What do you think mass applies to? Although a ton of feathers and a ton of lead weigh the same, drop them and the gravitational pull on the feathers is less because of the difference in density.

Newton's law deal with the change of momentum, thus f=ma.

From which frame of reference, the ball's? The ball leaves the pitcher's hand with velocity V, and with air resistance, maintains that speed. The ball strikes the bat, and returns with a new velocity, -v2. Assume the ball travels in a straight line and is hit directly back at the pitcher - then the total change in velocity is (V-V2). It's momentum changes from (mass of the ball is m) mV to mV2.

If the bat comes to a stop (it doesn't) and the ball receives 100% of that momentum (also doesn't), then you can say that (mass of bat is Mb, velocity is -Vb) -MbVb +mV=-mV2

V2=(MbVb-mV)/(m)

To maximize the speed of the ball coming off the bat, you want a heavy bat that is moving fast. Has anyone said anything different?