D&D 5E (2014) Wherein we discuss spells and other magical things.

[Satyrn]

You're correct. They're very different. Weight is just the force exerted on the ground by an object with mass in a gravitational field. You could have an object with mass but negligible weight relatively easily if you were outside the gravitational field or if some force blocked gravity for example. Such an object wouldn't weigh much, but you would still have to exert some force to move it.

Aye, although I think the equation [MENTION=6801845]Oofta[/MENTION] posted in reply to my question says that when gravity > 0 then a weight of 0 requires the mass also be 0.

I think the equation messes up though if we try to figure out what the is gravity when start by knowing that weight and mass are both 0. Which would be where the "it's magic" explanation comes in. Which leaves me wondering what physics has to do with anything in D&D

 

[Lanefan]

Whether or not the sphere could bounce would depend on its properties. A bouncing ball deforms when it impacts, turning kinetic energy into potential energy and then, when it returns to its original shape, releasing it into kinetic energy with a different velocity, sort of like a spring with infinite axes. The more the ball can deform without breaking, the more it will bounce, which is why rubber balls bounce more than rocks. In the case of the spell, it is not specific about whether the sphere can deform, though it cannot be damaged.

When I hit one pool ball with another, or one curling stone with another, neither is deformed yet the one being hit reacts by moving somewhere based on the angle of impact and all that, while the hitter's motion is also changed.

Same might go for the sphere vs. the planet; only this time the mass difference is so great that the planet wins every time, deflecting the sphere aside if it hits on any angle at all. If the fall is exactly at right angles to the ground, however, the sphere would stay put on landing, similar to a hit-and-stay in curling.

Lanefan

 

[Lanefan]

If you can push against the sphere's walls you can also splat against them.

Exactly.

If "hamstering" can't happen (and the sphere can't be moved by outside forces) then all this debate happily goes away. But it can, meaning that things both inside and outside* can in theory be splatted by the sphere should a light push ever become a very sudden (de/ac)celeration.

* - which raises another angle: what happens if a falling-from-great-height sphere lands on someone...or, getting slightly absurd, lands on a Tenser's Floating Disk?

Lanefan

 

[Satyrn]

* - which raises another angle: what happens if a falling-from-great-height sphere lands on someone...or, getting slightly absurd, lands on a Tenser's Floating Disk?

Lanefan

That's not at all absurd - nearly every fratboy in an Eberron wizard college steals his professor's wands so they can try that trick.

*And the cool professors actually set it up as an in-class demonstration.

** While the evil professor performs the demonstration with a fratboy inside the sphere, but lets his concentration lapse at the worst possible moment so he can cast charm person on a co-ed.

 

[Dannyalcatraz]

The ground is not adding any energy to the equation.

If you want a terminal velocity impact to be able to damage the occupant, it has to- F = MA: the massless sphere cannot be the source of the decelerating force against the occupant.

Energy is being transferred from the occupant to the sphere in the form of momentum. The occupants body will also deform to absorb some of the energy. The sphere may or may not violate basic conservation of momentum by not transferring any momentum to the ground.

My position since the first post is that the ORS, by negating physical damage across its barrier, violates that rule in both directions, with the spell's stated exceptions regarding moving the sphere from within or outside.

Either you are transferring energy* to the occupant (including gravitational) or you are not. Choose.

I've said it several times from my second post on:

According to the spell, the inability to transfer energy operates in both directions: kinetic energy from outside cannot enter, kinetic energy from inside cannot exit. Force in either direction is magically dispersed to zero*.

* with the exception of being able to roll the sphere like a hamster.

I say again- the ORS magically reduces harmful kinetic energy to zero.

The source of the problem pops up in every RPG system I can think of: in an effort to simplify the language of the game, complications are created. Law of unintended consequences.

The description of the spell's mechanical effects starts with an absolute statement: "Nothing...can pass through the barrier, in or out..." then immediately starts giving us exceptions. But it doesn't list all the exceptions- gamers simply imply them in. We asssume enough light and sound can pass to see and hear. But anything beyond that? Well, the spell says the occupant can't be harmed by effects from outside, soooo...

And of course, there's the kinetic energy problem we've been bouncing around for 9+ pages, when we know that somehow, the ORS can be moved by those inside or out.

IOW, the ORS doesn't obey the rules of physics as written. It seems to act somewhat like a speed limiter on a sports car: this much, and no more. But it is virtually silent about the effects of the occupant's actions beyond being able to move the ORS, and being equally incapable of damaging those outside it as they are him.

 

[Oofta]

I say again- the ORS magically reduces harmful kinetic energy to zero.

And I keep repeating that the spell does not say that. Either energy can be transferred to the occupant giving them velocity and therefore momentum or it is not. In addition, the sphere is not transferring kinetic energy to the occupant when it hits the ground.

I don't know where you get the concept that if you jump off a cliff that the ground transfers energy to the person hitting the ground, it's completely backwards. Energy (in the form of momentum) is transferred from the falling object to the ground, not the other way around. You hit the ground, the ground does not hit back. Well, unless it's an earth elemental.

