u want models on the lots. I will anticipate that's µg. However truely the models cancel out, so so long as they're consistant, that is ok. Exchange in momentum is 1µg x 5e7 + 1µg x zero.9 x 5e7 ∆P = 1µg x 5e7(1+zero.9) = 50(1.9) gm/s imparted to the gold, that is ∆P = 50(1.9) = 197µg x V V = 482000 m/s .