ARCHIMEDES

Archimedes was a Greek
scientist. He discovered the                                                

                                                                                 

principle, subsequently
named after him, after
noticing that the water in a
bathtub overflowed when he
stepped into it. He ran
through the streets shouting
“Eureka!”, which means “I
have got it”. This knowledge helped him to
determine the purity of the 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, wheelsand-
axle helped the Greek army in its war
with Roman army

ARCHIMEDES PRINCIPLE

When a body is immersed fully or partially               
in a fluid, it experiences an upward force that
is equal to the weight of the fluid displaced
by it.

RELATIVE DENSITY

As you know, the density of a substance is
defined as mass of a unit volume. The unit of
density is kilogram per metre cube (kg m–3).
The density of a given substance, under
specified conditions, remains the same.
Therefore the density of a substance is one
of its characteristic properties. It is different
for different substances. For example, the
density of gold is 19300 kg m-3 while that of
water is 1000 kg m-3. The density of a given
sample of a substance can help us to
determine its purity.
It is often convenient to express density
of a substance in comparison with that of
water. The relative density of a substance is
the ratio of its density to that of water:
Density of a substance
Relativedensity =
Density of water

WEIGHT

We know that the earth attracts every object
with a certain force and this force depends
on the mass (m) of the object and the
acceleration due to the gravity (g). The weight
of an object is the force with which it is
attracted towards the earth.
We know that
F = m × a, (10.13)
that is,
F = m × g. (10.14)
The force of attraction of the earth on an
object is known as the weight of the object. It
is denoted by W. Substituting the same in
Eq. (10.14), we have
W = m × g (10.15)
As the weight of an object is the force with
which it is attracted towards the earth, the
SI unit of weight is the same as that of force,
that is, newton (N). The weight is a force acting
vertically downwards; it has both magnitude
and direction.
We have learnt that the value of g is
constant at a given place. Therefore at a given
place, the weight of an object is directly
proportional to the mass, say m, of the object,
that is, W m. It is due to this reason that
at a given place, we can use the weight of an
object as a measure of its mass. The mass of
an object remains the same everywhere, that
is, on the earth and on any planet whereas
its weight depends on its location.

IMPORTANCE OF UNIVERSAL LAW OF GRAVITATION

The universal law of gravitation successfully
explained several phenomena which were
believed to be unconnected:
(i) the force that binds us to the earth;
(ii) the motion of the moon around the
earth;
(iii) the motion of planets around the Sun;
and
(iv) the tides due to the moon and the Sun.