A particle rests on the top of a hemisphere of radius R. The smallest horizontal velocity that must be imparted to the particle if it is to leave the hemisphere without sliding down is
$\sqrt{gR}$
$\sqrt{2gR}$
$\sqrt{3gR}$
$\sqrt{5gR}$
A particle rests on the top of a hemisphere of radius R. The smallest horizontal velocity that must be imparted to the particle if it is to leave the hemisphere without sliding down is