what is the maximum downward accelaration you can give your hand in m/s^2
i was thinking 10 due to gravity
No, it's much more than that. A top pitcher can throw a baseball V = 95 mph = 42 m/s while moving his hand forward about 3 meters. The time t required for the motion is (distance/(average speed) 3 m/21 m/s = 1/7 s and the acceleration is V/t = 42/(1/7) = 294 m/s^2. That is about 30 g's
An average male adult should be able to achiever an acceleration at least 100 m/s^2. The value will depend somewhat upon direction that it is thrown, because throwing downwards does present a problem maintaining footing.