NCERT Book Class 9 Science Chapter 7 Motion

C hapter 7
MOTION
In everyday life, we see some objects at rest Activity ______________ 7.1
and others in motion. Birds fly, fish swim,
• Discuss whether the walls of your
blood flows through veins and arteries, and
classroom are at rest or in motion.
cars move. Atoms, molecules, planets, stars
and galaxies are all in motion. We often Activity ______________ 7.2
perceive an object to be in motion when its
• Have you ever experienced that the
position changes with time. However, there train in which you are sitting appears
are situations where the motion is inferred to move while it is at rest?
through indirect evidences. For example, we • Discuss and share your experience.
infer the motion of air by observing the
movement of dust and the movement of leaves Think and Act
and branches of trees. What causes the
We sometimes are endangered by the
phenomena of sunrise, sunset and changing
motion of objects around us, especially
of seasons? Is it due to the motion of the
if that motion is erratic and
earth? If it is true, why don’t we directly uncontrolled as observed in a flooded
perceive the motion of the earth? river, a hurricane or a tsunami. On the
An object may appear to be moving for other hand, controlled motion can be a
one person and stationary for some other. For service to human beings such as in the
the passengers in a moving bus, the roadside generation of hydro-electric power. Do
trees appear to be moving backwards. A you feel the necessity to study the
person standing on the road–side perceives erratic motion of some objects and
the bus alongwith the passengers as moving. learn to control them?
However, a passenger inside the bus sees his
fellow passengers to be at rest. What do these 7.1 Describing Motion
observations indicate? We describe the location of an object by
Most motions are complex. Some objects specifying a reference point. Let us
may move in a straight line, others may take understand this by an example. Let us
a circular path. Some may rotate and a few assume that a school in a village is 2 km north
others may vibrate. There may be situations of the railway station. We have specified the
involving a combination of these. In this position of the school with respect to the
chapter, we shall first learn to describe the railway station. In this example, the railway
motion of objects along a straight line. We station is the reference point. We could have
shall also learn to express such motions also chosen other reference points according
through simple equations and graphs. Later, to our convenience. Therefore, to describe the
we shall discuss ways of describing position of an object we need to specify a
circular motion. reference point called the origin.