Conservation of Momentum with Collisions

[Sadiebunkins]

Homework Statement

A diesel engine weighs 8 times as much as a freight car. This diesel engine, moving on a horizontal track at 3 km/h, crashes into the freight car (which was moving at 2 km/h in the same direction). What is the velocity, in km/h, of the now linked together engine/car combination? You may assume no numerical values not given.

Homework Equations

V= m_dv_d + m_fv_f over M or m_d + m_f

The sub d is for diesel engine
The sub f is for freight car

The Attempt at a Solution

I am not really sure is the m's cancel and all that is left with is the v's. If so the final velocity would be 5 km/h

 

[brainpushups]

Try calling the mass of the car m and then write the mass of the engine as 8m. You might be able to see what ends up canceling afterward.