The general formula is: Momentum = (mass) x (speed)
I never like to just write a bunch of algebra without explaining it.
But in this particular case, there's really not much to say, and
I think the algebra will pretty well explain itself. I hope so:
Original momentum = (original mass) x (original speed)
New momentum = (2 x original mass) x (2 x original speed)
= (2) x (original mass) x (2) x (original speed)
= (2) x (2) x (original mass) x (original speed)
= (4) x (original mass) x (original speed)
= (4) x (original momentum).
