Finding van speed in collision with stationary compact car

[Iceclover]

Homework Statement

a 2575-kg van runs into the back of a 825-kg compact car at rest. they move off together at 8.5m/s. assuming no friction with the ground, find the initial speed of the van

 

[EP]

Write out conservation of momentum equation. The momuentum of the system before the collision on one side and the momentum of the entire system after the collision on the other.

 

[Iceclover]

how do i do that

 

[EP]

see other problem

 

[Iceclover]

how do i find out the momentum before the collision when i only have a velocity for after the collision?

 

[EP]

write out the equation like in the last problem. Fortunately that is your only unknown.

 

[Iceclover]

i got 11.2 m/s

 

[EP]

Good work!