I have seen a fair amount of catapult + folding boat questions and it seems like people are just looking for solutions for when players find out that is a thing. However, I am more curious as to whether it works fundamentally.

Does the spell catapult deal damage based on weight or based on level cast?

If it is based on weight, then using enlarge/reduce to shrink a 40-pound creature on turn one, then on turn two casting a first-level catapult and dropping concentration mid-flight (which is possible based on this Jeremy Crawford tweet) would result in an 8th-level-equivalent catapult collision dealing 10d8 worth of damage to each, assuming the target failed its Dex saving throw. This would seem to be far more impactful than the folding boat because one could do this at level 3.

Is this correct?

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    \$\begingroup\$ -1 for lack of research — the Catapult spell description clearly says how much damage does it deal \$\endgroup\$ – enkryptor Feb 10 '19 at 20:09
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    \$\begingroup\$ How are you launching a creature? Catapult only launches objects. \$\endgroup\$ – Ruse Feb 10 '19 at 20:59

It is based on spell level.

The Catapult spell description says:

At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, the maximum weight of objects that you can target with this spell increases by 5 pounds, and the damage increases by 1d8, for each slot level above 1st.

The increase in level increases damage and increases weight of the object that can be thrown. A level 9 spell would still do full damage even if it only launched a pebble.

From an RP/physics perspective, it's because Catapult is imparting a set amount of force onto an object. Based on K=MS, as mass increases, more energy is needed to accelerate it. The mass increasing would slow down the object as the momentum spreads. The second you released concentration, your creature would slow in midair like someone had pulled a parachute.

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The damage for Catapult is based on level cast.

As the spell description states:

Choose one object weighing 1 to 5 pounds within range that isn’t being worn or carried. [...] On a failed save, the object strikes the target and stops moving. When the object strikes something, the object and what it strikes each take 3d8 bludgeoning damage.

At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, the maximum weight of objects that you can target with this spell increases by 5 pounds, and the damage increases by 1d8, for each slot level above 1st.

If you didn't want people flinging boats, but perhaps wanted to reward the players once for their creativity, you could imply that the velocity of the object being thrown would be too high to accurately restore the object's mass and weight mid-flight to get the desired effect. That way, they might be lucky enough to nail it the first time, but no more than that. The rule of cool doesn't really apply on the 9th attempt.

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – SevenSidedDie Feb 10 '19 at 21:06
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    \$\begingroup\$ You answer doesn't explain why the spell damage is independent of weight. Quoting the whole spell is not helpful: you should highlight the relevant parts of the catapult quote, remove unnecessary parts from the quote, and explain what the quote means. \$\endgroup\$ – Ruse Feb 10 '19 at 21:13

Catapult's damage isn't based on weight, and it can't throw creatures

1.) Catapult's damage is based on the level it is cast at, not the weight of the projectile.

Although higher level castings of this spell will permit you to hurl heavier objects, that is not explicitly required to do more damage. Catapult states (XGtE, p. 150, bold added)

When you cast this spell using a spell slot of 2nd level or higher, the maximum weight of objects that you can target with this spell increases by 5 pounds, and the damage increases by 1d8, for each slot above first.

Although you are permitted to hurl heavier objects, upcasting this spell does not require a heavier object to do more damage. The spell specifies that you can throw a heavier object with an upcast version of this spell, but you definitely do more damage (whether or not the thrown object is heavier). By a strict reading of this spell's rules, you could throw a 1 pound object with a 3rd level casting of this spell, and it would do 5d8 damage (3d8 for the casting, +2d8 for the 3rd level slot).

As further evidence that the spell's damage is dependent exclusively on spell level, note that the object used by this spell when cast with a level 1 spell slot can be

one object weighing 1 to 5 pounds. (Ibid)

If weight determined damage, then the 5 pound object would presumably do five times the damage that a one pound object would. But this is not the case. The weight of the object does not determine the damage done by the spell. Only the spell slot used determines the damage: the fact that the spell slots also permit you to throw larger objects is incidental.

2.) Catapult throws objects, not creatures.

You mention throwing a 40 pound creature (reduced to 5 pounds via the Enlarge/Reduce spell) with the Catapult spell. This will not be possible as catapult permits you to throw objects, not creatures (see most recent quote). As funny as it would be to send a shrunken goblin flying with this spell, you'll have to satisfy yourself with smacking it very hard with a nearby object.

That being said...

There is explicit text in the Enlarge/Reduce spell indicating that causing a creature to grow to double its size will do the following:

The target’s weapons also grow to match its new size. While these weapons are enlarged, the target’s attacks with them deal 1d4 extra damage.

There is similar text indicating that shrinking will cause a target's weapons to do 1d4 less damage. That being the case, you might be able to talk the DM into letting your hurled small (5 pound) object which is returned to its true size (40 pounds) do an extra 1d4 damage, as implied by the text of Enlarge/Reduce. They would be under no obligation to do so, as the target of Enlarge/Reduce was not a creature (which is implied in the section on extra damage), nor is the object you are flinging with Catapult necessarily a "weapon." But I suspect a DM might be generous and let you have this one.

Especially since it would take two turns to set this strategy up, and it would require you to use both a first and second level spell slot. If you had just cast Catapult twice (once with a first level slot and once with a second), you could have done 7d8 damage total in these two turns (though not to two targets at once, as you're suggesting). Using your strategy, you'd do 3d8+1d4 damage (again, only to one target), which hardly seems worth it.

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