# Help with clarifying steps for 1-Dimensional Collisions without known mass

1. Nov 18, 2014

### princesspeach13

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

I have a practice question in front of me, with the answer and the steps to reach it but I am having a tough go trying to figure out how to do the steps myself. Can someone please clarify the outlined area of the following question?

In a crash, test car A, was initially moving 20m/s, and Truck B, with twice the mass of car A, was travelling in the opposite direction, during a head on collision. The vehicles were stuck together after the collision, and the final velocity of the car was 6m/s in the opposite direction.

2. Relevant equations

Pi = Pf
m1v1 + m2v2 = (m1+m2)Vf

3. The attempt at a solution
m(20) + 2mV(truck initial) = (m + 2m)(-6)
20m + 2MVti = -18m
20m/s + 2Vti = -18m/s
V2 = 19m/s

Where my brain doesn't click is where the author has solved out the masses. If you have two sets of masses on the left side of the equation, and only one on the left, how can you just drop them all together?
Im sure this is probably some stupidly easy answer but when I go back to this type of question it doesn't solve out properly in my head.

Tips??

2. Nov 18, 2014

### lep11

m's will cancel out
20m-2mvt=-18m
(20-2vt) m=-18m

Last edited: Nov 18, 2014