Gravitation
The force of attraction between any two
bodies of the universe is called gravitation.
• Gravitation was discovered by Newton in the year 1665
• Gravitational force is the weakest force among the four fundamental forces
of nature.
Newton's law of gravitation
Everybody in the universe attract every other body with a force which is
directly proportional to the product of their masses and is inversely
proportional to square of the distance between them.
Consider two bodies "A" and "B" of masses M1 and M2.
Let "r" be the distance between their centre and if be the force of attraction
between them.
• Gravitational constant is a scalar quantity.
• The value of gravitational constant is same throughout the universe and is
independent of the nature and size of the bodies as well as nature of the
medium between the bodies.
Evidence in support of Newton's law of gravitation.
• The rotation of earth around the Sun or moon around the earth is explained
on the basis of this law.
• The prediction about solar and lunar eclipse made on the basis of this law
always come out to be true.
• The formation of tides in the oceans is due to force of attraction between
moon and ocean water.
• The prediction about the orbits and the time period of the modern artificial
satellites made on the basis of this law are found to be very accurate.
Important characteristics of gravitational force :-
• According to Newton's law of gravitation, the gravitational force between
two bodies
1. Is independent of nature of intervening medium.
2. Is independent of the presence or absence of other bodies.
3. Is independent of the nature and size of the bodies, till their masses
remains the same and the distance between their centre is fixed.
4. Form action and reaction pair.
• The gravitational forces are Central forces as they act along the line
joining the centres of the bodies.
• The gravitational forces are conservative forces.
Gravity :-
The force of attraction exerted by Earth towards its centre on a body lying on
or near the surface of Earth.
• Gravity is a special case of gravitation and is also called Earth's
gravitational pull.
Weight :- the force of attraction exerted by the Earth on the body towards its
centre.
If "m" is the mass of a body placed on the surface of earth where acceleration
due to gravity is "g"
Then gravity pull = weight of body = mg
Acceleration due to gravity :-
The constant acceleration produced in a body when it falls freely under the
effect of gravity alone.
• It is denoted by " g"
• The SI unit of acceleration due to gravity is m/s2 or N/kg
Relation between "g" and "G" :-
Consider Earth to be a spherical body of mass "M", "Re" with centre "O" suppose a body of mass "m" placed on the surface of earth where, acceleration due to gravity is "g"
If density of earth is uniform, It's mass can be suppose to be concentrated at its centre "O"
Let F be the force of attraction between body and the earth.
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