Page 121 - Understanding NCERT Science 09

When a body is immersed fully or partially Archimedes’ principle has many
in a fluid, it experiences an upward force that applications. It is used in designing ships and
is equal to the weight of the fluid displaced submarines. Lactometers, which are used to
by it. determine the purity of a sample of milk and
Now, can you explain why a further hydrometers used for determining density of
decrease in the elongation of the string was liquids, are based on this principle.
not observed in activity 9.7, as the stone was
overflowed when he stepped Q on a weighing machine. Is your
fully immersed in water? uestions

Archimedes was a Greek scientist. He 1. You find your mass to be 42 kg
discovered the principle, subsequently named 2. You have a bag of cotton and an
mass more or less than 42 kg?
after him, after noticing that
the water in a bathtub
of 100 kg when measured on a
into it. He ran through the iron bar, each indicating a mass
streets shouting “Eureka!”, weighing machine. In reality,
which means “I have got it”. one is heavier than other. Can
This knowledge helped him to you say which one is heavier
determine the purity of the and why?
Archimedes
gold in the crown made for
the king.
His work in the field of Geometry and
Mechanics made him famous. His
understanding of levers, pulleys, wheels-
and-axle helped the Greek army in its war
with Roman army.

What

you have

learnt

• The law of gravitation states that the force of attraction
between any two objects is proportional to the product of
their masses and inversely proportional to the square of
the distance between them. The law applies to objects
anywhere in the universe. Such a law is said to be universal.
• Gravitation is a weak force unless large masses are involved.
• The force of gravity decreases with altitude. It also varies
on the surface of the earth, decreasing from poles to the
equator.
• The weight of a body is the force with which the earth
attracts it.

• The weight is equal to the product of mass and acceleration
due to gravity.
• The weight may vary from place to place but the mass stays
constant.

110 SCIENCE

Rationalised 2023-24