An object is allowed to fall from a height \( R \) above the earth, where \( R \) is the radius of earth. Its velocity when it strikes the earth's surface, ignoring air resistance, will be:
\( \sqrt{\dfrac{gR}{2}} \)
\( 2\sqrt{gR} \)
\( \sqrt{2gR} \)
\( \sqrt{gR} \)