Does Levitate cancel (horizontal) inertia?

Question

According to the description of the levitate spell:

Levitate allows you to move yourself, another creature, or an object up and down as you wish. A creature must be willing to be levitated, and an object must be unattended or possessed by a willing creature. You can mentally direct the recipient to move up or down as much as 20 feet each round; doing so is a move action. You cannot move the recipient horizontally, but the recipient could clamber along the face of a cliff, for example, or push against a ceiling to move laterally (generally at half its base land speed).

My understanding is that while the description says you can't make something/someone/oneself move horizontally through this spell, there's nothing stopping the object from moving horizontally by its own device.

So, imagine the following case:
A drow casts her levitate spell-like ability. The next round, she runs at her maximum speed, and jumps over a cliff, at that very same time using levitate to keep her "afloat".

1. Will the character's inertia make her move horizontally, while she keeps her altitude constant (or rising, or going down) by carefully using levitate?
2. If there's no obstacle, will she even stop?
3. How does that compare to the fly spell?
4. Bonus question: Can this effect be used by the drow to "jump over" a charm of 30 metres/100 feet?

Answer 1

Inertia is not part of the movement rules of the game

A drow runs at her maximum speed, and jumps over a cliff, activating her levitate spell-like ability.

If this drow runs at her maximum speed, she can't possibly activate her spell-like ability because that takes a Standard Action. She would need to have the ability either as a free action or as a swift action. Running, in Pathfinder, is a full-round action, and even if she were to simply double-move, that would also consume all of her actions for the turn. At best, she can make a single move action and activate her spell.

From the Magic chapter, on Spell-like Abilities:

A spell-like ability has a casting time of 1 standard action unless noted otherwise in the ability or spell description.

You are right in that she can move horizontally normally, but she would have to activate her Levitate ability before running into a cliff.

Levitate does not have any inertia cancelation effect, or even a gravity cancelation effect, and you cannot use momentum to sling-shot yourself (or another target) forward because that's not part of the spell's effect. The spell allows you to move the target by spending a Move Action, it doesn't even cancel their fall if they jump off a cliff with Levitate already cast on them:

Levitate allows you to move yourself, another creature, or an object up and down as you wish. A creature must be willing to be levitated, and an object must be unattended or possessed by a willing creature. You can mentally direct the recipient to move up or down as much as 20 feet each round; doing so is a move action. You cannot move the recipient horizontally, but the recipient could clamber along the face of a cliff, for example, or push against a ceiling to move laterally (generally at half its base land speed).

A levitating creature that attacks with a melee or ranged weapon finds itself increasingly unstable; the first attack has a -1 penalty on attack rolls, the second -2, and so on, to a maximum penalty of -5. A full round spent stabilizing allows the creature to begin again at -1.

From a rules-as-written perspective, the spell can only do what's in its description and whatever other rules it could interact with. As there are no rules about inertia in the game, the spell couldn't possibly be affected by that rule. Everything else is GM Fiat and likely also a house-rule.

Will the character's inertia make her move horizontally, while she keeps her altitude constant (or rising, or going down) by carefully using levitate?

She will not. The character who cast Levitate must spend a move action to make the movement from the Levitate spell. What she could do is keep using her move actions every turn to cross the obstacle by using the walls or ceiling to crawl/climb without any risk of falling, as described under the spell.

If there's no obstacle, will she even stop?

Obstacles are relevant to her up/down movement, a ceiling would prevent the target from moving up, the floor would prevent her from moving down. Similarly, another creature or object could prevent this movement. Otherwise, her movement stops (midair, mind you) as soon as all of her movement and jumping distance has been covered, falling to her death.

But there are gravity rules in the game

On the other hand, we have rules for Gravity, even though Low Gravity doesn't modify your jumping distance in any way:

In an area with only a third of standard gravity, for example, PCs can jump three times as high and as far and lift three times as much. (Movement speed, however, stays the same, as moving in great bounds can be awkward and difficult to control.)

However, in areas of No Gravity (which is actually very low gravity), your movement speed is actually affected and you could do exactly what you described in your answer and slingshot your character forward:

Movement is difficult, and creatures without something to push off from often find themselves floating helplessly. When a creature does manage to find something to propel itself off of, it can choose to move in any direction, but at half speed. Double-moves and charges are still possible, but running is not. If provided with sufficient handholds, a creature with a climb speed can move along a wall at full speed, as can any PC who succeeds at a DC 20 Climb check (adding her Dexterity bonus).

Note as well that a creature that moves in a given direction continues to move in that direction at the same speed each round (without the cost of a move action) unless it is able to change its motion by latching on to an object or creature, pushing off in a new direction, or creating thrust of some kind (all of which are considered move actions).

There is also a spell called Gravity Sphere (wizard 5), which allows you to create a 30-feet-radius sphere of no gravity. As such, it would allow you to propel yourself forward following the same rules for no gravity, as long as all the movement happens inside the area of the spell.

Can this effect be used by the drow to "jump over" a charm of 30 metres/100 feet?

It cannot. Again, there is no inertial movement unless you are in zero gravity environments, and the spell doesn't allow horizontal movements either.

