Two spheres ' $A$ ' and ' $B$ ' of equal mass lie at rest at the opposite ends of a diameter of a smooth…
Two spheres ' $A$ ' and ' $B$ ' of equal mass lie at rest at the opposite ends of a diameter of a smooth horizontal circular groove of radius ' $r$ '. 'A' moves and collides with ' $B$ ' after a time ' $t$ '. If ' $e$ ' is the coefficient of restitution, next collision between the spheres takes place after a time of ____