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. 

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If there was no gravity then their would be no downword force on the football to have it return the the grownd, and if there was no friction (mainly air friction) then there would be no opposing force the stop the ball from moving through the air allowing the ball to travle ∞.