Feel free to rule any way you want. This was an interesting thread and I do agree that the most logical conclusion may be that no energy is transferred to the occupant, including gravitational force which makes them weightless. The occupant of the sphere is removed from our space/time continuum in many ways. Well, except that time continues for the occupant which shouldn't happen but hey, it's magic.

Good gaming!

 

[Cognomen's Cassowary]

When I hit one pool ball with another, or one curling stone with another, neither is deformed yet the one being hit reacts by moving somewhere based on the angle of impact and all that, while the hitter's motion is also changed.

Same might go for the sphere vs. the planet; only this time the mass difference is so great that the planet wins every time, deflecting the sphere aside if it hits on any angle at all. If the fall is exactly at right angles to the ground, however, the sphere would stay put on landing, similar to a hit-and-stay in curling.

Lanefan

Well, in point of fact, they do deform when they strike. The two games you cite are both designed to have the most elastic collisions possible, meaning that as much energy is retained within the system as possible, and none of the moving bodies in billiards or curling is massive enough that its gravity perceptibly affects the motion of the other bodies.

The sphere striking the ground would, in all likelihood (though, again, we don't really know how the sphere behaves) be far less elastic, as both the rigidity of the sphere and the gravity of the planet would resist its bouncing. Assuming the planet is an actual planet as we think of them--i.e., if it has collapsed into a spheroid under its own mass and gravity--then the sphere will be drawn toward its center of mass at a right angle to its surface. The sort of glancing blow one sees in pool and curling is impossible. If the ground is level, the sphere either bounces if it can deform and regain its shape or does not if it cannot.

And if it strikes a slope? Exactly the same. It bounces (or deflects) if it can deform and spring back or it does not if it cannot. It might conserve some momentum by rolling, but, again, we don't really know. It could be sticky for all it matters.

If "hamstering" can't happen (and the sphere can't be moved by outside forces) then all this debate happily goes away. But it can, meaning that things both inside and outside* can in theory be splatted by the sphere should a light push ever become a very sudden (de/ac)celeration.

Except, as I mentioned earlier, the "hamstering" might simply be a magical effect of the spell and not a physical interaction with the ground. If the sphere is weightless, taking that to mean neutrally buoyant in air (assuming that the spell is cast in air, which is a whole other can of worms), then why should it cost an action and half the occupant's speed to move it? It would take almost no effort if this were a purely physical interaction.

A fun hypothetical: if the resilient sphere is cast on a flying creature, what happens? Can the creature move the sphere simply by flying against its side? It is, after all, weightless. Can the creature move it only at half speed? Why--what is resisting? If the creature cannot move the sphere, since it needs to be "hamstering" against the ground, and if the creature needs to move to keep flying, what happens? If the creature cannot continue flying and settles to the bottom of the sphere, does the sphere begin to fall? Why--how is this different from the creature pushing on its side? If the sphere falls, does it accelerate as though the creature were falling unimpeded, or does it use the sphere's flow characteristics? If the creature accelerates at its normal rate--i.e., if the air does not interact in any way with the sphere--and the creature reaches a speed at which it would normally be able to fly, can it gain lift and level off the descent of the sphere, or can can it still not move the sphere horizontally? Can it use its wings within the sphere to break its fall?

Or, instead, if the creature is able to move the sphere, can it keep flying despite the sphere preventing air from moving over its wings as normal? Is the creature able to keep flying by circulating the air within the sphere, basically flying on a treadmill? If so, does it keep moving forward?

Another fun hypothetical: a creature is in a resilient sphere on the ground. Somebody casts reverse gravity in the area. What happens? Does the creature fall up and the sphere fall with it, since the creature is affected by gravity? Or does the creature stay where it is, because spell effects cannot penetrate the barrier?

 

[Satyrn]

IOW, the ORS doesn't obey the rules of physics as written.

Oofta gave me the physics explanation for going from weightless to massless.

But if the sphere doesn't obey physics, I don't understand how you are concluding that the sphere is massless.

 

[Dannyalcatraz]

And I keep repeating that the spell does not say that.

No, it doesn't. But that's a description that is consistent with the known and uncontroversial behavior of the ORS.

I don't know where you get the concept that if you jump off a cliff that the ground transfers energy to the person hitting the ground, it's completely backwards. Energy (in the form of momentum) is transferred from the falling object to the ground, not the other way around. You hit the ground, the ground does not hit back.

Your understanding of Newtonian physics seems...flawed. The concept that the ground is applying equal and opposite force to the falling object is Newton's 3rd!

First law: In an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a force.

Second law: In an inertial reference frame, the vector sum of the forces F on an object is equal to the mass m of that object multiplied by the acceleration a of the object: F = ma.

Third law: When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.

(Emphasis mine.)

 

[Dannyalcatraz]

Oofta gave me the physics explanation for going from weightless to massless.

But if the sphere doesn't obey physics, I don't understand how you are concluding that the sphere is massless.

Well, it explicitly says it is weightless.

The sphere is weightless and just large enough to contain the creature or object inside.

As for the obeying of physics...

Any spell you look at obeys AND breaks laws of physics. Most magical fire is still hot and sheds light.

The question, then, is which rules get broken or obeyed, how, when and why?