So, as you can see, there are rules covering the effect you are looking for, and those mechanics, in no way, interact with Levitate, or even the Fly spell.

What about the fly spell?

Fly gives you a fly speed, as such, you couldn't use it to prevent you from falling, or even creating this slingshot effect mentioned previously, but you could use it to fly using part of your movement speed. Since the spell grants you 60 feet fly speed, you could spend a move action to walk towards a cliff, and then simply take another move action to fly over it for another 60 feet. Even though you should just double move or run while flying, to be honest.

But note that Fly, just like Levitate, does not prevent you from falling or taking falling damage. So simply running towards a cliff and spending all your actions will likely drop you into the cliff for the remaining of the round, falling down at least 200 feet (as described under Falling).

Answer 2

Pathfinder's rules for Jumping can be used to partially answer this circumstance, and are actually unaffected by the application of a Levitate spell.

Mechanically, if a character makes a long jump, the result of their Acrobatics check is the number of feet they jump forward. Essentially, this is the point where Pathfinder says your momentum stops. So, as a GM, if I were presented with this situation (with or without the Levitate), the result of an Acrobatics check to Jump would determine how far away from the cliff the horizontal movement would stop. After that point, the character would stop, and by default fall. With a Levitate spell in place, the character would then be able to adjust height as per the spell, but they should not move further forwards without some means of actual flight.

If you need a way to reconcile this with real-world physics, then the results of the jump check determine at what point the wind resistance overcomes the character's intertia.

Answer 3

What's been overlooked is this line from the levitate spell: "You can mentally direct the recipient to move up or down as much as 20 feet each round; doing so is a move action." That's what the levitate spell does, and everything else in the spell's description is predicated on having done that. In other words, until she takes a move action to mentally direct herself some distance up or down and stays there, the drow's levitate effect has no effect.

So when the question puts forth essentially the following…

A drow activates her spell-like ability levitate. On her next turn, she takes a full-round action to run and, at the cliff's edge, makes an Acrobatics skill check to jump. What happens?

…here's what happens: She falls.

The levitate effect in isolation doesn't put the target in a zero-G bubble or anything. The effect allows its creator to take a move action to move the affected target up or down, and, when that movement ends, the affected target stays there until the target somehow moves itself, until it's moved up or down by the effect's creator, or until the effect's duration expires. That's it.

To be clear, after she's used the spell-like ability levitate, the drow's not, for example, secretly levitating even while she's still in contact with the ground… were that even a thing. She's simply not doing what the spell enables her to do: she's not actually levitating yet, despite the presence of the levitate effect. She's interacting with the environment normally. (This also means that were she not yet actually levitating—the levitate effect present but unused—and the ground were to fall away beneath her feet, she'd also fall. Afterward, though,—if it's any consolation—she could take a move action to use the levitate effect to move back up or farther down.)

Spells do what they say they do, and the levitate spell effect has no effect on a target until the effect's creator takes a move action to make the affected target move up or down and stay up or down. Then the levitate effect follows its own rules. And if a target that's affected by a levitate effect stays on the ground, then there's no effect from the levitate effect, in much the same way that a flying creature doesn't benefit from its flight when it's in contact with the ground.

This author knows that this removes many fun physics tricks from the spell levitate spell, but this also makes the levitate spell playable.

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Note: When a creature has been lifted from the ground by a levitate effect, a GM may allow the creature to still jump up and land at the point of departure unless it grabs something above it. However, a GM is likely to forbid the creature from making a horizontal jump while so lifted as this should be as impossible as normal ground movement… of which jumping is a part. Likewise, a GM may rule that trying to turn sideways while so lifted so as to, for example, run across a cliff face is impossible: the levitating creature's essentially rendered itself prone, making it so the typical creature must take a move action to right itself before normal "ground" movement can occur… which can't occur because the creature's affected by a levitate effect.

Answer 4

In Advanced Dungeons & Dragons and other early role-playing games, ideas like this were common. Because those earlier games were played as creative exercises, ideas like the one in the question were rewarded. The person running the game would weigh the idea, and, were it found to have merit, allow the idea to work, perhaps modifying it to fit a different understanding.

While it could be frustrating at times to have a seemingly reasonable idea turned down, enjoyment and satisfaction was had from developing unique ideas.

Times change, though, and the gaming community's tenor changed with it. Early games said outright that the rules were guidelines, and the gamers using them took those games at their words, but the contemporary gamer often seems to view the rules as ironclad and beyond which no character may venture. As someone who gamed in campaigns during that earlier time, it feels like something's missing from contemporary play. It feels like gamers are playing role-playing games as if they were board games.

Take the leap and hope it works!

Thus, my answer—pulled from an earlier age of gaming history—is Yes, physics still applies. No "action economy" will stop you in my games from timing your leap off the cliff so that you can't have both forward momentum and the benefit of the spell simultaneously. Further, in my game, you'd have control over that levitate spell so you could cast it first, keep your self grounded, and apply the vertical change after you run off the cliff. Then we'd figure out how long you'd drift using factors like wind speed and direction and spell duration. Enjoy your (limited, sort-of out of control) flight!