How far does that zero-inch fist travel into the target? Wouldn't that make a non-zero "a" easy?
F1=m1a1 (nage - one who did the punch)
F2=m2a2 (uke - one who got launch)
F1 = F2
a1 is the one that's interesting. zero-inch punch = nage fist flat against uke body, no draw back any part of the body. this is one of the teacher tests, i believed.