Write the expression for the acceleration due to gravity at the surface of the earth.

The weight of any person on the moon is about $1/6$ times that on the earth. He can lift a mass of $15\, kg$ on the earth. What will be the maximum mass,which can be lifted by the same force applied by the person on the moon (in $, kg$)?

The value of $g$ on the earth's surface is $9.8 \, m s^{-2}$. Suppose the earth suddenly shrinks to one-third of its present size without losing any mass. What is the value of $g$ on the surface of the shrunk earth (in $, m s^{-2}$)?

$A$ man weighs $600\, N$ on the surface of the Earth. What would be his mass and weight on the surface of the Moon? $(g = 10\, m s^{-2})$

An apple falls from a tree because of gravitational attraction between the earth and the apple. If $F_1$ is the magnitude of force exerted by the earth on the apple and $F_2$ is the magnitude of force exerted by the apple on the earth,then:

Define the "universal law of gravitation".