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Message: <blockquote data-quote="Greenfield" data-source="post: 5606971" data-attributes="member: 6669384">No, not right.  The train "collided" with an object that had mass, and "less momentum" (i.e. was moving at a different speed).  That's not the same as an object, at speed, suddenly increasing its own mass.No, not right.  The rock collided with an object (your hand) that was moving at a slower relative velocity.  Your body then had the urge to pick up some speed, though in practice you then transferred this to the ground you were standing on.  Again, no object changed its mass. Separating mass into different segments with the same motion is far different from an object picking up mass from noplace, or losing it to noplace. Okay, let's look at that.First, presume that if you're going to invoke the laws of physics, you have to take the whole package, not just the parts that do what you like.Second, presume that you're on a planet that isn't the flat, stationary center of the universe.  That is, it rotates on its axis and orbits a star.  If it's earth-like in gravity, density, and length of day, then it's about 25,000 miles around, and rotates in about 24 hours.  Presuming you're someplace temperate (i.e. not at the poles or the equator) this gives us a surface rotational velocity of about 1,000 mph.  More if you're closer to the equator, less if you're more polar, but that's a good number to work with.Next, if the star is comparable to our sun, and the year is the same, that means you have an average orbital velocity of 62,500 mph.  Higher when the planet is closer to the sun, slower when farther, but 62,500 on average.  (Earth is close to the sun during the north hemisphere winter, for example).We could also count the velocity of the sun itself in it's orbit of the galactic center, but we have enough to give the general picture.Finally, we'll presume that in your physics driven universe, conservation of energy applies.  That is, you can't create or destroy energy or matter, just change it from one form to another.So if a moving object must lose velocity when it magically expands, but somehow fails to gain any when it shrinks, you are effectively destroying energy, and that isn't allowed. Whatever happens at one end has to be balanced at the other, right?So when you shrink 2000 lbs of stone by a factor of 4096 to get that half-pound sling stone, it has to multiply its momentum by that same factor.  And since the "stationary" view of that boulder on the ground is only an illusion of relative motion, it's new velocity should be 63,500 x 4096, for a grand total of 260,096,000 miles per hour, relative to the sun.Now subtract original planetary surface motion of 63,500, and you get 260,032,500 miles per hour relative to the caster of that spell.Now that presumes a midnight casting, when the rotational velocity will add to the orbital velocity.  If you do it ant noon the numbers are 61,500 x 4096, for a total of 251,904,000.  Subtracting out the original 61,500 we get a surface relative velocity of 251,842,500.Actual results will vary, of course, based on your exact latitude, altitude and time of day, and of course you should take care not to be standing in the path of the boulder.  Or near it.  Or in the same time zone, for that matter.  The vortex it will create as it takes off will be like a hurricane. <img src="https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/1f642.png" class="smilie smilie--emoji" loading="lazy" width="64" height="64" alt=":)" title="Smile    :)"  data-smilie="1"data-shortname=":)" />So okay, that's the math.  But let's scale it down from the ridiculous.When a person under Enlarge Person fires a bow, their arrow reverts to normal size and mass as soon as it leaves the bow.  That's a mass decrease of 8 to 1.  Do you play that it's speed increases by that amount?  That would take it supersonic and increase the range and damage immensely.  Do you do that?If not, why not?  Does Conservation of Momentum only apply when it's convenient?If you want to compare that to your rock and train example, someplace there should be a "rock" (i.e. the mass the arrow lost).  Do you have it shed dust, or split into 8 arrows?  In a real, physics driven world, "stationary" is an illusion.  Everything's in motion, always.  And there is no way to increase or decrease the size/mass of an object (outside of relativistic velocities).  All you can do is add another object to it, or split it into several objects and take one or more of them away.The "unnecessary complication" is to rule that a shrunken boulder changes speed when it grows.  Magic and physics can't co-exist.  Magic violates physics, pretty much every time.  That's why we call it magic.PS: The reason I referred to the "hamster canon" was because of what happens when you Polymorph an Elephant into a Hamster.  60,000 lbs reduced to 1/4 pound means a mass decrease of 240,000 to 1.  Hamster takes off at a relative velocity that's strictly theoretical, since it's faster than light. <img src="https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/1f642.png" class="smilie smilie--emoji" loading="lazy" width="64" height="64" alt=":)" title="Smile    :)"  data-smilie="1"data-shortname=":)" /></blockquote>

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