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In everyday life, we see some objects at rest
and others in motion. Birds fly, fish swim,
blood flows through veins and arteries, and
cars move. Atoms, molecules, planets, stars
and galaxies are all in motion. We often
perceive an object to be in motion when its
position changes with time. However, there
are situations where the motion is inferred
through indirect evidences. For example, we
infer the motion of air by observing the
movement of dust and the movement of leaves
and branches of trees. What causes the
phenomena of sunrise, sunset and changing
of seasons? Is it due to the motion of the
earth? If it is true, why don’t we directly
perceive the motion of the earth?
An object may appear to be moving for
one person and stationary for some other. For
the passengers in a moving bus, the roadside
trees appear to be moving backwards. A
person standing on the road–side perceives
the bus alongwith the passengers as moving.
However, a passenger inside the bus sees his
fellow passengers to be at rest. What do these
observations indicate?
Most motions are complex. Some objects
may move in a straight line, others may take
a circular path. Some may rotate and a few
others may vibrate. There may be situations
involving a combination of these. In this
chapter, we shall first learn to describe the
motion of objects along a straight line. We
shall also learn to express such  motions
through simple equations and graphs. Later,
we shall discuss ways of describing circular
motion.
Activity ______________ 8.1
•
Discuss whether the walls of your
classroom are at rest or in motion.
Activity ______________ 8.2
•
Have you ever experienced that the
train in which you are sitting appears
to move while it is at rest?
•
Discuss and share your experience.
Think and Act
We sometimes are endangered by the
motion of objects around us, especially
if that motion is erratic and
uncontrolled as observed in a flooded
river, a hurricane or a tsunami. On the
other hand, controlled motion can be a
service to human beings such as in the
generation of hydro-electric power. Do
you feel the necessity to study the
erratic motion of some objects and
learn to control them?
8.1 Describing Motion
We describe the location of an object by
specifying a reference point. Let us
understand this by an example. Let us
assume that a school in a village is 2 km north
of the railway station. We have specified the
position of the school with respect to the
railway station. In this example, the railway
station is the reference point. We could have
also chosen other reference points according
to our convenience. Therefore, to describe the
position of an object we need to specify a
reference point called the origin.
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Chapter
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CHAP 8.pmd

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