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I was just looking at a friend's sheet (a water-only sorceror) and saw the cantrip Shape Water. The spell specifies that you can move your 5'x5'x5' block of water (which is 125 feet^3) up to 5 feet in any direction. It is also specified that this movement lacks the force to do damage.

RAW can my friend use Shape Water as a vacuum bomb kind of thing, by expanding the water away from a center point so that when he lets it go, it snaps back, exploding violently?

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Briefly, no.

If you're going by physics, conservation of energy means that the energy from the vacuum bomb explosion has to come from whatever moves the water, which is the spell. Because the spell doesn't have the force to damage, any vacuum bomb you make also won't have enough energy to deal damage.

D&D is a tabletop RPG, not a physics simulator

D&D is a game that aims to achieve some semblance of balance, and is directed toward being a game. Additionally, the laws of physics don't always mesh with the rules (look at the lack of rules for damage from falling objects).

There are lots of magic effects that could create huge destructive effects from air pressure alone. For example, what about Misty Step? Wouldn't you leave a huge vacuum in the spot that you teleport from, and displace a ton of air where you appear? Could you open a portable hole or bag of holding at low altitude, then open it at high altitude to make a blast of wind? If you cast Gate and make portals between two areas of different altitudes, wouldn't the pressure differential make a powerful, constant wind (see the second to last question)?

These things don't happen in the game because it's a game, and because it's literally magic. If you think too much about the physics of D&D's magic, things break down very quickly. Instead, the world of D&D uses a kind of intuitive fantasy logic.

Of course, this is all up to your DM. If your DM thinks that making a vacuum bomb is a clever, justified use of the spell, then he/she can let it happen. I personally have let a few interesting interpretations of physics slide, just because I thought it would be cool (Fabricating a chunk of radioactive metal into a shape that immediately goes critical, maybe). However, such rulings are DM fiat, and not anything explicitly supported in the written rules.

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    \$\begingroup\$ Never mind gate at different altitudes, try different latitudes. The rotational speed softened between pole and equator is roughly 1,000 mph. You could flatten castles. \$\endgroup\$ – keithcurtis Nov 12 '17 at 16:47
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    \$\begingroup\$ I'd suggest a Gate to the bottom of the sea if you really want a hideously destructive use of Thinking With Portals. Even just 1000 feet down, you'd get a vast volume of water blasting out 3 times stronger than a fire hydrant. \$\endgroup\$ – Darth Pseudonym Nov 13 '17 at 16:51
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    \$\begingroup\$ There are lots of fun things you can do with D&D magic + physics. My point in this answer is to show the shenanigans you can do with air pressure alone. \$\endgroup\$ – Icyfire Nov 13 '17 at 17:02

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