We
have already known about rest, motion, displacement, velocity, acceleration
etc. We
can
see different types of objects around us. Some of them are in rest and some of
them
are
in motion. Among the bodies in rest, there are chair, table, houses, log of
wood etc.
Can
a body in rest move itself? Tonight your reading table is in a certain place.
Will it be
in
the same place in the next morning? What do we observe from these practical
experiences?
It is observed that the bodies at rest are still at rest. These bodies can’t
move
by themselves. Suppose that one of your friends is riding a bi-cycle on a plane
road.
At any moment he stopped paddling the cycle. Will the cycle stop at once? We
find
that the cycle will stop slowly after crossing a small distance. Would the
cycle move
continuously
if there is no air resistance and friction?
From
this evidence we can understand that everybody has the tendency to remain in
the
same
state as it is. If the body is at rest it continues to be at rest. On the other
hand if the
body
is in motion it continues to be in motion. This tendency or property of a body
to
maintain
its own state is called inertia. Thus the tendency of a body to maintain its
own
state
for ever in which the body is at present or the property of a body to preserve
that
state
is defined as inertia.
The
inertia of a body depends on its mass. That is, mass is the measure of its
inertia. The
more
the mass, the more the inertia of a body. In other words, it is difficult to
move,
increase
or decrease velocity or change the direction of velocity of a body whose
inertia
is
large.
Do yourself
• Keep a pen and a book on a table. Now
strike the pen with finger. What do you
observe?
The pen moved a small distance on the table.
• Now repeat this procedure for the book.
The book won’t move. Now push the book
with
your hand. The book will be displaced from its original position.
Between
the pen and the book, it needs more effort to move the book as the mass i.e.
the
inertia
of the book is greater.
Example of inertia
If
a bus at rest suddenly starts moving the passengers lean backwards. The reason
behind
this
is inertia. When the bus is at rest, the bodies of the passengers are also at
rest. When
the
bus starts moving, the part of the passenger’s body attached to the bus also
moves
with
it. But the upper part of the body tends to remain stationary due to inertia of
rest.
Thus
the upper part of the body lags behind with respect to the lower part. As a
result the
passenger
inclines backwards. On the other hand, when a sudden brake is applied in a
moving
bus the passengers lean forwards. When a bus is in motion, the passenger of the
bus
is also in the same speed of the bus. When the bus stops suddenly, the lower
part of
50
Physics
the
body also becomes stationary with the bus. But the upper part of the
passenger’s
body
moves forward due to inertia of motion.
During
car driving the drivers wear seat belt for safety. What is the reason? The
reason
behind
this is inertia. If he doesn’t use seat belt, he would lean forward due to
inertia of
motion
for applying fast brake. For this reason he would hit the steering and other
bodies
in
front of him, for this a serious accident may happen. Not only have the
drivers, the
passengers
also had to wear the seat belt if there is an arrangement of seat belt in the
car.
We
have a general concept about force from our daily life experiences. When we
pull or
push
a body, then we say a force is applied upon the body. This applied force can
create
or
tends to create motion in a stationary body. If the body is in motion, then the
applied
force
can cease the motion of the body or tries to increase its velocity. In both
cases to
apply
force, it needs direct contact between the force applier and the body. These
types
of
force are known as contact force. If the two bodies are not in direct contact,
still they
can
apply force on each other. Such a well-known force is the force of gravitation,
whose
practical example is the weight of a body. When a body falls from your hand,
then
it quickly touches the ground. What is the reason? It happens due to the weight
or
this
way we feel the existence of different type of forces in different events of
nature.
For
example- the magnetic force between two poles of a magnet, the electrical force
between
two charged bodies, nuclear force within the nucleus.
Now
we will see how we can get concept about inertia and force from Newton’s first
law
of motion. Newton’s first law of motion is-
‘Every
object will continue in its state of rest or of uniform motion in a straight
line
unless
an external force is applied to it.’
Newton’s
first law expresses the property of inertia of matter.
From
Newton’s first law of motion we observe that a body cannot change its state on
its
own.
If the body is at rest, it tends to remain at rest forever and if it is in
motion it tends
to
keep on motion with uniform speed for all time. This property of a body is
termed as
inertia.
Thus from the Newton’s first law of motion we get the concept of inertia.
Again
from Newton’s first law we see that to change the state of a body something
external
must be applied. That is, the external cause which changes or tends to change
the
state of an object is called force. Thus from Newton’s first law we get
qualitative
definition
of force. According to Newton’s first law, a force is that which acting on a
body
at rest produces or tends to produce motion or acting on a moving body changes
or
tends to
change its state of motion.
