Help elastic collistion, cons of momentum problem

[nchin]

Block 1 moves with speed of 10m/s to right. It hits block 2 which has twice the mass of block 1 and speed of 5m/s to right. compute the magnitude and direction of block 1 for a perfectly elastic collision.

solution:

u₁ = 10m/s
u₂ = 5m/s

m₁v₁ + m₂v₂ = m₁u₁ + m₂u₂ ---->
v₁ + 2v₂ = u₁ + 2u₂
...= 10 + 2(5) = 20

(1) v₁ + 2v₂ = 20

why did u₂ and v₂ get multiplied by two and why did the m cross out?

 

[haruspex]

why did u₂ and v₂ get multiplied by two and why did the m cross out?

Are you saying you've just copied these steps from somewhere else and don't know the logic behind them? If those steps were your own work, surely you know why you made them.