# Homework Help: Head on collision

1. Oct 31, 2004

### 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.

2. Oct 31, 2004

### austin zitro

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

3. Oct 31, 2004

### 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