• A block of mass $28$ kg$,$ sliding with a speed of $12$ m$/$s on a smooth level surface, makes ahead$-$on collision with a block of unknown mass, initially at rest. The two stick together. Afterthe collision the velocity of the mega$-$block is $4\mathrm{.}9$ m$/$s in the same direction as the first block$’$s velocity. Determine the mass of the $2$nd block.