What Is the Final Velocity of Two Colliding Putty Wads?

[uno]

1. Two wads of putty are propelled horizontally directly toward each other. Wad 1 has a mass of 5.00 kg and is traveling at 21.0 m/s south. Wad 2, which is 5.90 kg, is moving at 16.0 m/s north. They collide and stick together. What is the velocity of the joint mass of putty?



2. M1 V1i + M2 V2i = (M1 + M2)Vf



3. I used M1 = 5kg, V1 = 21m/s, M2 = 5.90kg, and V2 = 16m/s. I used the equation to solve for Vf, but I must be doing something wrong because I am not getting the correct answer. Any suggestions? Thanks.

 

[Redbelly98]

Note that the two wads are traveling in opposite directions before colliding.

+21 m/s and +16 m/s indicate two objects traveling in the same direction. So at least one of those velocities is wrong.