A satellite of mass $m$ revolves around the earth of radius $R$ at a height $x$ from its surface. If $g$ is the acceleration due to gravity on the surface of the earth, the orbital speed of the satellite is\\
(1) $g x$\\
(2) $\frac { g R } { R - x }$\\
(3) $\frac { g ^ { 2 } } { R + x }$\\
(4) $\left( \frac { g R ^ { 2 } } { R + x } \right) ^ { 1 / 2 }$