# Momentum and collisions

Hanga
This is not dircectly a coursework question, rather I need someone to tell me what is correct and what is not correct in my assumption of Momentum and collisions, esp conservation of linear momentum.

For some reason I stand to believe there are two diffirent equations for conservation of linear momentum Pbefore=Pafter. In one case we have

m1u1+m2u2 = m1v1m1v2

In the other we have
1/2m1u1^2 + 1/2m2u2^2 = 1/2m1v1^2 + 1/2m2v2^2

For some reson I stand to believe that they are for Perfectly inelastic collisions and Elastic collisions respectivly. Is this the case? Is there some "key word" in a work question that I can look for to say "Ah! I'm going to have use equation x"?

$m_1 u_1 + m_2 u_2= m_1 v_1 + m_2 v_2$ and $\frac{1}{2}m_1 u_1^2 + \frac{1}{2}m_2 u_2^2 = \frac{1}{2}m_1 v_1^2 + \frac{1}{2}m_2 v_2^2$
$m_1 u_1 + m_2 u_2 = (m_1 + m_2) v^2$