Question

Newton’s universal law of gravitation applies to:
A. Small bodies only
B. Planets only
C. Both small and big bodies.
D. Only valid for the solar system.

Answer 1

Hint: The word universal means here that this law is valid for each and every body in this universe that has mass. The law applies on objects as small as $1gm$ to as large as a million kilograms.

Complete step by step answer:
According to Newton’s law of gravitation, there exists a force between any two bodies in the universe. The gravitational force which acts between them depends on two factors.
Let ${m_1}$and ${m_2}$be the masses of the 2 bodies placed at a distance $r$ from each other. Let $F$ be the force of gravitation which acts between them.
$F \propto {m_1}{m_2}$(force is proportional to the product of the two masses)
$F \propto \dfrac{1}{{{r^2}}}$(force is inversely proportional to the distance between the masses)
On combining the above equation, we get,
$F \propto \dfrac{{{m_1}{m_2}}}{{{r^2}}}$
$F = G\dfrac{{{m_1}{m_2}}}{{{r^2}}}$
Where G is known as the gravitational constant.
From this equation we can see that there is no limit in the masses of the bodies between which we have to calculate the force of gravitation. The masses could be as small as a cricket ball to as large as the planets.
c) is correct.

Additional information: as we can see from the above equation that there is no limit on the mass of the objects while calculating the force of gravitation between them. A man exerts the same amount of force on earth as the earth exerts on them.

Note: Newton’s universal law of gravitation is not a perfect theory and it fails at many places. For example, Newton’s universal law of gravitation does not predict the existence of blackholes which are later predicted by the famous Einstein’s general theory of relativity. Einstein’s general theory of relativity is a more complete theory which tells us the true nature of gravity.