1. The problem statement, all variables and given/known data ok so I was trying to figure out how to derive the formula for true elastic collision and I came up with the following but I want to make sure I didn't make a mistake. 2. Relevant equations (K1+K2)before=(K1+K2)after 1/2 m1 v1^2 + 1/2 m2 u1^2 = 1/2 m1 v2^2 + 1/2 m2 u2^2 m1v1 + m2u1=m1v2 + m2 u2 3. The attempt at a solution (1/2 m1 v1^2 + 1/2 m2 u1^2 = 1/2 m1 v2^2 + 1/2 m2 u2^2)*2 m1v1^2+m2 u1^2=m1v2^2+m2 u2^2 m1(v1^2-v2^2)=m2(u2^2-u1^2) *(1) m1v1 +m2u1=m1v2 + m2 u2 m1(v1-v2)=m2(u2-u1) *(2) (1)/(2) = v1+v2=u1+u2 u2=v1+v2-u1 m1v1 + m2u1=m1v2 + m2 (v1+v2-u1) m1v1 + m2u1=m1v2 + m2 v1+m2 v2 - m2 u1 2m2u1 +m1v1 -m2v1 =v2 (m1+m2) v2= (2m2u1 +v1(m1-m2)) / (m1+m2) EDIT: the point of this is that my friend told me that the derivation will take more than 3 pages so I am thinking there must be something incorrect here or I ate some lines lol.