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6.2 Friction
What is friction?
 Friction acts on an object which slides
or tends to slide over another object.
What is the effect of
friction on our lives?
e.g. The car cannot
travel without friction.
6.2 Friction
How can we measure the
friction between a sliding
block and a table?
Pull the block with a
spring balance
6.2 Friction
pulling force
friction
When the pulling force is small,
pulling force = friction
The block does not move.
6.2 Friction
pulling force
friction
Increase the pulling force such that
maximum friction between
pulling force =
the block and the table
The block still does not move.
6.2 Friction
pulling force
friction
Increase the pulling force such that
maximum friction between
pulling force >
the block and the table
The block moves.
Simulation: Friction
6.2 Friction
 Friction depends on the nature of the
sliding surfaces.
tyre
brake pad
Rubber is used in tyres and brake pad on a
bicycle
It can exert a large friction.

6.2 Friction
 Special carved patterns help to
increase friction.
The grooved pattern underneath a hiking shoe
6.2 Friction
Reducing friction
Can you give daily
examples of reducing
friction?
Lubricants used
in bicycle gears
Rolling motion in
ball bearings
6.2 Friction
A levitated object experiences negligible friction
during motion.
An ‘air cushion ball’ game
A Maglev
6.2 Friction
Fluid friction
 Fluid friction – resistive force exerted on
a body moving in a gas or a liquid.
e.g. air resistance, water resistance
 speed of the object
fluid friction
Can you give daily
examples of reducing
fluid friction?
6.2 Friction
Airplanes are in
streamlined shapes to
minimize air resistance.
Swimmer wear
smooth suit to reduce
water resistance.
6.2 Friction
Terminal velocity
EXTENSION
When a skydiver just jumps out of an aircraft …
velocity = 0
force acting on him:
weight
net force = weight
weight
acceleration = g
(downwards)
6.2 Friction
When he moves faster...
velocity
air resistance
force acting on him:
weight (constant)
air resistance
weight
(
, < weight)
net force = weight – air resistance (
acceleration
)
6.2 Friction
Air resistance continues to increase such that
air resistance
weight
air resistance = weight
net force
= weight – air resistance
=0
acceleration = 0
he moves at a
constant velocity
Terminal velocity
Simulation: Skydiver
6.2 Friction
 Terminal velocity depends on the
(1) weight of the falling object
heavy object
large
gravitational
force
large
terminal
velocity
(2) surface area of the falling object.
object with
large surface
area
large air
resistance
small
terminal
velocity
6.2 Friction
Why can a parachute
make the skydiver land
safely?
A parachute has a
large surface area
Terminal velocity of the
skydiver
6.2 Friction
That’s the end of Section 6.2
Check Point
Key Ideas
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