Page 128 - Understanding NCERT Science 09

James Prescott
Joule was an
o u t s t a n d i n g
British physicist.
He is best known
for his research in
electricity and
thermodynamics.
Amongst other
things, he
formulated a law
for the heating
James Prescott Joule
(1818 – 1889) effect of electric
current. He also
verified experimentally the law of
conservation of energy and discovered
the value of the mechanical equivalent
of heat. The unit of energy and work
called joule, is named after him.
10.2.2 KINETIC ENERGY
Activity _____________10.6
• Take a heavy ball. Drop it on a thick
bed of sand. A wet bed of sand would
be better. Drop the ball on the sand
bed from height of about 25 cm. The
ball creates a depression.
• Repeat this activity from heights of
50 cm, 1m and 1.5 m.
• Ensure that all the depressions are
distinctly visible.
• Mark the depressions to indicate the
height from which the ball was
dropped.
• Compare their depths.
• Which one of them is deepest?
• Which one is shallowest? Why?
• What has caused the ball to make a
deeper dent?
• Discuss and analyse.
Activity _____________10.7
• Set up the apparatus as shown in
Fig. 10.5.
• Place a wooden block of known mass
in front of the trolley at a convenient definition, we say that the kinetic energy of a
fixed distance.
• Place a known mass on the pan so
that the trolley starts moving.
WORK AND ENERGY
Rationalised 2023-24
Fig. 10.5
• The trolley moves forward and hits the
wooden block.
• Fix a stop on the table in such a
manner that the trolley stops after
hitting the block. The block gets
displaced.
• Note down the displacement of the
block. This means work is done on the
block by the trolley as the block has
gained energy.
• From where does this energy come?
• Repeat this activity by increasing the
mass on the pan. In which case is the
displacement more?
• In which case is the work done more?
• In this activity, the moving trolley does
work and hence it possesses energy.
A moving object can do work. An object
moving faster can do more work than an
identical object moving relatively slow. A
moving bullet, blowing wind, a rotating wheel,
a speeding stone can do work. How does a
bullet pierce the target? How does the wind
move the blades of a windmill? Objects in
motion possess energy. We call this energy
kinetic energy.
A falling coconut, a speeding car, a rolling
stone, a flying aircraft, flowing water, blowing
wind, a running athlete etc. possess kinetic
energy. In short, kinetic energy is the energy
possessed by an object due to its motion. The
kinetic energy of an object increases with its
speed.
How much energy is possessed by a
moving body by virtue of its motion? By
body moving with a certain velocity is equal to
the work done on it to make it acquire
that velocity.
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