Physics
posted by Daniel .
A 821kg car stopped at an intersection is rearended by a 1810kg truck moving with a speed of 11.5 m/s. If the car was in neutral and its brakes were off, so that the collision is approximately elastic, find the final speed of both vehicles after the collision.

Use the eqn:
v1'=v1*(m1m2)/(m1+m2)
M2=neutral car, its v2=0 since its at rest.
m1=truck, v1=11.5.
Then, once you have v1' you can sub it into m1v1+m2v2=m1v1'+m2v2' and find v2'.
The first eqn is a special eqn only to be used when v2=0 and the collision is completely elastic. NEVER under anyother circumstances. They are specialy derived for solely this case. 
Vf2=(m1+m2)(Vo1)/m1
m1=truck; m2=car; Vo1=initial speed of truck.
mo1Vo1=mf1Vf1+mf2Vf2; substitute Vf2 to find Vf1