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I saw an answer on an old question, and was curious as to spells and how they work. It brought me to an old comment (Oh to be young again) suggesting that wet paper could block a spell. If spells have mass and acceleration, then it couldn't be blocked by something so flimsy. However, if they don't, then it seems there is a "range" (as in all spells are Area of Effect to different degrees) at which they are effective. This leads me to the obvious question:

Do spells have force (as in mass times acceleration)?

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    Objects in general don't have "force", but carry momentum, differences of which are perceived as a force, eg a car hitting a wall etc. Therefore, it would be more precise to ask if spells carry momentum. – Rebel-Scum Aug 17 '17 at 15:50
  • If they do not have mass then they have to move at the speed of light :-D – SJuan76 Aug 17 '17 at 16:16
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    Even massless particles, eg photons, carry momentum. Judging from the fact that spells don't move at the speed of light, at least in the movies, we can surmise they have a mass. :-P – Rebel-Scum Aug 17 '17 at 16:41
  • @sJuan76 It's Magic with a capital M, we have no basis that spells follow the laws of Physics. – Anoplexian Aug 17 '17 at 17:55
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    Given that spells (say Avada Kedavra or Stupefy) often impact the target in the chest, and the target is wearing clothes...I think it's fair to say that the spells have at least some form of penetrating effect... – Doc Aug 17 '17 at 20:54
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+100

Some spells at least have force, if they don't hit the target they can destroy what they do hit.

A Killing Curse that Dumbledore avoided hit a statue instead, and destroyed part of the statue.

“Another jet of green light flew from behind the silver shield. This time it was the one-armed centaur, galloping in front of Dumbledore, that took the blast and shattered into a hundred pieces, but before the fragments had even hit the floor, Dumbledore had drawn back his wand and waved it as though brandishing a whip.” - *Harry Potter and the Order of the Phoenix, Chapter 36 (The Only One He Ever Feared)

Another Killing Curse hits a desk and catches it on fire. Then Dumbledore casts a spell that Harry described as having a lot of force behind it.

“By which time I shall be gone, and you will be dead!’ spat Voldemort. He sent another killing curse at Dumbledore but missed, instead hitting the security guard’s desk, which burst into flame. Dumbledore flicked his own wand: the force of the spell that emanated from it was such that Harry, though shielded by his golden guard, felt his hair stand on end as it passed and this time Voldemort was forced to conjure a shining silver shield out of thin air to deflect it. The spell, whatever it was, caused no visible damage to the shield, though a deep, gong-like note reverberated from it – an oddly chilling sound.” - *Harry Potter and the Order of the Phoenix, Chapter 36 (The Only One He Ever Feared)

Impedimenta also has some force behind it as well.

“Not until somebody in the vicinity yelled ‘Impedimenta!’ and he was knocked over backwards by the force of the spell, did he abandon the attempt to punch every inch of Malfoy he could reach.” - *Harry Potter and the Order of the Phoenix, Chapter 19 (The Lion and the Serpent)

So does Stupefy, since it shattered a cabinet.

“STUPEFY!’ The jet of red light flew right over the Death Eater’s shoulder and hit a glass-fronted cabinet on the wall full of variously shaped hour-glasses; the cabinet fell to the floor and burst apart, glass flying everywhere, sprang back up on to the wall, fully mended, then fell down again, and shattered –”

As far as what this means for whether magic and physics are cohesive, they probably aren't.

This doesn't necessarily mean that magic doesn't bend the rules of physics. It just means that spells have enough impact behind them to prevent their being blocked by every little thing in their way, which seems to be the question driving this question.

To answer the implied question, most spells probably can't be blocked by flimsy objects.

The impact behind Avada Kedavra, Stupefy and Impedimenta, at least, would be far too strong to be stopped by a wet piece of paper, or any other fairly small and flimsy objects. Trying to use a wet piece of paper to block a Killing Curse would quite literally be a grave mistake.

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    Given that spells (say Avada Kedavra or Stupefy) often impact the target in the chest, and the target is wearing clothes...I think it's fair to say that the spells have at least some form of penetrating effect... – Doc Aug 17 '17 at 20:52
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    What if we think of them like bullets? Blocking them with statues is possible, with wet paper: not so much. – Thomas Aug 17 '17 at 21:02
  • What about Bombarda Maxima? (Hopefully I spelled that correctly.) It sounds like a spell with force against the first obstacle that gets in the way regardless of what it is. – Doddy Aug 17 '17 at 22:36
  • @Doddy It would have impact, but I don't think its effects are limited to the first object it hits. It seems to be more like an explosion, where anything within a certain range will get blasted. – Bellatrix Aug 17 '17 at 22:44
  • And those 3 disarming spells that knocked out Snape... – marcellothearcane Aug 20 '17 at 10:36
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I can't say definitively for all spells, but at least one - the Stunning spell - appears to have some force behind it:

No fewer than four Stunners had shot from the figures around the cabin toward Professor McGonagall. Halfway between cabin and castle the red beams collided with her. For a moment she looked luminous, illuminated by an eerie red glow, then was lifted right off her feet, landed hard on her back, and moved no more.
Harry Potter and the Order of the Phoenix, Chapter Thirty-One - O.W.L.S

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I don't think we can think of spells in these terms. While the fact that I don't remember a single instance where terms like mass or acceleration - or even basic mechanics - were applied to spells doesn't prove anything - you can't prove a negative anyway - HP magic doesn't seem to interact with physics in explicit ways, or else it would have to be coherant with physics.

I explain : either spells are considered to have physical properties that are simply not mentioned or they are just some special instruction traveling through a sort of magic fabric and provoke effects when they hit something.

I go for the later, because if they were considered to have physical properties, then the next step is to make them compatible with physics. And given that the apparition or disparition of mass either absorbs or projects an insane amount of energy, the physical consequences of any spell would by far outmatch its magical effects by several orders of magnitude.

Something that makes that conclusion appealing to me is that despite it's impossible to create food, there are spells which make various objects to appear, including flowers - which oddly don't count as food... -, birds, water, wine... And spells to make object to disappear - evanesco. If E=mc² applied here, each casting of evanesco would be like a small nuclear fission and any conjuration of object would suck a comparable amount of energy in order to be carried out.

That's why I believe that spells have nothing to do with mechanics : it would create way more troubles than it would solve.

  • Best case scenario you suddenly create a thing-shaped vacuum, the collapsing of which could be pretty catastrophic as well depending on the size of the thing. – Dave Aug 17 '17 at 22:35
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    Best case scenario :) – ksjohn Aug 17 '17 at 23:01

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