How do you calculate the momentum of two objects immediately after a perfectly elastic collision?
Do you need to find the velocities of the two objects after collision and then use the formula of p=mv. where p is momentum vector, m is mass of the object, and v is the velocity vector?
Or is there another way to find the linear momentum?

Question

How do you calculate the momentum of two objects immediately after a perfectly elastic collision? 
Do you need to find the velocities of the two objects after collision and then use the formula of p=mv. where p is momentum vector, m is mass of the object, and v is the velocity vector?
Or is there another way to find the linear momentum?

shane_b · Answer

It would be easier to explain with an actual problem but for the most part, you just have to remember that momentum and KE are conserved in a perfectly elastic collision. This should help: http://hyperphysics.phy-astr.gsu.edu/hbase/colsta.html and http://hyperphysics.phy-astr.gsu.edu/hbase/elacol.html#c4

phoenixfire · Answer

I have a problem. 
object 1: m=10kg v=(6, 1) p=mv=(60, 10)
object 2: m=100kg v=(-6, -2) p=mv=(-600, -200)
They collide, what's the momentum of each object immediately after collision.

phoenixfire · Answer

So what I got from those two links was basically that I do need to calculate the final velocities. 
$$v_{1f}={(m_1 - m_2)v_{1i} + 2m_2 v_{2i} \over m_1+m_2}$$$$v_{2f}={(m_2-m_1)v_{2i}+2m_1v_{1i} \over m_1+m_2}$$
Then apply the momentum formula$$\mathbf {\vec p}=m {\mathbf{\vec v}}$$

Kinda strange since the next question asks me to find the velocities.. You'd think they would put it in the right order to help you learn the method.

shane_b · Answer

The problem you posted it's a 2-dimensional collision and momentum is still conserved but you have to account for the momentum in 2 dimensions.  

That's a bit tougher and I admit that I haven't done one of those in a while :)