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

[Lanefan]

Do you distinguish between violent movement of the sphere caused by impact with the ground, and violent movement of the sphere caused by impact with a dragon's tail for example?

No, not really. Dragon tail or giant golf club are likely to cause violent movement vaguely horizontally...maybe also ending violently if there happens to be a handy cliff face or cavern wall nearby.

Lan-"and if the dragon tail punts the sphere over a cliff...well...nice knowin' ya"-efan

 

[Oofta]

I'm not sure this silly question about a silly game is worth continuing to hash out, but . . . If the sphere stops energy from going through either way, how will the energy of the fall get inside?

Edit because I just saw this:


This is not correct. An object cannot have weight without having mass, but an absence of weight is not evidence of an absence of mass.

Weight is mass times gravitational force. Assuming that the game rules are based on gravity being a factor, a weightless object can't have mass. Magic can mess with weight and gravitational forces, of course.

An object may have neutral buoyancy and appear to have no weight. But even the contents of an "empty" container weighs something unless the it's a vaccuum because of the air molecules.

Of course I think much of this discussion is silly because virtually all other references to energy in the book are references to magical energy, not physical energy and I see no reason to believe this is an exception.

 

[Lanefan]

Of course I think much of this discussion is silly because virtually all other references to energy in the book are references to magical energy, not physical energy and I see no reason to believe this is an exception.

What's more interesting - and maybe worth its own thread - is how different DMs might see magical "energy" fitting in with the physics of the game world.

I've got my own ideas, which I won't bore you with here; but if there's interest I can start a new thread (and if I do it'll be in General RP as it'll apply to more than just 5e).

Lanefan

 

[Cognomen's Cassowary]

However, we have a sphere that is weightless so therefore has no mass.

This is not correct. As you say in your next post, magic can affect gravity as easily as it can mass. Also, after all this Newtonian talk, the definition of "weight" that you give in your next post is decidedly non-Newtonian. "Weightless," as used in the spell description, is a much more coherent term in the Newtonian sense, where "weight" is effectively the force a body could exert on a scale in the same state of motion. Thus, a body in free fall is weightless. Similarly, anything neutrally buoyant is weightless.

When the sphere falls off the edge of the cliff, the sphere does not technically fall (it has no mass and is unaffected by gravitational force). It is pushed by the occupant. The occupant accelerates towards the ground as normal because gravity sucks and it's been stated that the occupant can move the sphere.

Well, technically, it has been stated that the occupant can move the sphere up to half his or her speed by using an action. Perhaps he or she has to use an action to "fall" at half his or her speed.

The occupant of the sphere does not hit the ground. The sphere hits the ground, but it has no momentum so it does not transfer any momentum to the ground. There is no equal and opposite reaction because no momentum was transferred. It's velocity still becomes zero relative to the ground. Because magic.

So you are now at the point where you have violated or ignored the law of the conservation of momentum (by adding momentum to a body in a closed system), Newton's first law of motion (by causing an object at rest to not stay at rest), and Newton's third (by allowing an action without a reaction) in order to make this work the way you want it to. And you say it works this way because magic, after having made a case for the magic working this way because of physics. Okay, just want to make sure we've got that straight. Now the big leap to the conclusion . . .

The occupant still has momentum so they hit interior of the sphere. They cannot transfer momentum to the sphere, there can be no equal and opposite reaction in the standard Newtonian sense. They stop because magic.

Except that the occupant never had momentum relative to the interior of the sphere. If the sphere stops magically due to a non-Newtonian physical interaction, there is little grounds to insist that the occupant continues to act as a Newtonian body in a Newtonian universe and continues hurtling downward until it has a non-Newtonian physical interaction with the non-Newtonian body of the sphere.

If their velocity was high enough when they stop, and the stop is sudden enough they take damage. The damage does not originate from outside of the sphere - it happens because of the occupant's sudden deceleration caused by the magic of the sphere and the occupant's momentum.

I cannot even conceive of an action without a reaction, and yet you are stating a sequence of a reaction without an action followed by an action without a reaction would have exactly the same consequence as an ordinary action and reaction.

There are all sorts of issues with ORS if you say you cannot transfer any energy from the exterior to the interior - basically the occupant would never gain momentum (change their velocity).

