Conservation of Linear Momentum problem

[mp9191]

1. Problem
A 2.3 kg cart is rolling across a frictionless, horizontal track towards a 1.5 kg cart that is initially held at rest. The carts are loaded with strong magnets that cause them to attract one another. Thus, the speed of each cart increases. At a certain instant before the carts collide, the first cart's velocity is +4.9 m/s and the second cart's velocity is -1.9 m/s.
a) What is the total momentum of the system of the two carts at this instant?
b) What was the velocity of the first cart when the second cart was still at rest?
2. Homework Equations :
p = m1v1 + m2v2
3. Solution:

a) p= 2.3*4.5 + 1.5*(-1.9) = 7.5 kg*m/s
b) 7.5 = 2.3v + 0
v= 7.5/2.3 = 3.26 m/s

Could someone please verify this solution? For some reason my prof gave me the answers
a) 2.15 kg*m/s
b) .935 m/s

which I'm pretty sure are incorrect... thanks in advance

 

[tiny-tim]

welcome to pf!

hi mp9191! welcome to pf!

A 2.3 kg cart is rolling across a frictionless, horizontal track towards a 1.5 kg cart that is initially held at rest. The carts are loaded with strong magnets that cause them to attract one another. Thus, the speed of each cart increases. At a certain instant before the carts collide, the first cart's velocity is +4.9 m/s and the second cart's velocity is -1.9 m/s.
a) What is the total momentum of the system of the two carts at this instant?
b) What was the velocity of the first cart when the second cart was still at rest?
…
a) p= 2.3*4.5 + 1.5*(-1.9) = 7.5 kg*m/s
b) 7.5 = 2.3v + 0
v= 7.5/2.3 = 3.26 m/s

Your answers look correct to me.

(except should that be 4.9 or 4.5 ??)

 

[mp9191]

Oh, my bad. In part a) that should be 4.9 not 4.5.

Thank you! I thought I was going insane because I was like...there's no way it can be 2.15...