First off - English isn't my native language, so please go easy on me if my translations are wrong. The problem: A ball with the mass 20g and the speed 3.0m/s collides with another ball with the mass 80g which is standing still. The collision is a sentral, fully elastic collision. Find the speed of the two balls after the collision. I thought it was a question about the combined speed of the two after the collision, but appearently it's asking about both of the balls individual speeds after the collision. PS: I know the answere, but I'd like to understand the equations. The two formulas that is relevant for me (according to the book itself) is: P(after)=P(before) P=mv and E(k(after))=E(k(before)) E(k)=1/2mv^2 I'm using 'u' as the speed after the collision and 'v' as the speed before the collision. First ball is ball (A) and the second is ball (B). m(A)=20g=0.02kg m(B)=80g=0.08kg v(A)=3.0m/s v(B)=0m/s u(A)= unknown u(B)= unknown Seeking u(A) and u(B) (separately, not the combined speed) I've tried several different setups, but the one I think I'm supposed to use is something like this: (1) m(A)u(A)+m(B)u(B) = m(A)v(A)+m(B)v(B) (2) 1/2m(A)u^2(A)+1/2m(B)u^2(B) = 1/2m(A)v^2(A)+1/2m(B)v^2(B) and then switch them around, divide/multiply, and so on until I'm left with the correct equations, but I'll spare you for that now as I've failed so far. I've reached a point where I see that my equation is totally off, and I've asked quite a few of the other students, but they haven't had any luck with it either. In advance, thanks for any tips.