What is the Solution to a Perfectly Inelastic Collision Problem?

[BrainMan]

Homework Statement

A 90-kg halfback running north with a speed of 9 m/s is tackled by a 120-kg opponent running south with a speed of 3 m/s. If the collision is perfectly inelastic and head-on, calculate. (a) the velocity of the players just after the tackle and (b) the total energy lost as a result of the collision.

Homework Equations

m₁v_1i + m₂v_2i = (m₁ + m₂)v_f

The Attempt at a Solution

I plugged the masses and velocities into the equation for perfectly inelastic collisions and solved for v_f. I got 5.57 m/s and the answer is 2.14 m/s.

 

[.Scott]

Which direction have you picked as positive, north or south?

 

[BrainMan]

OK I see what I did. I didn't take into account negative velocity. Thanks!