Friction notes for class 8

1. The force acting along the two surfaces in contact which
opposes the motion of one body is called force of friction.
The force of friction is commonly called friction.
Friction always opposes the relative motion between the
two bodies in contact irrespective of the direction of motion.
When a person applies brakes to a moving bicycle, the
surface of brakes offers friction to the surface of wheel and
the bicycle stops because of friction.
The friction between any two surfaces is mainly caused
either due to interlocking of surfaces or due to adhesion
between two surfaces.
Friction Due to Interlocking of Surfaces
All solid surfaces are rough. The degree of roughness varies
from surface to surface.
When a body is pulled over another, these ‘hills’ and
‘valleys’ interlock with each other and oppose the relative
motion between two bodies. This gives rise to a frictional
force.
Thus, friction is due to the roughness of two surfaces in
contact.
Rougher surface offers more force of friction.
Friction Notes

2. Friction Due to Force of Adhesion
between Two Surfaces
Two rough surfaces when placed together meet only at
certain points, The atoms or molecules present at such
points of contact attract each other due to electrostatic
attraction. This attraction is called force of adhesion.
When one body is made to move over the other, the force
of adhesion opposes the motion. This force which opposes
the motion also gives rise to frictional force. So, the force
of adhesion between the two surfaces gives rise to
friction.
Factors Affecting Friction
i. On a horizontal surface, the force of friction is
directly proportional to the weight of the bodies
which moves. If the weight is increased, irregularities
between the surface got interlocked tightly due to
pressure and the surface offers more resistance or
friction.
ii. The force of friction depends on the nature of
surface in contact.

3. Activity to show that a rough surface
exerts greater frictional force than a
smooth surface
Make an inclined plane on a smooth floor, or on a table. You
may use a wooden board supported by bricks, or books. Put a
mark with a pen at any point A on the inclined plane. Now let a
pencil cell move down from this point. How far does it move on
the table before coming to rest? Note down the distance. Now
spread a piece of cloth over the table. Make sure that there are
no wrinkles in the cloth. Try the activity again. Repeat this
activity by spreading a thin layer of sand over the table.
Maintain the same slope throughout the activity.

4. Classification of friction
Friction can be static, sliding or rolling according to the state of
motion of the object.
Static Friction
The force required to overcome friction at the instant an object
starts moving from rest is a measure of static friction. For
example, the friction caused to move a stationary heavy box is
the measure of static friction. In fact, static friction comes into
play when a stationary object is moved.
Sliding (or Kinetic) Friction
The force of friction between the two surfaces in contact when
one of the just slides over the other is called sliding (or kinetic)
friction.
Rolling Friction
The force of friction between the two surfaces in contact when
one of them is rolling on the other is called rolling friction. For
example, the friction between a ball rolling on a floor is rolling

5. friction.
The force required to keep a body in motion is less than the
force required to start the motion. This is because the
irregularities on the surface of already moving object do not get
sufficient time to entangle with the irregularities of another
surface over which it is sliding. Therefore sliding (or kinetic)
friction is less than the limiting static friction. For example,
sliding a mat or a box over ground is easier than moving it
directly.Rolling friction is less than the sliding friction. It means
Static friction > Sliding friction > Rolling friction,

6. Friction: A Necessary Evil
Friction has both harmful and beneficial effects and hence it is
called a necessary evil. One can walk because ground offers
friction. In the case of less friction, walking becomes almost
impossible.
Advantages of friction
Friction enables us to:
1.Walk without slipping
2.Move a car without skidding
3.Apply brakes and slow down a moving car
4.Write and draw on paper
5.Pick up and hold things in our hands
6.Fix the nails in the wall

7. 7.Light a matchstick
8.Cut wood with a saw

8. Disadvantages of friction
1.Friction wears away the soles of our shoes
2.The tyres of vehicles wear out gradually due to friction
3.Friction wears out the rubbing machine parts
4.Friction wears out the brake pads of vehicles gradually
5.Friction wears out steps of staircases in buildings and foot over
bridges
6.Friction produces heat which may damage machines
7.Friction reduces the efficiency of machines
8.Friction slows down motion
Methods of Increasing Friction
1.Grooves are made in the soles to increase friction and prevent
slipping.
2.Treads are made in tyres of vehicles to increase friction and
prevent skidding of vehicles on wet roads.
3.Spike are provided in the shoes of players and athletes to
increase friction and prevent slipping.
4.Gymnasts apply some coarse substance on their hands to
increase friction for better grip.
5.Machine belts are made of special materials to increase friction
and drive machine wheels properly.
Methods of Decreasing Friction
1.Friction can be reduced by making surfaces smooth by polishing.

9. 2.Friction can be reduced by applying lubricants to the rubbing
surfaces.
3.Frictions can be used by using wheels to moving objects.
4.Friction can be used by using ball bearings to the moving parts.