1. The problem statement, all variables and given/known data So I have this practice problem with the solution, but I don't understand how: "A particle of mass m1 and speed v1 in the +x direction collides with another particle of mass m2. Mass m2 is at rest before the collision occurs, thus v2 = 0. After the collision, the particles have velocities v'1 and v'2 . in the xy plane with directions θ1 and θ2 with the x axis as shown below. There are no external forces acting on the system. Express all of your answers in terms of m1, m2, v1, θ1 and θ2. Q:What is the ratio of the speeds v'2/v'1 ?" The answer goes like this: "Conservation of momentum in the y direction: m1v'1 sin θ1 = m2v'2sin θ2 and by solving for ratio we get: m1sinθ1/m2sinθ2" Why did we choose the conservation in the y not the x direction? Can this problem be solved any other way? 2. Relevant equations - 3. The attempt at a solution When I first attempted the problem, I thought of solving it like this: v1'=v1'(sqrt[(cos(θ1)+sin(θ1))^2]) v2'=v2'(sqrt[(cos(θ2)+sin(θ2))^2]) And since momentum is conserved, m1v1=m1v1'(sqrt[(cos(θ1)+sin(θ1))^2])+m2v2'(sqrt[(cos(θ2)+sin(θ2))^2]) But this feels wrong, and I don't know what's with it. Please feel free to correct me and give me more info about momentum and collision if needed, I understand it but I don't feel like having a full grasp of the concept.