The distance between the centers of the moon and the earth is $D$. If the mass of the earth is $81$ times that of the moon,at what distance from the center of the earth will the gravitational field be zero?

Infinite particles,each of mass $3 \ kg$,are placed at distances of $1 \ m, 2 \ m, 4 \ m, 8 \ m, \dots$ from point $O$. What is the gravitational intensity at point $O$?

Explain gravitational intensity.

$A$ spherical shell is cut into two pieces along a plane as shown in the figure. $P$ is a point on the plane of the cut. The gravitational field at $P$ due to the upper part is $I_1$ and that due to the lower part is $I_2$. What is the relation between them?

Find the gravitational force of attraction between the ring and the sphere as shown in the diagram. The plane of the ring is perpendicular to the line joining the centres. The distance between the centres of the ring (mass $m$) and the sphere (mass $M$) is $\sqrt{8}R$,where both have an equal radius $R$.

Two point masses having mass $m$ and $2m$ are placed at a distance $d$. The point on the line joining the point masses,where the gravitational field intensity is zero,will be at a distance ............