Friction - Introduction

1. (PLEASE WATCH THE VIDEO BEFORE GOING THROUGH THIS PRESENTATION.)
• Whenever an object moves, or tends to move
,over the surface of another object, there is a
force acting between the two surfaces in
contact. This force is called Friction or Force
of Friction.
• It always opposes or tends to oppose the
relative motion between two surfaces in contact.
• It is a Contact Force and is always directed
along the surfaces in contact i.e.it acts in
tangential direction.
FRICTION

2. Ex- *A ball rolling on ground gradually slows down and comes to rest due to force of
friction between the ball and the ground.
*If we stop pedalling a bicycle, it gradually slows down and comes to stop due to
force of friction between the wheel and the road.

3. AIM : To explore the relation between friction and nature of the surfaces in contact.
PROCEDURE : As discussed in the video.
OBSERVATIONS: On which surface does the object move with greater ease ?
S.No Type of surfaces Distance covered by the object
(cm)
1.Floor
2.Newspaper
3.Polythene sheet
4.Crushed Aluminium foil
5.Hand Towel
CONCLUSION: The more the roughness between surfaces, less is the distance that the object covers and
more is the friction.

4. CAUSE OF FRICTION
• Friction is associated with roughness of the surfaces in contact.
• All surfaces have roughness in them.
• Rough surfaces have large irregularities (ups and downs ) in them.
• Even smooth looking surfaces have minute irregularities when seen under
powerful microscope.
• When two surfaces come in contact, the irregularities on one surface get
interlocked with the irregularities on the other surface (which is the basic cause
of friction). So, we have to apply a force to unlock this interlocking of two
surfaces and thereby, enable them to move with respect to each other.
Area of actual contact

5. FACTORS AFFECTING FRICTION
Friction depends on two factors. They are:
1. NATURE OF TWO SURFACES IN CONTACT.
More will be the roughness between two surfaces, more will be the friction.

7. 2. FORCE PRESSING THE TWO SURFACES TOGETHER.
More will be the force (weight) pressing the two surfaces together, more will be the friction.

8. AIM: To observe that Force of Friction increases with increase in the weight of the body.
PROCEDURE : *Tie a string around a wooden block. Pull the block by a spring balance as shown in the figure.
Note down the reading on the spring balance.
*Now keep a book on the block. Again pull the block by the spring balance. Note down the reading on spring
balance.
OBSERVATIONS: In which case the reading of the spring balance is more ?
In the SECOND CASE, reading on spring balance would be more.
CONCLUSION : When a body moves over a horizontal surface, it presses down against the surface by a force
equal to its ‘weight’. So, with increase in the weight of the body the force of friction will increase .

11. PART A. Fill in the blanks:
1. Friction always opposes any relative motion
between the two surfaces that are in contact with each
other.
PART B. State True or False :
1) When a body slides over smooth and wet
surfaces, the amount of interlocking of irregularities,
of the two surfaces in contact , increases. False
2) If a car moves eastwards, the force of friction acts
southwards. False
3) Friction depends on how hard the two surfaces
press against each other. True
PRACTICE QUESTIONS:

12. PART C.
6. A block is sliding on a horizontal surface. The
force of friction between the two can be increased by:
a) Decreasing the area of contact of the block with
the surface.
b) Applying a layer of oil on the surface.
c) By polishing the surface and making it smooth.
d) By putting a second identical block on top of the
given block.
Answer: d)
PART D.
3. State the likely cause of Friction.
Answer: Friction is caused by the interlocking of
irregularities of the two surfaces in contact.

13. PART E.
1.Define Force of Friction. List the factors affecting the force of friction. Explain with examples.
Answer: Whenever an object moves, or tends to move, over the surface of another object, there is
a force acting between the two surfaces in contact. This force is called Force of Friction. It always
opposes or tends to oppose the relative motion between two surfaces in contact.
Factors affecting the force of friction are:
1. NATURE OF TWO SURFACES IN CONTACT.
More will be the roughness between the two surfaces, more will be the friction. Ex- a ball rolling on
a marble floor will travel more distance as compared to a ball rolling on a road. This is because,
the road has a rough surface, so friction between the road and the ball will be more.
2. FORCE PRESSING THE TWO SURFACES TOGETHER.
More will be the force (weight) pressing the two surfaces together, more will be the friction. Ex- It
is easier to push a lighter box than a similar heavy box on the same floor, because to move the
heavier box, a greater force is required as friction will be more.

14. 4.Give reasons for the following:
a) We tend to slip when we step on a banana peel.
We tend to slip when we step on a banana peel because the inner surface of
banana peel is smooth and slippery, which reduces the friction between the feet
and the ground.
b) It is easier to push a lighter box than a similar heavy box on the same floor.
Friction increases with the increase in weight of the object. So, to move the
lighter box, a lesser force is required. However to move the heavier box, a
greater force is required as friction will be more.
Visit the link and enjoy the video:
https://www.youtube.com/watch?v=cMDV-uhfR9A