of
your school. If you say your school is in Jheeltuli it is absolutely true but
we cannot
know
the exact position of your school from this information. We can know the exact
position
only when you mention the position, direction and distance of your school with
respect
to a particulars place. At first we have to consider a known point or an object
with
respect to that the position of another point or object is to be determined.
For
example,
if said your school is one kilometer apart in the east from the gate of your
residence,
its position can be told certainly. We have to consider a particular point to
find
out the position of our village or town, country or the world or the universe
around
us.
This point is called reference point or origin and fixed object with respect to
which
we
find out the position, rest and motion of another object is known as reference
frame.
Any
point can be considered as reference point for our convenience. In the above
example
we could consider any other point as reference point.
Rest and motion : We see many object around us of them some are rest and some
are in
motion.
What do we actually mean by rest and moving object.
What
is there around the pen ? Does the position of the pen get changed with respect
to
other
objects around ? It does not. The pen in your has a particulars distance and
direction
with respect to the objects around you such as your chair, table, book, copy,
house,
door, window and to all other things. So the position of your pen is fixed with
respect
to other objects around you. The position of the pen is not changing with the
change
of time. We can say the pen is in rest to the surroundings. The act of remain
rest
of
the pen is called rest. Therefore, a body is said to be static or at rest with
respect to its
surroundings
when it does not change its position with time.
Ani
is standing by the side of the road. He said that, houses, plants and trees,
electric
poles
etc are standing at rest. Why does he say so ? According to Ani those objects
are
not
changing their positions with respect to time.
Does
the position of the pen change with respect to the objects around, the position
and
direction
of the pen is gradually changing with respects to each object of its
surrounding.
The
position of the pen is changing with time. We say the pen is in motion with
respect
to
the surroundings. A body is said to be in motion with respect to its
surroundings when
it
changes its position with time. And this change of position with time is called
motion.
We
discussed earlier that to understand whether an object is in rest or in motion
it is
necessary
to chose a reference object or reference frame. If the relative positions of
this
Do by yourself : Hold a pen in your hand.
Do by yourself : Keep the pen in your hand moving to and fro.
28
Physics
considered
object and the reference object remain unchanged with time, the considered
object
is said to be in rest with respect to reference object. If the considered
object and
the
reference object move along the same direction with same velocity even though
the
distance
between the objects, remain unchanged despite the object, being in motion. If
two
friends remain seated face to face in a running train, then the relative
position of one
with
respect to other does not change. So one is said to have in rest with respect
to other.
When
a person observes these two friends standing by the side of the train line he
will
see
their position is changing with respect to him. That is both the two friends
are in
motion
with respect to the person standing by the side of the train line.
So
we see that an object is actually in rest or not depends on the reference
object. If the
reference
frame is actually in rest, object in rest will be : actually in rest with
respect to
that
frame. This type of rest is called absolute rest. A body is said to be in
absolutely rest
when
it is in rest with respect to an absolutely rest object.
Similarly
absolute motion of a body is its motion with respect to a reference object
absolutely
at rest. But in this universe it is not possible to get a reference object
which is
at
absolute rest. Since the earth is continuously moving round the sun, while the
sun
itself
is moving along the galaxy with its planets and satellites. Thus when we say
that a
body
is at rest or in motion, we mean it is to be so with respect to a body
apparently at
rest.
So we can say that in this universe all rest and all motion are relative. No
rest or no
motion
is absolute.
Mitu
is waiting for a bus at the bus stand to go somewhere. She notices her friend
Roni
crossing
her by rickshaw. She says that the rickshaw is in motion because it is changing
its
position continuously with respect to time.
