One of my players wants to create a sling of doom, shooting boulders out of it. In fact this was a project of mine in the previous saga that never came to fruition, so he is picking it up. We are discussing what level would it be to transform a pebble into a boulder. That would seem to be MuTe with a Cr requisite. I argue that it will be level 5 as default (kincrease weight a thousandfold and size in the same proportion) but he argues that it should not be that weird.
+1 stone, +1 diameter, +1 touch.
He argues for level 15 (base 4)
I argue for level 20 (base 5)
This is the first spell in a series of spells and slings related. The character is a tytalus and former sheperd, quite adept at shooting stuff with his sling. The spell is to be named "David's Slingstone", even if it would not be able to affect Goliath (Goliath is likely to have had MR)
Judging from 'Object of Increased Size', it's base 4 to double the dimensions of an object in every direction and multiply the weight by 8x. I'd suggest adding a magnitude for each doubling beyond that, so... base 5 would multiply it by 4x in every direction, base 10 for 8x, base 15 for 16x, and base 20 for 32x. Assuming that the pebble is about half an inch in diameter, that gives you a sixteen inch wide boulder - a bit small, but still, impressive enough for a sling bullet. Since the pebble and the boulder are both below the Terram 'Individual' target size, there's no need to add a size modifier to the spell.
So, it becomes Muto Terram 30 (base 20, +1 touch, +1 diameter, +0 individual), to change a pebble into a boulder.
A sling enchanted to perform this effect would probably want more levels for multiple uses and penetration, of course, and you may want to increase the duration to diameter if you're trying to throw the pebble very far.
The damage looks like it'd be along the lines of +15, for a pebble-turned-boulder. A bit low for the spell level, perhaps. On the other hand, a Rego(Creo) Terram to create a boulder and fling it like a slingshot would be Re(Cr)Te30 for +15 damage too, judging from the guidelines in societates.
Another option might be to use Rego Terram base 15, to just throw a normal sling bullet hard enough to deal +15 damage. This additionally doesn't require a duration longer than momentary, so it ends up dropping it to ReTe20(base 15, +1 touch, +0 momentary, +0 individual). This lacks the 'Oh my god he's throwing boulders at me!' effect, though.
My take on it would be to have the sling turn normal boulders into pebbles (range touch duration concentration item maintains concentration) when the "pebble" leaves the sling it resumes its mundane boulder form.
Are you ruling that momentum is conserved for game balance reasons?
Eight times previous volume is base level 4. This might be enough depending upon how flexible his definition if boulder is.
Level 5 might be ten times that (80x original size) if you are comfortable with the size alteration rules applying to muto spells.
Yes, as Erik says, it's by far more efficient to turn boulders into pebbles and then fire them.. for a start, it's a difference between having to make another penetration roll.
We apply the basic x10 volume size modifiers to all spells in our saga, it just seems to make more sense if things are consistent. I also ended up altering the base terram sizes.. so they all fit together properly.. .. cause it's damn annoying that shrinking stone will use a different set of values than growing a gem (which just has non decimalised arbitrary value)
It is a spell, not an enchanted item. The spell is cast while you shot the sling (concentration of 9+) and is designed as an anti-mundane spell, not an anti-MR spell. The boulder to pebble to boulder again Sling oif Death will come later. And yes, we rule that momentum is kept.
Right. That's conservation of speed in my book. In what I remember of physics, conservation of momentum implies the mass x speed total stays the same when one or the other changes, not speed alone... thus my confusion.
Then again, modern physics does not work in Mythic Europe.
Erik's comment seems to come from a deeper use of medieval physics than what most gamers use. We don't do it that way, but IIRC, according to medieval physics (that are NEVER described in the rules, BTW, so I prefer not to force them on my players) it seems that the pebble would fall to the ground the moment you transform it into a boulder since momentum is not kept between states. We conserve movement between transformations so a 40mph peeble turns into a 40mph boulder.
The actuall spell is "turns a pebble into a boulder. If you cast it on a pebble you are shooting with a sling (concentration roll of 9+) the transformed boulder causes +20 damage on impact."
If real-world physics applies, despite the depravations of magic, you could have a lot more fun than that.
Enlarge a small pebble by a factor of about 15 to make it the size of a sling bullet -- just for a moment -- and fire it from a sling. As it leaves the sling it turns back into a pebble, with the pebble's mass but the momentum imparted by the sling. That's one very fast pebble.
A skilled sling user can throw a stone with a velocity, as the stone leaves the sling, of up to around 27m/s (the figure I've seen is "in excess of 60 mph" as an upper limit, but I don't know for what weight of stone). If the stone's mass were subsequently reduced by a factor of 15 with no loss of momentum the stone would then be travelling at 405m/s -- just over the speed of sound (330m/s)
What's more, once the stone has returned to its normal, small, size it will no longer be magical, and so will not be deflected by Parma.
It's not all as simple as this, of course. If the stone is to be about the size of a normal sling-bullet when fired (so that the 27m/s figure will be about right) it will be very small (about pea-sized?) and much lighter once the magic wears off. A supersonic pea will have the ability to do a great deal of damage at short range, but as it lacks the mass of a normal sling bullet it will be slowed down by air resistance much more easily and the effective range will be much shorter. At supersonic speeds one must also consider the effect of turbulence in air, which will cause considerable drag on the stone and slow it down still more quickly.
Conversely, if you start with a sling bullet and enlarge that significantly you will get big rock that weighs too much to fire efficiently from a sling. There is some leeway, though, as sling stones and bullets with weights from 25g up to 500g or so have been found in archaeological sites, and those stones were presumably chosen/made to be optimal for the style of sling use in the area. The best use of the trick might be to take a (say) 30g sling bullet and magic it momentarily to 450g, fire it from a heavy-duty sling -- almost certainly at less than 27m/s because of its size -- and have it transform back into a 30g bullet, now travelling at more than 27m/s (but probably less than supersonic).
All this is very interesting but, of course, what allows us to make a stone ten times larger and heavier in the first place is magic, which is unknown in the Real World and therefore is not obliged to be describable in terms of its science. If the effects of magic cannot be described by science, there is no reason to suppose that the law of conservation of momentum should apply to magical effects.
You mean you just invented the hermetic equivalent of a gun? Using the oldest thrown weapon of history?
We consider that mass or weight do not affect speed for simplicity, but in your case it sounds like if they would, the process can be pretty much guaranteed to kill anyone hit by it but terram mages. Sudddenly ReTe (stone) wards sound quite interesting.....