Q.

Two bodies of mass m and 9m are placed at a distance R. The gravitational potential on the line joining the bodies where the gravitational field equals zero, will be (G = gravitational constant)           [2023]

1 -16GmR  
2 -20GmR  
3 -8GmR  
4 -12GmR  

Ans.

(1)

Let gravitational field at point P is zero which is situated at a distance x from mass m.

    Gmx2=G(9m)(R-x)2

or,  (R-x)2=9x2

or,  R-xx=3 or x=R4                                ....(i)

Gravitational potential at point P, VP=-Gmx-G(9m)R-x

or  VP=-GmR/4-G(9m)3R/4  (using (i))

or   VP=-GmR(4+12)=-16GmR