Observations on Gravity

The following material is taken from a question and my response to it, posed on TSR's AOL message boards when that was still their primary internet presence.  I've taken the liberty of revising it here for clarity and brevity.   Though I don't think you'll find an archive of those boards anywhere I've got a few highlights on file.  The original headers are:

Subj: Man Overboard
Dates: 9/7/96 11:43:48 AM to 9/16/96 12:10:01 PM
From: LapuLapu
Posted on: America Online

Lien Guyang, a rocketeer (shoots them, not rides them), finds himself overboard.  His ship is in the upper atmosphere of the planet Toril, and he is now in danger of drifting away and burning up as he falls or dying from the impact with the ground, miles below.  He fell over the rail, pushed by an explosion, and bobs up and down in a diminishing cycle until he comes to rest at the gravity plane.  Then he starts a slow slide, outward, away from the ship.  Once he leaves the ships gravity envelope he will feel the inevitable pull through the sky awaiting him from Toril's gravity.

[snip a digression on being able to throw objects in a loop around a ship]

Trevean, another man on board the ship sees Lien fall and attempts to rescue him.  He grabs a rope attached to a yardarm and leaps over the side to somehow aid in his rescue.

Lien Guyang struggles to "swim" back to the ship from panic.  Trevean passes through the gravity plane a time or two and reaches him from underneath just as Lien feels the pull of the planet and they both fall.

Then Solin, an elven mage lets loose a Web spell from his staff directed at the two to help keep them attached to the ship.  The webs attach to Trevs boots with the other end remaining attached to Solin's staff and now Solin has to haul them in.  Solin is now a bit like a fisherman whose "catch" looks as though it was going to take him over the side as well and he struggles to keep his footing.

The captain calls orders to the helmsman to continue a climb and to roll the ship so that the overboard crewmen will possibly fall onto the side of the ship rather than to their deaths.  Other sailors on deck start throwing ropes with weighted ends and shooting thin lines from crossbows to the helpless pair.

The ships hull shifts, turning around it's long axis.  The mountainous horizon, then the barely perceptible curvature of Toril, began to "rotate" until they were perpendicular over the bow.  An unsettling visual to those less experienced in spelljamming.

Lien Guyang feels his weight shift again as the two overboard sailors became suspended in the margins of the air envelope "high" off the starboard side.  One force pushes them out, towards the bluish-black skies of wildspace, the other force attempts to pull them along the "skin" of the air bubble.  Lien regains his composure and grasps one of the lengths of cord now floating nearby.  The sailors are hauled aboard and saved from a deadly fall to earth.

My Response:

To begin with, tossing things in a 360 degree loop about the ship is a contradiction of the way gravity is supposed to work.  This was first described in the original Spelljammer boxed material, but it was contradictory then too.   Once an object is given a heave and is headed out, away from the ship it will continue to move that way - away from the ship.  Although it may repeatedly pass through the gravity plane reversing its up and down directions there is no reason for it to suddenly "reverse" its' bobbing back IN towards the ship instead of AWAY.   Gravity pulls down toward the plane NOT towards the center of the ship.   Indeed, as it hits the gravity plane the speed with which it moves away from the ship should increase ever so slightly due to the outward drift phenomenon for objects resting at the gravity plane.  The closest you should be able to come to the "loop" around the ship would be to lean out over the rail and throw the object with enough force and enough high arc so that someone leaning out from the opposite rail could catch it as it "fell up" from below, but still close enough to the opposite side of the ship to reach it.  And if the catcher misses it, the thrown object continues to move away from him and away from the ship.

I'd like to mention at this point that the very non-real nature of a gravity plane and the wide variety of circumstances that can apply to an object in motion in a ships gravity envelope make it very difficult to come up with some kind of formula that can be easily applied.  The arc of an object and the distance it has to fall towards the gravity plane would seem to figure quite significantly since it should tell you how much of the objects' forward momentum gets converted into "downward" momentum (or rather, into momentum towards the gravity plane).   The best I've been able to do is simply visualize each instance and make a judgement call.

Next, when the victim in the "Man Overboard" example and his would-be rescuer slip beyond the ships air/gravity envelope the ships gravity plane no longer influences them.  They should begin to fall towards the planet but swing underneath the ship hanging by the webs (this is assuming that the ship IS oriented traditionally to the up and down of the planet).  The ships gravity only pulls on the portion of the webs and rope that are within its gravity well which is not much in comparison to the two men hanging at the other end being pulled by the gravity of the planet.  So, there is a small amount of "pull" being generated by the ship which would help lift the two men up, away from the planet but their weight and that of any ropes and webs that are outside of the ships pull is being pulled down by the planet.   So, they would continue to hang helplessly outside the gravity of the ship only now they would swing free below the keel instead of off the beam.

The position of the men outside the ships gravity is important to know because when they re-enter the ships gravity that position will determine how far they then "fall" to the gravity plane of the ship.  If they are hanging below the keel and are pulled up into the ships gravity - they "fall" onto the ships keel!  Depending on the physical size of the ship, and therefore the size of the envelope, it could be quite a hit.  If they are positioned to remain at the edge of the ships gravity plane though, they don't "fall" anywhere when they re-enter it.

So, rotating the ship could have a couple of results here.   Let's say the men are hanging by the webs and rope over the port railing.   Rotating the gravity plane is not in and of itself of any help since the ships gravity plane is not pulling on the men - only the planets gravity is pulling on them.   If, however, the ship is rotated to port so that the web/rope is parallel or roughly in line with the gravity plane when the men ARE pulled back into the ships gravity envelope at the plane itself they can be easily pulled back to the ship as they will be floating weightless there.  The difficulty is that this means shifting the orientation of the pull on the webs and rope for those on deck trying to help pull them aboard.  It shifts from downward to straight out over the rail which would make it difficult to keep your footing indeed.  It would be like shifting from pulling a man up from the bottom of a well to being in a tug of war with him.  The mage, tugging on his webs, will likely have to end up "standing" on the inside of the rail to keep from being pulled overboard himself.  In fact, it should be very difficult for him to remain standing even while the two men hang below since it's the weight of two men versus the weight of one - the one is likely to lose the tug-of-war.

In the "Man Overboard" example there are crewmen who can pull the men aboard.  But if they weren't there consider this: rotating the ship to starboard will have the effect of wrapping the ropes around the bulk of the ship, which shortens the length of the rope, which effectively pulls the men back towards the ship. The ship is thus acting like the drum on a winch taking up the slack.  The important question to ask in this case is where do the men re-enter the ships envelope?  If they are "above" the bottom hull of the ship they will "fall up" and hit it.   Further options to make this maneuver more attractive might be to adjust the amount of line,  and having the two hapless victims fall towards a clear gravity plane rather than a hard wood hull; or to rotate just enough so that the two are half-in/half-out of the envelope, effectively balancing them at it's limits until the ship can rise up out of the effects of the planets gravity whereupon they will be safe as kittens (relatively).

This last bit I would personally consider to be too fine a degree of control of the ship, the balance of the men between two gravities, and the length of the line to be at all practical but... BTW, a ring of feather fall would float you gently to the gravity plane of the ship - so long as you were within the ships gravity envelope.   Remember that all other things being equal an object is influenced by the gravity plane it is closest to.

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