Actually, that would be entirely consistent with the ruling that the occupant takes no falling damage, since he or she does not move relative to the sphere--or that the sphere does not fall at all.

Not to argue. I just find it a fun exercise. I would simply rule that the occupant takes no damage. Because magic.

 

[Satyrn]

This is not correct. As you say in your next post, magic can affect gravity as easily as it can mass.

But he's already explained to me (twice! I'm dense) how he gets to weightless also being massless, providing a formula that proves that an object of 0 weight in a gravity of >0 has a mass of 0. Since we know the object is weightless, and we're discussing how this works on a planet (thus assuming some gravity) he has shown that the sphere ought to be massless.

But more significantly, he's also regularly saying he might indeed be wrong on that (that the sphere may be weightless by being buoyant like a balloon) so I see no reason at all to tell him he's wrong.


. . . Also, we can assume the sphere is massless by deciding from the spell description that it's essentially a force field, a fantasy creation that would in any fiction be massless. And this way shortcut past any silly physics talk.

 

[Sorcerers Apprentice]

I'm not sure this silly question about a silly game is worth continuing to hash out, but . . . If the sphere stops energy from going through either way, how will the energy of the fall get inside?

The energy is already inside the sphere, it's just converted from potential to kinetic energy as the wizard accellerates down the gravity well.

 

[Oofta]

Not to argue. I just find it a fun exercise. I would simply rule that the occupant takes no damage. Because magic.

I think we can agree that the sphere magical. There is no need for the sphere itself to obey Newtonian physics. Correct?

Probably my biggest mistake is trying to describe the walls of the sphere in terms of a physical construct, because it is not. It has influence on some aspects of the physical world, but it is not what we would consider normal matter and does not need to obey Newtonian laws of physics. (BTW massless particles do break Newtonian laws, there was this guy Einstein who had to make up new rules because of it.)

We know the sphere weighs nothing. It is impervious to weapons, [magical] energy and spells. We know it (and therefore it's occupant) can be moved. I always assumed it was simply a spherical wall of force that surrounded a creature that can move and be moved (unlike a wall of force). Normal weapon attacks are not going to jostle the sphere enough to cause damage to the occupant, falling is an edge case that is not mentioned in the rules.

I see no reason to believe the occupant of the sphere would not obey basic laws of momentum, velocity and acceleration.

Hence the creature follows Newton's first law: an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

If there is a change in velocity of the sphere, the occupant will continue moving in the same direction at the same velocity until acted on by an unbalanced force. That "unbalanced force" in this case is the interior wall of the sphere. There doesn't have to be any conservation of momentum, force or energy in the traditional sense because the occupant is not interacting with a physical object.

If you want to state that the sphere has magical inertial dampers, or drifts down slowly to the ground, that's fine.

If you say that the occupant of the sphere is effectively in a non-Newtonian dimension while in the sphere, that is also works. At that point I would say the sphere and occupant are now effectively weightless (or an insignificant weight), and any movement is simply a property of the magic.

I still like to think of the sphere is a spherical wall of force. You could hit a wall of force with a freight train and the wall would absorb the impact in a way a physical wall could not. If you had a horizontal wall of force and fell onto that wall from a significant height, you would go splat.

However ... it being simply a spherical wall of force does mean a giant could hamster ball over a halfling and crush the little guy. Who happens to be unconscious and prone at the time. With no allies between him and the giant because they're over fighting the giant's shaman and they thought the halfling was safe for the moment. Because it's an evil giant. Whether that's breaking the spirit of spell is up to the DM.

 

[Dannyalcatraz]

The source of your confusion is that your are treating the sphere with wizard as a single body, when they are actually two separate bodies. So when the wizard splats against the inside of the sphere after the sphere has been stopped by hitting the ground, it's not the ground that's the source of the opposite force, but the sphere.

Actually, no I'm not- as I explicitly pointed out multiple times, neither the occupant nor the sphere can be the source of the decelleration force. The same problem exists regardless of whether we treat the ORS & occupant are treated as a single body or two.

In the single body problem, the question is simple: how do you suffer deceleration damage from the ground when the ORS state's the occupant can't be damaged by forces originating outside the ORS. By the terms of the spell, the occupant can't be.

