Head-on collision with 4.5 kg and 11 kg objects

[TraceBusta]

A 4.5 kg object with a speed of 4.0 m/s collides head-on with a 11 kg object moving toward it with a speed of 3.0 m/s. The 11 kg object stops dead after the collision.

(a) What is the final speed of the 4.5 kg object?
(b) Is the collision elastic?

so i tried initial momentum = final momentum but I get the wrong answer.
(4.5 kg)(4.0 m/s) + (11 kg)(3.0 m/s)=(4.5 kg)(Vf)
and solving for Vf = 11.333 m/s, but that is the wrong answer.

 

[austin zitro]

Take into account that they have different directions. One momentum will be positive while the other negative.

 

[Lyuokdea]

Conservation of Momentum is the correct property to use, but you forgot to factor in that the two objects are moving at different directions before the collision, so initial momentum = 4.5kg * 4 m/s - 11kg * 3 m/s

For the second part, you should just see if Conservation of Energy is also upheld in the collision.

~Lyuokdea