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.