In the double body problem, the ORS is stopped by the ground, then milliseconds later, the occupant smacks into the ORS's inner surface. We know from Newton's 3rd, that the falling occupant can only decelerate if an equal and opposite force is applied to his momentum. So we have to ID the source of the force.

1) the occupant can't be the source- that would mean he is not only supplying his kinetic energy, but somehow supplying energy to stop himself. How can he do this- doubling his energy- without thrusters?

2) the sphere can't be the source- for all intents & purposes, it is massless, and cannot deliver enough energy to decelerate the occupant. If you hit a relatively massless thing with something heavy, the massless thing will deflect off at a high velocity and the more massive object willhardly be slowed at all. The double body problem is essentially the falling boulder illustration twice- first, the great mass of the ground halts the ORS without noticably slowing, then the same sphere is struck by something else with a great mass compared to itself. If it could, it would simply start moving again, the occupant hardly slowing.

But it can't move because it is resting on the earth, you'll say. This is true, which gets us to:

3) the ground is the source of the deceleration force.

It's the only thing that can supply it.

But this outside deceleration force, originating as it does from outside the ORS, can't damage the occupant...the same answer as in the single body problem.

The energy is already inside the sphere, it's just converted from potential to kinetic energy as the wizard accellerates down the gravity well.

And

I see no reason to believe the occupant of the sphere would not obey basic laws of momentum, velocity and acceleration.

Hence the creature follows Newton's first law: an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

If there is a change in velocity of the sphere, the occupant will continue moving in the same direction at the same velocity until acted on by an unbalanced force. That "unbalanced force" in this case is the interior wall of the sphere. There doesn't have to be any conservation of momentum, force or energy in the traditional sense because the occupant is not interacting with a physical object.

See above. The deceleration energy can't "already be inside the sphere". There is nothing there to provide it.

The ORS by itself lacks the mass to resist the occupant's momentum in any meaningful way.

 

[Oofta]

In the double body problem, the ORS is stopped by the ground, then milliseconds later, the occupant smacks into the ORS's inner surface. We know from Newton's 3rd, that the falling occupant can only decelerate if an equal and opposite force is applied to his momentum. So we have to ID the source of the force.

We have. The magic of the sphere stops the person from escaping the sphere.

3) the ground is the source of the deceleration force.

Except that it can't be. Something that has no mass cannot have momentum. Therefore you cannot transfer momentum through a massless object (assuming the sphere is treated as an object).

The ORS by itself lacks the mass to resist the occupant's momentum in any meaningful way.

It doesn't have to. It's magic. We know the occupant cannot physically pass through the sphere. They cannot interact with the ground. However, if the sphere decelerates to a speed relative to the ground of zero, the occupant is also going to decelerate to a speed of zero relative to the sphere.

There doesn't have to be conservation of energy or momentum or any logical consequence that would happen if someone fell onto a physical object. Why? Because ... repeat after me ... magic is stopping the occupant, not a physical object.

In my campaign from a metaphysical standpoint, I would say that the energy behind a spell is pulled from the ether. The ether surrounds us, some creatures such as dragons naturally pull power from the ether as readily as they breath, magic users pull energy from the ether to power their spells or to create objects. The momentum of the creature hitting the wall of the sphere is transferred to the ether which transfers an equal amount of force back to the creature.

A related question: what would happen if someone jumped from a great height onto a wall of force? Nothing can physically pass through a wall of force. You could drop a mountain on top of one and nothing would happen to someone standing underneath. Yet a wall of force can be free floating. What happens to the momentum of the falling mountain? Are you saying that someone that falls onto a wall of force would take no damage?

 

[steeldragons]

But really, what I want to know is...

...if the guy inside the sphere is not going splat when he hits the ground, it stands to reason the energy generated from the sphere with the guy in it hitting the ground gets sent out from impact point. Seeing as none of that energy -which would normally turn the body in the sphere to shattered broken mush- is being utilized and has to go somewhere...

Does the single plane of kinetic energy speeding away from the sphere's point of impact slice the 10 orcs waiting around having cigarettes at the bottom of the cliff off at the ankles...or simply shatter their ankles and shin bones inside their skin?