# Explain gravitational force by a suitable example.

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Hint: We can obtain the relationship of gravitational force with mass and radius and write its formula through which we can explain the universal constant of gravitation. Afterwards, we will apply Newton's third law of motion on the gravitational force and hence prove that two masses can apply force onto each other of equal magnitude.

According to this law, the force of particle 1 with mass ${m_1}$ on a particle 2, with mass${m_2}$is given by the following formula:
${\vec F_{(12)}} = \dfrac{{G{m_1}{m_2}}}{{{{\vec r}_{(12)}}^2}}$ ,
Where, G is a constant and it is known as the universal constant of gravitation and ${\vec r_{(12)}}$ is the distance between particle 1 and particle 2. G has the same value at all places in the universe and at all times. The value of G was first obtained by Cavendish and its value is $6.67 \times 10^{-11}$ $\dfrac{{N{m^2}}}{{k{g^2}}}$. The dimensional formula of G is$[{M^{( - 1)}}{L^3}{T^{( - 2)}}]$
${\vec F_{(21)}} = - {\vec F_{(12)}} = - \dfrac{{G{m_1}{m_2}}}{{{{\vec r}_{(12)}}^2}}$