# Homework Help: Momentum of car and truck sytem.

1. Oct 25, 2011

### JJRKnights

1. The problem statement, all variables and given/known data

A 1 175 kg car traveling initially with a speed of 25.0 m/s in an easterly direction crashes into the back of a 8 900 kg truck moving in the same direction at 20.0 m/s. The velocity of the car right after the collision is 18.0 m/s to the east.

(b) What is the change in mechanical energy of the car–truck system in the collision? (Use input values with an adequate number of significant figures to calculate this answer.)

m1 = 1175kg
m2 = 8900kg
v1i = 25m/s
v2i = 20m/s
vf = 20.583m/s

2. Relevant equations

K = 1/2mv^2
ME = U + K
U = mgh

3. The attempt at a solution

ΔK = (1/2m1v1f^2 + 1/2m2v2f^2) - (1/2m1v1i^2 + 1/2m2v2i^2)

am I on the right tracks?
if so how do i find v1f and v2f.
I know the relation:
20.583 = √(v1f^2 + v2f^2)
but there's 2 unknowns.

Last edited: Oct 25, 2011
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