Gravitation
The force of attraction between any two bodies in the universe is called force of gravitation or gravitation.
Newton's law of gravitation
Newton’s law of gravitation states that, “everybody in this universe attracts every other body with a force which is directly proportional to the product of their masses and inversely proportional to square of the distance between their centers”.
Consider two bodies having masses M1 and M2. Let‘d’ be the distance between their centers. If ‘F’ is the gravitational force of attraction between these bodies, then according to Newton’s law of gravitation
The force of attraction between two bodies is directly proportional to product of their masses, that is
F α M1M2 ……… (i)
And the force of attraction between two bodies is inversely proportional to the square of distance between their center, that is
F α 1/d2 ……….. (ii)
Combining equation (i) and (ii)
F α (M1M2)/d2
F = GM1M2/d2
Where ‘G’ is a proportionality constant called gravitational constant.
Definition of universal gravitational constant (G) :- As we know that Newton’s law of gravitation,
F = GM1M2/d2
If M1 = M2 = 1 kg and d = 1 m, then
F = Gx1x1/12
Then, F = G
Thus universal gravitational constant is equal to the force of attraction between two bodies of mass 1 kg each which are kept unit distance apart.
SI unit of G :- As we know that,
F = GM1M2/d2
Since SI unit of Force is N, mass is kg and distance is m. Therefore,
F = GM1M2/d2
Or, G = Fd2/M1M2
Or, G = Nm2/Kg.Kg
Or, G = Nm2/Kg2
Therefore SI unit of G is Nm2/Kg2
Value of G :- Experimentally the value of G was found by Henry Cavendish and the value is
G = 6.6734 X 10-11 Nm2Kg2
Effects of gravitational force :-
- Revolution of heavenly bodies.
- Tides in ocean.
- Rolling objects.
- Fruits falling from trees.
- Presence of gases in Sun.
- Presence of atmosphere around the planet.
Gravity
Gravity is the force of attraction exerted by any planets or satellite towards its center on a body lying on or near the surface of that planets or satellite.
Effects of a gravity :-
- Human activities like waking, running, playing, etc are possible only due to gravity.
- Construction of tall infrastructure which stands on the surface of the earth is only possible due to gravity.
- Water flows from mountains to ocean is due to gravity.
- Natural disaster like landslide is due to gravity.
Difference between gravitation and gravity
Gravitation | Gravity |
It is the force of attraction between any two objects in the universe. | Gravity is the force of attraction exerted by any planets or satellite towards its center. |
This force is very small. | This force is large. |
Its value cannot be zero unless distance between two bodies becomes infinity. | The force of gravity at the center of any planets or satellite is zero. |
Acceleration due to gravity
The uniform acceleration produced on a freely falling body under the action of gravity alone is known as acceleration due to gravity. It is denoted by letter ‘g’ and it’s SI unit is m/s2.
Consider an object of mass ‘m’ is placed on the surface of the earth having mass ‘M’ and radius ‘R’.
According to Newton’s law of gravitation, the force of attraction between the body and the earth is given by
F = GMm/R2
The force on mass ‘m’ due to earth’s gravity is given by
F = mass x acceleration
F = mg
Where ‘g’ is the acceleration due to gravity.
Combining equation (i) and (ii)
mg = GMm/R2
g = GM/R2
Above equation tells us that ‘g’ is independent of shape, size and mass of the body but depends upon mass and radius of the earth (planet) or satellite.
Difference between Gravity and acceleration due to gravity.
Gravity | Acceleration due to gravity |
Gravity is the force of attraction exerted by any planets or satellite towards its center on a body lying on or near the surface of that planets or satellite. | The uniform acceleration produced on a freely falling body under the action of gravity alone is known as acceleration due to gravity. |
Its SI unit is Newton. | Its SI unit is m/s2 |
It is the cause of acceleration due to gravity. | It is the effect of gravity. |