Haha you must have missed that day in school Newton’s 2 law
Not sure how F=MA (newtons second law) has anything to do with bullet selection, outside of felt recoil... the acceleration on the receiving end of the bullet has a lot of factors that can widely change the felt force. Energy on the other hand, is easier to calculate, since it only relies on the target containing the shot (not passing through, which would result in wasted energy of the bullet. The force on a piece of armor plate is much higher than if it hit soft tissue for example, but the end effect to the target is drastically different (and in this case, the armor plate target has almost infinitely higher felt force, but walks away, while the lower input force is likely killed). The energy both targets feel however, is the same, assuming the bullet does not pass entirely through the soft target.
Velocity is much more important than weight/mass in most scenarios. KE = 1/2*mass*velocity^2... and P (momentum) = mass*velocity
In terms of energy, a 110gr bullet at 2000fps has a bit more energy than a 165gr bullet at 1500fps (~975ft-lbs versus ~825ft-lbs)
The guy wearing the armor plate is going to get more of a "push" from the smaller, faster bullet, than the heavier, slower bullet...
That said, to answer Sincerd's question, it all depends on what you want to do with said projectile, they aren't all created the same. A light partition round will basically explode on impact of soft tissue, causing incredible damage, but without major penetration (hence why its great on smaller game that doesn't require penetrating a thick hide). Conversely, a heavy, solid projectile, or thick FMJ bullet, will penetrate deep, but with little to no expansion, and risks a lack of energy transfer into the target (it passes through cleanly).
So again, what do you intend to do with the gun/load. If varmint hunting, a light bullet is great, flat shooting, and with the high velocity, will drop the animal cleanly assuming it's expending all it's energy in the target (PSP/Partition). Hog or deer hunting, you'll want something a bit heavier 110gr-150gr, with a bit slower expansion (JHP or Tipped hunting bullet, like the ELD-X or the likes).
