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    Gravitation
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    According to Newton’s law of gravitation everybody in this universe attracts every other body. But we do not see bodies on the surface of the earth moving towards one another on account of this interaction. Why?
    According to Newton’s law of gravitation,
    According to Newton’s law of gravitation,In this equation G = 6.67x
    In this equation G = 6.67x10-11 Nm2/kg2. Since the masses of two ordinary bodies on the surface of the earth is small i.e. of the order of 102 kg, therefore, the force of attraction between two bodies, each of mass 100 kg separated by 1m, is of the order of 10-6N which is too small to overcome the force of friction between the body and the surface. That is why we do not see the bodies on the surface of the earth moving towards each other.
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    Answer
    What are the units and dimensions of G?
    According to Newton’s law of gravitation, 

                       straight F equals straight G fraction numerator straight M subscript 1 straight M subscript 2 over denominator straight r squared end fraction 

    rightwards double arrow            space space space space space straight G equals fraction numerator Fr squared over denominator straight M subscript 1 straight M subscript 2 end fraction 

    Dimensional formula of G is, 

     straight G equals fraction numerator left square bracket MLT to the power of negative 2 end exponent right square bracket left square bracket straight L squared right square bracket over denominator left square bracket straight M right square bracket left square bracket straight M right square bracket end fraction equals left square bracket straight M to the power of negative 1 end exponent straight L cubed straight T to the power of negative 2 end exponent right square bracket 

    SI units of G is Nm2/kg2 or m3/kg/sec2 

    CGS units of G is dyne cm    2/gm2 or cm3/gm/sec2.
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    Answer
    Any two bodies on the surface of the earth should move towards each other due to force of attraction acting between them but they do not. Why?
    The masses of the bodies are very small as compared to the mass of the earth. 

    Hence, the force of attraction between the two bodies is very small as compared to the force of friction between the bodies and the surface of the earth.

    Hence, they do not move towards each other. 
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    Answer
    Define gravitational constant.

    Force of gravitation, straight F equals straight G fraction numerator straight m subscript 1 straight m subscript 2 over denominator straight r squared end fraction 

    Let,

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    and               straight r equals 1 straight m 

    Then,          straight F equals straight G fraction numerator 1 cross times 1 over denominator left parenthesis 1 right parenthesis squared end fraction equals straight G 

    Therefore, gravitational constant is equal to the force between two bodies, each of mass 1 kg when their centres of mass are separated 1 metre apart.
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    When the object falls on the ground, the earth moves up. Why the earth’s motion is not noticeable?
                                        OR 
    If the force of gravity between the earth and the body is same and mutual, then does the body fall towards the earth and the earth does not move towards body?

    When a body falls on the ground, both the body and the earth move towards each other. But the ratio of distance travelled by each is inversely proportional to the mass.

    i.e.,  

    where s, S are distances travelled by the body and the earth. 

    Since,               M>>>m 

    ∴                       s>>>S 

    Therefore motion of the earth is not noticeable.
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