A rocket is fired from the earth towards the sun. At what distance from the earth’s center is the gravitational force on the rocket zero? Mass of the sun = 2×1030 kg, mass of the earth = 6×1024 kg. Neglect the effect of other planets etc. (orbital radius = 1.5 × 1011 m).

The net gravitational force on the rocket will be zero when attractive gravitational force on Rocket due to sun is equal in magnitude to attractive gravitational force by Earth, so it is between earth and sun, suppose distance between earth and sun or the orbital radius is R, and rocket is at a distance r from earth so it is at (R-r) distance from sun and attractive pull balances attractive pull of earth

The situation has been depicted in the figure



The gravitational force on Rocket due to sun Fs is equal in magnitude to attractive gravitational force by Earth Fe and opposite in direction as can be seen in the figure


we know gravitational force on a body is given as



Where F is the gravitational force


G is universal gravitational Constant


m1 is mass of the first body


m2 is the mass of the second body


and r is the distance between the two bodies


Now gravitation force on rocket due to earth will be



Where Me and Mr are masses of earth and rocket and r is a separation between them


Similarly, gravitation force on rocket due to Sun will be



Where Ms and Mr are masses of Sun and rocket and separation between Sun and rocket is (R-r)


Since both forces should be equal in magnitude, equating both


Fe = Fs


i.e.


solving and canceling terms we get



Or we can say



We are given


Mass of the sun, Ms = 2×1030 kg


Mass of the earth, Me = 6×1024 kg


The distance between Earth and Sun or orbital Radius


R = 1.5 × 1011 m


So putting these values to find the distance between earth and rocket r



577.3r = 1.5 × 1011 m – r


578.3r = 1.5 × 1011 m


r = (1.5 × 1011 m)/ 578.3 = 2.59 × 103 m


so the rocket is at a distance of 2.59 × 103 m from earth’s centre


12