posted by brett on .
Phrank (36.4kg) runs at 2.2 m/s and jumps onto a large stationary skateboard (2.3kg) on which Phrancine (22.7kg) is sitting. After the collision, Phrank, Phrancine, and the skateboard move at 1.3 m/s. Find the momentum before and after the collision along with the percent difference.
Before hopping on,
Momentum = 36.4* 2.2 = 80.08 kg m/s
After hopping on,
Momentum = (36.4 + 2.3 + 22.7)*1.3 = 79.82 m/s
Since there are only two significant figures in your numbers, the 0.3% computed decrease in momentum is not significant. No momentum should have been lost at all. There is a law that says there should be no total momentum decrease if there are no "outside forces" , but friction while getting on may have reduced it slightly.