Class 9
Physics
Chapter
3
GRAVITATION
The Force with which the earth pulls the objects toward it is called
the gravitational force of earth.
Universal law of Gravitational
1. The force
between two bodies directly
proportional to the product of their
masses.{F
proportional to m1m2}
2. The force
between two bodies is inversely
proportional to
the square of the distance
between them. .{F
proportional to 1/r2}
3. Gravitational force , F= G*m1m2/r2
Gravitational Constant , G
1. G=F*r2/ m1m2
2. Unit = Nm2/kg2 (Nm2kg-2)
3. Value of
gravitational constant G = 6.67*10-11 Nm2/Kg2
Newton’s Third Law of Motion and Gravitation
The Newton’s third law of motion also holds good for the
force of gravitation. This means that when earth exerts a force of attraction
on an object , then the object also exerts an equal force on the earth, in the
opposite direction.
Free
Fall
The Falling of a body (or object) form a height towards
the earth under the gravitational force of earth (with no other forces acting
on it) is called free fall.
NOTE::: The
acceleration of the object falling freely towards the earth does not depend of
The Mass of the object.
Acceleration
due to Gravity
The uniform acceleration produced in a freely
falling body due to the gravitational force of the earth is knows as Acceleration
due to Gravity.
The acceleration due to gravity ,g =9.8 m/s2 .
g= G*M/R2 (R= radius
of earth G= constant M= mass of the earth)
EQUATI0N OF MOTION FOR FREELY FALLING BODIES
1) v= u +gt
2)
h=ut+1/2gt2 (h= height )
3)
v2=u2+2gh
Note
:
a) when a body
is falling vertically downward, its velocity is increasing , so the
acceleration due to gravity, g, is taken as positive
(+).
b) When a body
is thrown vertically upward, its velocity is decreasing, so the acceleration
due to gravity, g, is taken as negative (-).
c) When a body dropped freely from a height, its initial
velocity (u) is zero.
d) When a body is thrown vertically upward, its final velocity
(v) is zero.
MASS
a)
The mass an object is the quantity of matter contained
in it.
b)
The SI unit of mass is (kg).
c)
The mass of an object is constant.
d)
The mass an object can never be zero.
WEIGHT {w=mg}
1) The weight
of an object is the force with which it is attracted towards center of the
earth.
2) The SI unit
of the weight is Newton.
3) The weight
of the object is not constant. It change with the change in acceleration due to
gravity
4) The weight
of an object can be zero.
Ex.
Interplanetary space, where g=0 , the weight of
an object become zero.
{Weight
of an object on the moon is 1/6 of its weight o the earth.}
PRESSURE
Pressure is the force acting perpendicularly on the unit area
of the object.
Pressure = force/ Area.
Its SI unit is PASCAL (Pa) OR (N/M2) .
PRESSURE IN FLUIDS
Those Substance which can flow easily are called fluids.
The fluid exerts pressure in all direction – even upward.
BUOYANCY (UP THRUST)
The tendency of an liquid to exert an upward force on an
object placed in it , Is called Buoyancy.
Buoyant force
The upward force acting on an object immersed in liquid is
called buoyant force.
FACTORS EFFECTING BUOYANT FORCE
1) Volume of
object immersed in the liquid, and
2) Density of
the liquid.
·
As the volume of object immersed inside the
liquid increases, the upward “ buoyant
force ”also increase.
·
The density of a liquid increases, the buoyant force
exerted by it also increases.
ARCHIMEDES PRINCIPLE
When an object is
partially immersed in a liquid, it experiences a buoyant force which is equal to the weight
of liquid displaced by object.
·
Buoyant force acting on an obj. = weight of
liquid displaced by object.
Application of Archimedes
Principle
1)
Archimedes’ principle
is used in determining the density the relative density of a Substance .
2)
The hydrometers used
for determining the density of liquids are based on Archimedes’ principle
3)
The lactometers used
for determining the purity of milk are based on Archimedes’ principle.
4)
Archimedes’ principle
is used in designing ships and submarines.
Why
Objects Float or Sink in a Liquid
·
An object will float
in a liquid if its density (or average density) is less
then that of the liquid.
·
An object will also float in a liquid if its density is equal to that of the
liquid.
·
An object will sink in a liquid if its density is more then that of the
liquid.
DENSITY
The density of a substance is defined as mass of the
substance per unit volume.
·
Density= mass of
the substance / volume of the substance
·
Density= M/VOLUME
RELATIVE DENSITY
The relative of a substance is the ratio of its density to
that of water.
·
Relative density of the substance =
density of the substance /density of water.
