According to Catapult's description, the spell is limited by the weight of the object with some variance based on the level in which the spell was cast. With the spell Levitate, could you interpret this to mean that an object under the effects of Levitate weighs effective 0 and then be subjected to the effects of Catapult?
2 Answers
Levitate does not change the weight of an object
Just because an object is held aloft doesn't mean it is weightless. If you pick up a rock, does it become weightless until you put it down? Nothing in the text of the Levitate spell changes the weight of the levitated object. There are spells that can change the weight of something, such as Enlarge/Reduce, but Levitate is not one of them.
Hence, if you levitate a 500-pound object and then attempt to cast Catapult on it, the spell will fail, because the target object weighs more than 10 pounds.
On a valid levitated target, Catapult works normally
Catapult works on
one object weighing 1 to 5 pounds within range that isn’t being worn or carried.
If such an object is currently under the effect of a Levitate spell and suspended in mid-air, that object is neither worn nor carried, so it remains a valid target for Catapult. Furthermore Levitate does not hold the target in place or in any way prevent it from being moved, so nothing inhibits the effect of Catapult when cast on this object. If you do this, the object will start from a point in midair and move 90 feet in the designated direction, stopping at the first thing it hits, where it will once again remain suspended in midair.
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\$\begingroup\$ For the sake of completeness, if we were to take an object that weighs at 5lbs or less, within Catapult's specifications, would there still be an interaction between the casting of Catapult on a levitating object? \$\endgroup\$ Jan 3, 2020 at 21:16
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\$\begingroup\$ @SomethingSomething That's a good point. I'll edit my answer to address it. \$\endgroup\$ Jan 3, 2020 at 21:56
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\$\begingroup\$ It could be argued that an object under the effects of levitate is carried (def: "support and move (someone or something) from one place to another"). \$\endgroup\$– SzegaJan 4, 2020 at 1:37
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\$\begingroup\$ @Szega Generally the 5e rules use "carried" to mean "in the possession of a creature", whether held in hand or stowed in a pack the creature is carrying. See e.g. Telekinesis, which can be used on carried objects. \$\endgroup\$ Jan 4, 2020 at 5:08
This initially started as a response to Ryan's answer, but I think that this explanation requires a full answer.
Levitate depending on your DM's ruling, could make an object weightless
In scientific terms weight is a definition of force, not mass. The text for Levitate does not specify the exact mechanism at play, so, if your DM rules that the Levitate spell simply removes the force of gravity on a creature, Levitate would indeed make the object weightless.
Either way, you still cannot use Catapult on the object
Catapult's definition has the following qualifier:
Choose one object weighing 1 to 5 pounds
As a Levitated object is weightless (meaning it weighs 0 pounds), it cannot be used as the target for Catapult.
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2\$\begingroup\$ That is an incorrect definition of weight, even per your source (2nd paragraph). An object floating aloft still has weight so long as it has mass with gravity acting on it, even if it has no apparent weight. Regardless, the game is very definitely not referring to definition of weight you give, as such would imply that every object not in motion (including those rest on the ground) would also be weightless (and thus invalid for Catapult), since the forces always cancel out when at rest. \$\endgroup\$– Mwr247Jan 3, 2020 at 22:18
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1\$\begingroup\$ @Mwr247 As this answer says, levitate does not specify its exact mechanism; if it works by turning off gravity for the object, then it would indeed be weightless. \$\endgroup\$ Jan 4, 2020 at 0:00
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\$\begingroup\$ @MarkWells My comment was made pre-edit. While I would agree with that interpretation if the spell were worded as such, the problem is that spells only do what they say they do. Since Levitate does not describe the object as weightless or the mechanism as nullifying gravity, it is not weightless (short of DM fiat, but that'd be the exception, not the rule). \$\endgroup\$– Mwr247Jan 4, 2020 at 4:18
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\$\begingroup\$ @Mwr247 What you're proposing is that, since levitate doesn't say what its mechanism is, it has no mechanism: the object isn't weightless, nor is it supported by a force. This approach makes it impossible to even make a ruling on edge cases, and that's the problem with this "Spells do only what they say they do" nonsense. \$\endgroup\$ Jan 4, 2020 at 20:51
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\$\begingroup\$ @MarkWells Not "no mechanism", but that the mechanism is irrelevant. Otherwise, "it would say so if it did". Weightlessness requires assumptions of the mechanism, while the object retaining it's weight does not. I don't claim to know how it holds it up, but if it was intended to change the weight, I should expect it to say so. If a DM decides to define a mechanism then that's fine, but this answer presently states "Either way, you still cannot use Catapult on the object", which is not true. \$\endgroup\$– Mwr247Jan 5, 2020 at 0:33