I asked you not to make me do this, but once again I'll break out the Hamster Canon (tm).
I posted this a few months ago, so if it sounds familiar that's why.
I had the Conservation of Momentum question come up once before, when dealing with an epic monster, a colossal construct of some type. It had a DR50/+5 (It was a 3.0 critter), and an anti-magic field that went out 50 feet, so there pretty much wasn't a +5 weapon anywhere near it.
My solution was to take a mule and load it down with a pack saddle and as much stone as it could hold. It doesn't have to walk with it, just be able to stand up. We figured the fully loaded mule weighed a ton.
Now Polymorph it to a hamster. Since hamsters don't wear pack saddles, the pack saddle and boulders get absorbed into the new form, and effectively vanish. Rinse/repeat until you have a bag full of hamsters.
Load hamster into sling and fire. As soon as it hits the AMF, it turns back into a fully loaded mule, and hits like a ton of bricks (literally).
And, of course, someone argued for the application of a physics principle to a situation where it could never occur. Conservation of Momentum was invoked.
In response, I brought the following argument: If the game world runs on physics, then it's a round world, about earth sized since it has similar gravity and day/night cycles. It's spinning on its axis, and orbiting the sun at about the same distance.
The DM agreed.
So, I argued, the surface of said world is moving at a bit over 1000 mph, since it has a circumference of 25,000 miles and rotates in 24 hours. It has an orbital velocity of about 62,5000 mph, in order to have an orbital cycle the same as Earth's.
The DM agreed.
When I do that Polymorph, we're converting 2000 lbs to about 4 ounces, or 1/4 pound. That's a mass drop of 8000 to 1. If Conservation of Momentum applies, and we're doing this at midnight, when the surface rotational velocity adds to the orbital velocity, the hamster should suddenly acquire a lot more velocity. As in, 8,000 x 63,500 mph of velocity, or about 75% of the speed of light.
The sudden departure of the hamster from the planet's surface (it's now moving well in excess of even Solar escape velocity) should create a shock wave that will devastate pretty much everything within miles of its path, and tear away approximately 25% of the planet's atmosphere.
If we did it at noon, when the rotational and orbitals speeds aren't combined, the effect is slightly less. At sunset the direction is more vertical than horizontal, so the path of destruction should be much less, but if you do it at sunrise the course would be straight down.
Think of the meteor impact that punched the Earth's crust in the north Atlantic, and formed Iceland. The point on the surface where the mule was standing would now be more or less theoretical, as the impact crater would kind of destroy all reference points around it.
Now, as you struggle to grasp the enormity of the energy we're talking about, we won't discuss the velocity of the solar system in its orbit around the center of the galaxy. Nor will we even begin to attempt the math needed if we were to have used an elephant instead of a mule. Relativity begins to come into play, and we'd be arguing whether the hamster is now traveling at 45 times the speed of light, or whether Lorentz-Fitzgerald contractions come into play, and time distortions limit it to 99.999% of the speed of light.
And all from a single 4th level spell, and the very very bad idea of trying to apply poorly understood physics principles to a situation where they couldn't occur.
Remember, it's a game, and pretty much all magic already ignores the laws of physics. Trying to apply those laws, selectively, is just a bad idea. There's always someone like me to come along and ask why it only applies half the time.
<edit> The last time I brought this up, we got into a discussion on the aerodynamic properties of hamsters, and whether he should be pointed nose-first or butt first. You don't want to go there, trust me.
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