Physics question about moving through air and water.

[Whimsical]

I allow a person using the Fly spell to propel himself through water with the spell. But I would like to have an appropriate movement penalty for this, and I want to refer to real physics to set this penalty.

So, I just need to know how much slower would a person move through water if they are being propelled by the same amount of force that would propel them through air.

Oh, and if you are going to post to ridicule me for trying to include physics reality into my fantasy game world, don't bother. I genuinely don't care about your opinion.

 

[DanMcS]

I allow a person using the Fly spell to propel himself through water with the spell. But I would like to have an appropriate movement penalty for this, and I want to refer to real physics to set this penalty.

So, I just need to know how much slower would a person move through water if they are being propelled by the same amount of force that would propel them through air.

Fly spells don't really provide force, since they include wonky things like instant accelleration and deceleration (perfect maneuverability). So it's hard to say. It might be that the fly spell actually provides velocity, not force directly, so that someone under the effects of 'fly' travels at the same speed in any medium that isn't solid- from vacuum to water, I suppose. Saves you from trying to figure out how fast they can go in a vacuum, too.

Water at 20 C has a viscosity of 1 cP (the unit isn't worth wondering about). Air has 0.0182 cP, about 1/50th of water. So you could say someone travelling with a fly spell has 1/50th the speed in water that they would in air. Is that real-world-physicsy enough?

 

[CRGreathouse]

Water at 20 C has a viscosity of 1 cP (the unit isn't worth wondering about). Air has 0.0182 cP, about 1/50th of water. So you could say someone travelling with a fly spell has 1/50th the speed in water that they would in air. Is that real-world-physicsy enough?

It may be RW enough for the original question, but it would still bug me... the same argument would suggest that the fly spell would propell the character at infinite speeds in a vacuum.

 

[Piratecat]

I would think that how streamlined a person is would make the biggest difference. There's no movement penalty between an elephant with the fly spell and a halfling. Should there be, either in the air or underwater?

 

[DanMcS]

I would think that how streamlined a person is would make the biggest difference. There's no movement penalty between an elephant with the fly spell and a halfling. Should there be, either in the air or underwater?

The viscosity formulas relate to the surface area you present to the liquid, in the direction you are travelling, such that a big flat thing travelling through a liquid has much more resistance than a small pointy thing, for instance.

But, as you noted, Fly works the same for everybody, which is why I figure it just supplies a velocity, not a force.

 

[ARandomGod]

It may be RW enough for the original question, but it would still bug me... the same argument would suggest that the fly spell would propell the character at infinite speeds in a vacuum.

And that they could swim faster than they would fly. In my opinion fly doesn't work underwater. But that doesn't matter, as it was stated that in this world it would... So saying, if you wanted to apply a penalty to move speed I think that would be completely appropriate. Personally I'd make it 1/2 speed, just for simplicity. Making it 1/50 (as would suggest by one interpretation of physics) would just be silly. And, of course, you can't apply the physics properly, because fly isn't written out well enough for that.

 

[Dr. Harry]

Making it 1/50 (as would suggest by one interpretation of physics) would just be silly.

Actually, one of the things that I like about physics is that one can't simply say "one interpretation of physics", or rather, one can, but all interpretations but one (at best) will be wrong. It is a case of is the interpretation correct or not?

But, as was correctly noted:

And, of course, you can't apply the physics properly, because fly isn't written out well enough for that.

If fly provided a thrust force then I could see a way to analyze this, but as we are dealing with magical thinking, then as long as there is some half-baked rationale, then many answers could work. I would say that some side-effect of the spell magically reduces the drag of the overall medium, allowing the fly spell to work at half-speed.

 

[Macbeth]

I would apply a 1/4th speed penalty: it abstracts out some of the surface area stuff (so no 1/50th speed), but still makes a huge difference.

 

[Morrus]

I've never let Fly work underwater; as far as I'm concerned, "Flying" is mving through the air. It's just magic!

 

[Vaxalon]

Morrus has it right. Sheesh, when are you guys going to learn that when you mix real world science with magic, you get nothing but trouble?