Initial Velocity of Second Rail Car in Inelastic Collision

[Jay G.]

Homework Statement

A Rail car with a mass= 27000kg's moving at 4m/s collides with another rail car with mass 80,000kg that was moving towards the first. After the collision, both cars come to a complete stop, what was the initial speed of the second rail car?

Homework Equations

m1v1+m2v2=(m1+v1)v'

The Attempt at a Solution

This was a seemingly simple test question, but while most of the class got the same absolute value answer of 1.33m/s, half of us got -1.3m/s and the other half +1.3m/s. Logically the second car hand a negative momentum, but algebraically it seems like a positive answer is correct. Any clarification?

 

[haruspex]

Homework Statement

A Rail car with a mass= 27000kg's moving at 4m/s collides with another rail car with mass 80,000kg that was moving towards the first. After the collision, both cars come to a complete stop, what was the initial speed of the second rail car?

Homework Equations

m1v1+m2v2=(m1+v1)v'

The Attempt at a Solution

This was a seemingly simple test question, but while most of the class got the same absolute value answer of 1.33m/s, half of us got -1.3m/s and the other half +1.3m/s. Logically the second car hand a negative momentum, but algebraically it seems like a positive answer is correct. Any clarification?

You are asked for a speed, but the conservation law applies to velocities. If, as is generally advisable, you define the positive direction to be the same for all parallel velocities, displacements and accelerations then it will happen that some velocities will be negative. Speeds, however, are generally taken to be pure magnitudes, so are not quoted with negative values.