What is gravitational potential energy ? What is zero potential energy ?

A satellite of mass $m,$ initially at rest on the earth, is launched into a circular orbit at a height equal to the radius of the earth. The minimum energy required is

A particle of mass $m$ is thrown upwards from the surface of the earth, with a velocity $u.$ The mass and the radius of the earth are, respectively, $M$ and $R.$  $G$ is gravitational constant and $g$ is acceleration due to gravity on the surface of the earth. The minimum value of $u$ so that the particle does not return back to earth, is

The mass of a spaceship is $1000\ kg$. It is to be launched from the earth's surface out into free space. The value of $g$ and $R$( radius of earth) are $10\ m/s^2 $ and $6400\ km$ respectively. The required energy for this work will be

Two identical satellites $A$ and $B$ are circulating round the earth at the height of $R$ and $2R$ respectively, (where $R$ is radius of the earth). The ratio of kinetic energy of $A$ to that of $B$ is