Momentum

I am still seeing errors where people calculate delta p incorrectly.  AGAIN delta means final - initial and AGAIN direction matters. If something bounces back it has a reversed direction and reversed sign. For instance a 1.5 kg ball hitting a wall at 10 m/s and bouncing straight back at 8 m/s has a change in velocity of (  -8 -+10) = -18 m/s and a change in momentum of -27 kg m/s.