A $4\mathrm{.}2-$mm$-$diameter merry$-$go$-$round is rotating freely with an angular velocity of $0\mathrm{.}75$ rad$/$srad$/$s $.$ Its total moment of inertia is $1380$ kg$$m$2$kg$$m$2.$ Four people standing on the ground, each of mass $60$ kgkg $,$ suddenly step onto the edge of the merry$-$go$-$round.