The parking brake on a 1000 kg cadillac has failed, and it is rolling slowly, at 1 mph , toward a group of small children. seeing the situation, you realize you have just enough time to drive your 2000 kg volkswagen head-on into the cadillac and save the children.

Um of momenta = 0( Since both the cars stop) 
Let m1 be the mass of the cadillac and m2 be the mass of the volkswagen. 
then, v1=speed of cadillac and v2=speed of volkswagen. 
Therefore, (m1)*(v1) + (m2)*(v2) = 0 
Substituting, 
(1000*4) + (2000*v2) = 0 
4000 = - 2000 v2 
Therefore, v2= - 2 m/s.