# Really confused vector momentum problem

vector momentum problem

## Homework Statement

A car moving North at 9.0 m/s strikes a stationary car of equal mass. The first car moves off after the collision at an angle of 30 degrees West of North with a speed of 6.0 m/s.

(a) What is the speed of the second car after the collision? (3 marks)

(b) Show that the collision is inelastic. (2 marks)

(c) Explain how dents, skid marks, etc., show that kinetic energy has been lost (2 marks)

(d) What would the speeds of the cars be if the first car moved off at 30 degrees from a perfectly elastic collision? (3 marks)

## Homework Equations

v1 + v2 = v1' + v2'

## The Attempt at a Solution

I'm pretty sure I messed up on my calculation when I used the cosine law. The variable I'm trying to solve for is v2'

9 + 0 = 6 + v2'

Here was the calculation I made for part a:

v2'^2 = v1^2 - v1'^2 - 2(v1)(v1')(cos30)

Which I don't think is right.

I'm stumped at the other parts as well.

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## Answers and Replies

I don't believe law of cosines will get you anywhere. Think about breaking your velocity vectors into their components.

Since i'm trying to solve for the velocity of a stationairy object after a collision. I'm adding vectors?

Also if the collision is inelastic how do I know what angle the second car moved off at?

You're not "adding vectors." You need to apply conservation of momentum. Momentum is a vector quantity. Thus if you know the momentum vector before the collision, the sum of the individual momenta afterwards must equal what?