Shrink Item Stones

[Dandu]

On the other hand, why not have the raven fly the stone at the enemy, then you command it back to size? It will still hit like a ton of bricks.

Yes, that is probably the best option.

 

[Iku Rex]

It's been a while since I had any physics classes, but I'm going to give this a try anyway.

Ah, yet another place where the Hamster Cannon (tm) is needed.

Conservation of momentum doesn't apply. It's magic, which involves an event that can't happen in a world where physics works: An object spontaneously gaining or losing mass.

I don't understand what you mean by "spontaneously" in this context.

Simple thought experiment with a moving train (focusing on momentum):

A rock is is dropped straight down into one of the cars. The train as a whole has now gained mass. It will slow down. Right?

Same train, only this time you grab and quickly yank out the rock. The train has lost mass. It will not move any faster as the rock is still moving forward as it is removed. Right?

I showed the full math on this before, and can drag it up if you need me to, but I'll give you the short form: In a physics driven world, the planet rotates on its axis and orbits the sun. Adding those two velocities together (as would happen at midnight), and multiplying by 4096 (as would happen when you shrank the item to begin with) results in the boulder ne slingstone taking off at about 37% of the speed of light. Do it at noon, when the two velocities don't add together, and the take off speed will be about a third the speed of light.

So no, the stone won't lose all momentum when Shrink Item ends, any more than it would achieve solar escape velocity when it began.

Yes, I'd like to see the math, or, more importantly, the assumptions you used to do it.

I have a pile of dirt. I use a shovel to remove mass from the pile, making it smaller than before. Does this make the pile of dirt move? Why must it be different [barring special help from the magic] if I use incomprehensible magic to shrink the pile?

Also, even if the spell somehow does something with conservation of momentum when it shrinks the item, it does not follow that the spell must transfer a potentially enormous amount of energy to the item as it grows back. That seems like an unnecessary complication, without any support whatsoever in the spell description. (Remember, once the item is restored the spell ends, so the magic has to do its thing before then.)

 

[thompsja]

More fun with Shrink Item.

I'd agree with the people that would have it do sling damage, then it expands and creates some sort of obstacle. It does get rude when you can drop it on the roof of a building and then a much heavier stone then falls down to ground level. Another use for Mage Hand. If you take improvised weapon (1d3), and you increase it four sizes, it does 2d6. I'd probably use catapult rules. Getting hit by a one ton boulder should hurt more than a greatsword.

Other ideas include getting a large bottle full of sand and reduce it. You throw it at a foe, and he's partially trapped in sand when it breaks.

Thinking further, we can make a perpetual motion machine:

Imagine a wheel. Attached to it is a weight on a chain. Now, imagine some sort of flat surface that casts Shrink Item on the item that touches it. The weight pulls down the wheel, and touches the spell trigger item. It now weighs far less. The momentum pulls the wheel and weight up, until it gets to the top of the wheel, where it hits a flap, causing the weight to regain full mass. The weight is heavy on the downside, and light on the upside. Not cheap, since you have to have a use-activated 3rd level spell.

 

[Dandu]

Isn't it easier to achieve perpetual motion with a permanancied Gust of Wind spell?

 

[Dannyalcatraz]

Isn't it easier to achieve perpetual motion with a permanancied Gust of Wind spell?

Zombie on a treadmill powering gears.

 

[the Jester]

I showed the full math on this before, and can drag it up if you need me to, but I'll give you the short form: In a physics driven world, the planet rotates on its axis and orbits the sun. Adding those two velocities together (as would happen at midnight), and multiplying by 4096 (as would happen when you shrank the item to begin with) results in the boulder ne slingstone taking off at about 37% of the speed of light. Do it at noon, when the two velocities don't add together, and the take off speed will be about a third the speed of light.

I too would like to see the math. I find this sort of thing fascinating.

There are several problems that I see with your assumption, starting with the idea that the physics are the same as in our world (I would argue that is demonstrably false in any D&D world that runs by anything close to the RAW).

 

[Greenfield]

It's been a while since I had any physics classes, but I'm going to give this a try anyway.

I don't understand what you mean by "spontaneously" in this context.

Simple thought experiment with a moving train (focusing on momentum):

A rock is is dropped straight down into one of the cars. The train as a whole has now gained mass. It will slow down. Right?

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.

Same train, only this time you grab and quickly yank out the rock. The train has lost mass. It will not move any faster as the rock is still moving forward as it is removed. Right?

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.

Yes, I'd like to see the math, or, more importantly, the assumptions you used to do it.

I have a pile of dirt. I use a shovel to remove mass from the pile, making it smaller than before. Does this make the pile of dirt move? Why must it be different [barring special help from the magic] if I use incomprehensible magic to shrink the pile?

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.

Also, even if the spell somehow does something with conservation of momentum when it shrinks the item, it does not follow that the spell must transfer a potentially enormous amount of energy to the item as it grows back. That seems like an unnecessary complication, without any support whatsoever in the spell description. (Remember, once the item is restored the spell ends, so the magic has to do its thing before then.)

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.

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.

 

[thompsja]

Zombie on a treadmill powering gears.

I like the zombies better, much cheaper and easier than my implementation. Contiuously casting shrink item means spending 1200 XP as part of making an item, and 15K gold. Permanency on a Gust of Wind costs nothing but 1,500XP and a ninth level caster. Animate Dead just is one spell, with periodic replacement. Can be done with a fifth level caster.

I love this sort of thought exploration.

 

[Theo R Cwithin]

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.

While I do agree with this (which is why I find these sorts of threads so tiresome), I'll also note that rulings like this ideally should have nothing to do with physics; but rather should rely solely upon what the what the DM determines is appropriate. If he wants to rule that an un-shrinked boulder loses all forward momentum, fine; he is no more or less wrong than the DM who rules the opposite. The only "complication" should be in maintaining consistency in the setting, not real world physics considerations.

Rule 0 is more powerful than the whims of magic and the laws of physics combined!

 

[Greenfield]

Yeah, the DM can rule that your sling stone/boulder idea doesn't work. He/she can rule that lightning came from a clear blue sky and delivered pizza, for that matter. But that's just a matter of DM fiat, and/or being a jerk. It has nothing to do with physics.

And I agree that this kind of noise gets tiresome real fast. I guess you could say that bad physics in games is a pet peeve of mine.