CLASS -9 PHYSICS
CHAPTER – 10
GRAVITATION
# GRAVITATION:
è Gravitation is the force of attraction between
two objects in the universe.
Eg: - Attraction between the earth and
moon.
Attraction between the sun and planets.
# GRAVITY:
è Force
of attraction between earth and objects lying on or near the surface of the
earth.
Eg: - If a body is dropped from a
certain height, it falls downwards due to earth’s gravity.
# CENTRIPETAL
FORCE
è When
a body moves in a circular path, it changes its direction at every point. This
causes change in velocity and involves acceleration. This acceleration is
because of the force which keeps the body in the circular path acts towards the
centre of the circle. This force is called CENTRIPETAL FORCE and the
corresponding acceleration is CENTRIPETAL ACCELERATION
è If there is NO centripetal force, the body
will move in a straight line along the tangent to the circular path.
# UNIVERSAL LAW OF
GRAVITATION:-
è The
universal law of gravitation states that, ‘every object in the universe
attracts every other object with a force which is directly proportional to
product of the masses (M and m) and inversely
proportional to the square of the distance (d) between them.’
è
F α M x m and
F α
1/d2
Combining the two - F α M x m
/ d2
Or F = G M m / d2
è where
G is a constant of proportionality called UNIVERSAL GRAVITATION CONSTANT.
è The
SI unit of G is N m2 kg -2 and its value is 6.673 x 10-11 N m2 kg -2
# IMPORTANCE OF
THE UNIVERSAL LAW OF GRAVITATION
è This law explained
o The
force that binds us to earth.
o Motion
of moon around the earth
o Motion
of planets around the sun.
# FREE FALL
è When
the object falls freely under the influence of gravitational force alone, then
the object is free fall.
è While
falling there is no change in direction but velocity changes and involves
acceleration. This acceleration is due to the earth’s gravitational force. Thus
this acceleration is called THE ACCELERATION DUE TO GRAVITY ‘g’.
è
From the second law of motion, force is the
product of mass and acceleration.
F = ma
For
free fall, force is the product of mass and acceleration due to
gravity.
F = mg
m g = G M x m/d2
g = G m/d2
Where
M is the mass of the earth and d is the distance between the object and the
earth.
For
objects near or on the surface of the earth d is equal to the radius of the
earth R
mg = G
M x m/ R2 or g = G M/ R2
è
The value of g is 9.8 ms -2.
.
è
The value of ‘g’ becomes greater at poles than
at equators.
è
All the objects whether they are hollow or
solid, big or small should fall at the same rate (in absence of air
resistance). Because ‘g’ is independent of mass.
è
Equation of motion of free fall object:
o
v = u + gt
o
v2 – u2 = 2gh
o
h = ut + gt2
2
# MASS
è
The mass of a body is the measure of its inertia or mass can
be defined as amount of matter contained in body.
è
If the mass of a body is more its inertia is
more.
è
The mass of a body is constant and does not change from place
to place.
è
The SI unit of mass is kg.
è
It is a vector quantity.
# WEIGHT
è
The
weight of a body is the force with which the earth attracts the body.
è
The force with which a body is attracted by the
earth depends on its mass m and acceleration due to
gravity g.
F = m g
Since
weight of a body is the force with which the earth attracts the body,
W = m g
è
Since g at a place is
constant , W α m
è
The weight of a body changes from place to
place.
è
The SI unit of weight is the same as force –
Newton (N).
è
It is a vector quantity.
# WEIGHT OF THE OBJECT ON
THE MOON
è
The weight of an object on the earth is the
force with which the earth attracts the object and the weight of an object on
the moon is the force with which the moon attracts the object.
è
The mass of the moon is less than the mass of
the earth. So the moon exerts lesser force on the objects than the earth. Thus
the weight of the object on moon is less.
è
The weight of an object on the moon is
one sixth (1/6th) of its weight on the earth.
è
Let the mass of an object be ‘m’ and its weight on moon is Wm
, then by universal law of gravitation
Its weight on the earth will be
We = G m M / R2 where, M is the mass of
the earth, and R its radius
=
1.474 x 1011 x G x m
and the weight on the moon is: Wm = G m Mm / Rm2
Dividing the two weights Wm/We,
we get
= 1 / 6
Thus, Wm = WE / 6
No comments:
Post a Comment