GRAVITY
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| GRAVITY |
The gravitational force
of earth is called gravity.i.e., gravity is force by which earth pulls a body
towards its centre. The acceleration produced in a body due to force of gravity
is called acceleration due to gravity and its value is 9.8 m/s2. Gravity, also
called gravitation, in mechanics, the universal force of attraction acting
between all matter. On Earth all bodies have a weight, or downward force
of gravity, proportional to their mass, which Earth's mass exerts on
them. Gravity is measured by the acceleration that it gives to freely
falling objects.
NEWTON'S LAW OF GRAVITATION
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| NEWTON'S LAW OF GRAVITATION |
The
force of gravitational attraction between two point bodies is directly
proportional to the product of their masses and inversely proportional to the
square of the distance between them.
i.e.,F = G ✕ m1m2/r2
where
G is universal gravitational constant, it's value is 6.67✕10-11 N-m2 kg-2
VARIATION IN GRAVITY (g)
Value of g decreases
with height or depth from earth's surface.
g is maximum at poles.
g is minimum at equator.
g decreases due to
rotation of earth.
g decreases if angular
speed of earth increases and increases, if angular speed of earth decreases.
SATELLITE
Satellite are natural or
artificial bodies revolving around a planet under its gravitational attraction.
Orbital speed of a
satellite is independent of its mass and depends upon the radius of orbit.
The orbital speed of a
satellite revolving near the surface of earth is 7.9 km/s.
Time period of
revolution of satellite revolving near the surface of earth is 1 hr 24 min (84
min).
Period of revolution of
a satellite depends upon the height of satellite from the surface of earth.
Every body inside the
satellite is in a state of weightlessness.
Period of revolution of
a satellite is independent of its mass.
If a satellite revolves
in equatorial plane in the direction of earth's rotation i.e., fro west to east
with a period of revolution equal to time period of rotation of earth on its
own axis i.e., 24 hours, then the satellite will appear stationary satellite.Such
a satellite revolves around the earth at a height of 3600 km.
Polar satellite revolves
around the earth in polar orbit at a height of 800 km. Time periods of the
satellite is 84 min.
These are used for
weather forecasting and mapping etc.
ESCAPE VELOCITY
Escape velocity is that
minimum velocity with which a body should be projected from the surface of
earth, so as it goes out of gravitation field of earth and never return to
earth.
Escape velocity at the
earth surface is 11.2 km/s.
The escape velocity at
moon surface is 2.4 km/s. So,there is no atmosphere.
The value of g on the
moon is 1/6th of that on the earth.
MASS AND WEIGHT
The mass of a body is
the quantity of matter contained in it.
It is scalar quantity
and its SI unit is kg. Mass does not change from place to place and remains
constant.
The weight of body is
the force with which it is attracted towards the centre of the earth.
Weight of the body w =
mg
Weight of a body is not
constant it changes from place to place.
WEIGHT OF A BODY IN LIFT
If lift is stationary of
moving with uniform speed (either upward or downward), the apparent weight of a
body is equal to its true weight.
If lift is going up with
acceleration, the apparent weight of a body is more than the true weight.
If lift is going down
with acceleration, the apparent weight of a body is less than the true weight.
If the rod of the lift
is broken, it fall freely.In this situation, the weight of a body in the lift
become zero.This is the situation of weightlessness.
SIMPLE HARMONIC MOTION
If a particle repeats
its motion about a fixed point after a regular time interval in such a way that
at any moment the acceleration of the particle is directly proportional to its
displacement from the fixed point at that moment and is always directed towards
the fixed point than the motion of the particle is called simple harmonic
motion.
When a particle
executing SHM passes through the mean position
- no force acts on the particle.
- acceleration of the particle is zero.
- velocity is maximum.
- kinetic energy is maximum.
- potential energy is maximum.
When a particle
executing SHM is at the extreme end,then
- acceleration of the particle is maximum.
- restoring force acting on particle is maximum.
- velocity of particle is zero.
- kinetic energy of particle is zero.
- potential energy is maximum.
PERIODIC MOTION
A motion which repeats
itself after equal intervals of time is called a periodic motion.
OSCILLATORY MOTION
A motion in which an
object moves back and forth repeatedly about a fixed position is called an
oscillatory motion.
SIMPLE PENDULUM
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| SIMPLE PENDULUM |
A simple pendulum
consists of a small metal ball suspended by a long thread from a rigid support,
such that the both is free to swing back and forth.
The distance of the
pendulum from its mean position is called its displacement.
The maximum displacement
of the pendulum on either side of its mean position is called the amplitude of
oscillations.
Time period is the time
taken by the pendulum to complete one full oscillation.
Frequency is the number
of full oscillations completed by the pendulum in one second.
Second pendulum is one
whose time period is 2 seconds.
Time period of simple
pendulum T=2ℼ√L/g where L is length
of thread.The time period of a simple pendulum of infinite length 84.6 min.A
pendulum clock goes slow in summer and fast in winter.
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