The parking brake on a $2000$ kg Cadillac has failed, and is rolling at $2$ mph toward a group of small innocent children. As you see the situation, you realize there is just enough time for you to drive your $1000$ kg Volkswagen head$-$on into the Cadillac and thus save the children. With what speed should you impact the Cadillac in order to bring it to a halt?

$2$ mph

$4$ mph

$2\mathrm{.}8$ mph

$1\mathrm{.}4$ mph