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Minor Alchemy allows you to transmute one object into a different material. What happens if the object is in motion (example: arrow in flight) when it reverts? What is the effect of mass changing while in motion? Does it stay at the same speed, and somehow the reverting just gives it a huge increase in momentum? Does it slow down, conserving the amount of momentum? If the rules specify (doubtful) then great. If not, I'd like some sort of a physics perspective on the question.

I'm mostly worried about two things:

  1. If it doesn't change speed, you could just throw a very heavy object that has been transmuted to be really light, then release the transmutation right at the end, landing with a hugely powerful amount of force?
  2. If it does change speed based on mass and momentum, then you could shoot a ballista with a spear of a very light metal transmuted into a very heavy one, then reverted once it was airborn, giving it a huge speed boost. No way you would normally be able to send that light of a spear with that much force.

Either way, either you get an advantage from going from light to heavy, or you get an advantage in going from heavy to light.

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    \$\begingroup\$ I seriously doubt there will be rules to cover this, meaning it falls under the DM's purview. And of course, d&d isn't a physics simulator, which you can see quite clearly with the introduction of magic. \$\endgroup\$ – D. Ben Knoble Oct 19 '16 at 17:45
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    \$\begingroup\$ @DavidBenKnoble While your points are true, Patrick vD's question is a good one, as the implications are tremendous with either ruling. Seems to me a good answer would not simply be "DM's call" which is literally true about all rules in 5th edition, but would give consideration to game (im)balance implications. \$\endgroup\$ – Lexible Oct 19 '16 at 17:58
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    \$\begingroup\$ @Lexible agreed. I merely sought to point out the lack of a definitive ruling. An examination of possible scenarios would still be valuable. \$\endgroup\$ – D. Ben Knoble Oct 19 '16 at 17:59
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    \$\begingroup\$ @DavidBenKnoble That's cool. I guess am starting to feel that "that's the DM's call" is not really that helpful a critique of interesting questions on the SE. That said, there's plenty of questions that (a) fit within the RPG.SE guidelines, and (b) must have "that's the DM's call" as an answer. \$\endgroup\$ – Lexible Oct 19 '16 at 18:01
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    \$\begingroup\$ Closely related: How do I handle my players killing things by catapulting a folding boat? (Different method, same mass/velocity question, but different method affects the answer.) \$\endgroup\$ – SevenSidedDie Oct 19 '16 at 19:18
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Alright, as both a physicist and a DM these are the questions that also haunt me. I'll answer the second case first.

Althought it'll never happen in real life (an object of macroscopic size won't change mass drastically in an instant without that mass going somewhere or being converted into energy), there should be no difference in the energy of a heavy bolt being shot by a ballista and transmuting into a lighter bolt than in the energy of a light bolt being shot in the first place. This is simply due to the law of conservation of momentum and to the mechanics involved in an inelastic collision. This is due to the fact that a heavy bolt shot by a ballista will initially be going much slower than the light bolt, and the transformation between the two should just compensate for this. Same thing for a light bolt being shot then becoming a heavy bolt, it'll lose speed so that the momentum is conserved. This is keeping in mind that the momentum affects the force of the impact, and therefore there should be no difference in impact damage. This would be the physically accurate answer, but it's also the less entertaining one for gameplay. As @Lexible mentioned in the comments, in heavy-to-light transmutation, the range could be doubled, but that is up to you the DM.

In the first case, well then that's your call as a DM. This could be hugely fun for your characters (polymorph a tiger into an ant then throw it and release polymorph, have your barbarian throw it to create a pseudo tiger catapult), but could also be a handicap if the enemy does the same. The rules don't cover this, and since D&D is by design open to this kind of thing, don't hesitate to break some of the laws of physics to improve gameplay!

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    \$\begingroup\$ Your first paragraph almost covers it. The important difference is not in the penetration/damage of the heavy projectile that become light and fast, it is in the range of that lightened projectile. The much faster traveling spear should, perhaps, have it's range multiplied, potentially by quite a lot. For example, my spear now travels miles. \$\endgroup\$ – Lexible Oct 19 '16 at 18:12
  • \$\begingroup\$ Whoops, should have written second paragraph. :) \$\endgroup\$ – Lexible Oct 19 '16 at 18:19
  • \$\begingroup\$ Well, if we're gonna get physical about it, while it would be greater I wouldn't say the range is increased by all that much for two reasons: wind resistance, which will slow down the lighter spear more quickly, and the fact that the presence of cold/warm cross air currents and the coriolis effect would affect its accuracy on a range of almost a mile (much like a sniper's bullet). So yes increased range, but doubled at most. \$\endgroup\$ – QuantumDM Oct 19 '16 at 18:19
  • \$\begingroup\$ I have a question, since I'm not a physicist I may have some misunderstandings: does the conservation of momentum implies that the (des)acceleration will be the same? I ask because as far as I know f=ma, if m changes but a is constant, would not it mean a change in "damage"? \$\endgroup\$ – Chepelink Oct 19 '16 at 18:24
  • \$\begingroup\$ The force of impact is the equivalence of the change in momentum over time, which we'll assume to be constant since those calculations are too complex for this kind of exercice. But conservation of momentum implies that the heavy spear will go slower, and therefore while m increases (decreases), a would decrease (increase). \$\endgroup\$ – QuantumDM Oct 19 '16 at 18:27
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Neither condition results in different damage per se, but both conditions result in changes to the range of the missile.

Assuming momentum is conserved, condition 1 would result in a light-to-heavy missile dramatically slowing, but potentially being placed so as to easily drop on those below. If the timing of the flip can be precisely known by those hurling the missile, then in game-play terms they could effectively create either a longer range for their heavy object if it transforms at the end of its flight (e.g. I can easily throw a ball 120 feet, but I cannot easily throw a sledgehammer 120 feet; now if that ball transforms back into a sledgehammer at 119 feet...), or could effectively create a "hammer drop" maneuver (like the overhand Frisbee hammer throw) for the heavy-to-light missile if it transforms back into heavy mid flight and effectively dropping it's velocity to near zero, allowing precise targeting from above for a practiced attacker.

Assuming momentum is conserved, condition 2 does not change the amount of force of the projectile, it changes the velocity of the projectile. Any resulting damage will be a consequence of velocity * mass. The game-play consequence is that with greater velocity your heavy-then-light projectile travels farther. The earlier in the flight the transition occurs, the faster the light missile will fly. The DM will need to rule on how the added velocity affects the missile's range.

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