Collision of Two Carts: What Speed After Impact?

[dherm56]

A cart (m1 = 110 kg) is moving to the right along a track at v1i = 17 m/s when it hits a stationary cart (m2 = 390 kg) and rebounds with a speed of v1f = 7 m/s in the opposite direction.
a) With what speed does the 390 kg cart move after the collision?

I used conservation of momentum, m1v1=m2v2, because there are no net external forces acting on the system. However, my answer of 1.97 is incorrect. Is there more to this problem that I'm not seeing?

 

[LowlyPion]

Conservation of momentum means that the sum of the momenta of the objects before collision equal the sum afterward. You are missing a term in the after collision side of your equation

 

[dherm56]

I changed my equation to m1v1i + m2v2i = m1v1f + m2v2f

There is no initial velocity for m2 so that term cancels out. However, I was still unable to obtain the final velocity of m2

 

[djeitnstine]

Since there is no change in momentum, then $$p_i=p_f$$. $$m_1v_{i}+m_2v_{i}=m_1v_{2f}+m_2v_{2f}$$. Note that $$v_i$$ of m2 is 0

edit: you guys got it before I posted.

 

[dherm56]

The conservation of momentum concept I understand. However, my answer of 2.82 is not correct.

I got the correct answer. I forgot that vf of m1 is negative in reference to the lab frame.

Thank you for the help