in the water.

  • Qs11  It is easier to drag a carpet from the floor, but it is difficult the carpet to drag when someone is sitting on it, explain the reason behind it. 

 Ans11 If the two surfaces are pressed harder, then the force of friction will increase. So, it is easy to drag a carpet when nobody is sitting on it but when a person is sitting on it, then it produces more friction. 

  • Qs12  Give a reason for calling friction as a necessary evil

Ans 12 Friction is a necessary evil because it causes a lot of wear and tear, but without friction we cannot do our daily work like walking, writing, holding etc.

  • Qs13  Mention the moment when the rolling friction comes into play. 

 Ans13  The rolling friction comes into play when one body rolls over the surface of another.

  • Qs14The pieces of luggage are fitted with rollers, why?

Ans14, In order to reduce single friction, pieces of luggage, are fitted with rollers so that even a child can pull the luggage easily by overcoming friction.

  • Qs15 Can we reduce friction to zero by polishing surface or using a large amount of lubricants? 

 Ans15  No, friction can never be entirely eliminated. Some irregularities are always there 

  • Qs16  If a body is moving on rough horizontal surface towards east, then in whuch direction the frictional force will act?

Ans16 Frictional force always act in the direction opposite to the direction of motion, so, it will act towards the west. 

  • Qs17  Why an arrow with a pointed tip can move faster in air than that with a flat tip? 

 Ans17 The arrow with a pointed tip suffers small air resistance due to its pointed shape, so it moves faster.

  • Qs18  State the reason, why do we slip when we step on a banana peel?
  • Ans18  As we step on a banana peel, then there is no grip between our foot and the banana peel due to very less friction. So, we slip accidentally. 
  • Qs19 Can we eliminate friction completely? 
  • Ans19 NO, we cannot eliminate friction completely because it is the characteristics property or molecular property of of material
  • Qs20 Why is it difficult to move on a wet marble floor?

Ans20 On a wet marble floor, frictional force between the foot and floor decreases making it slippery, thus we feel difficult to move on a wet marble floor.


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