NCERT Book Class 9 Science Chapter 11 Sound

C hapter 11
SOUND
Everyday we hear sounds from various
sources like humans, birds, bells, machines,
vehicles, televisions, radios etc. Sound is a
form of energy which produces a sensation
of hearing in our ears. There are also other
forms of energy like mechanical energy, light
energy, etc. We have talked about mechanical
energy in the previous chapters. You have
been taught about conservation of energy,
which states that we can neither create nor
destroy energy. We can just change it from
one form to another. When you clap, a sound
is produced. Can you produce sound without
utilising your energy? Which form of energy
did you use to produce sound? In this Fig. 11.1: Vibrating tuning fork just touching the
chapter we are going to learn how sound is suspended table tennis ball.
produced and how it is transmitted through
a medium and received by our ears.
Activity _____________ 11.2
11.1 Production of Sound • Fill water in a beaker or a glass up to
the brim. Gently touch the water surface
with one of the prongs of the vibrating
Activity _____________ 11.1 tuning fork, as shown in Fig. 11.2.
• Take a tuning fork and set it vibrating • Next dip the prongs of the vibrating
by striking its prong on a rubber pad. tuning fork in water, as shown in Fig.
Bring it near your ear. 11.3.
• Do you hear any sound? • Observe what happens in both the
• Touch one of the prongs of the vibrating cases.
tuning fork with your finger and share • Discuss with your friends why this
your experience with your friends. happens.
• Now, suspend a table tennis ball or a
small plastic ball by a thread from a
support [Take a big needle and a
thread, put a knot at one end of the
thread, and then with the help of the
needle pass the thread through the
ball]. Touch the ball gently with the
pr ong of a vibrating tuning
fork (Fig. 11.1).
• Observe what happens and discuss
with your friends. Fig. 11.2: One of the prongs of the vibrating tuning
fork touching the water surface.