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If a creature casts Otiluke's resilient sphere on himself while he is walking or running, does the momentum stop when the spell is cast?

I'm thinking about a scenario where a creature is surrounded by a prismatic barrier. It can't be seen through so to get through it by teleportation misty step is out of the picture. People are split on whether dimension door (having to target the area you are making the door to) is blocked due to the barrier blocking line of effect. The spell 'teleportation' is a riskier form of teleportation that is also a higher level spell and is thus more difficult to access. So my thoughts turned to a spell my artificer does have: Otiluke's Resilient Sphere.

Resilient sphere tells you how it can be moved:

An enclosed creature can use its action to push against the sphere's walls and thus roll the sphere at up to half the creature's speed. Similarly, the globe can be picked up and moved by other creatures.

The words of the sphere only include a way for the creature inside to move by expending an action, it would however be fallacious to say this must be an exhaustive list of what ways the sphere can be moved. As it doesn't say it can't be moved by other means. These words would indicate that for an creature inside to start moving the sphere it will require its action.

I want to know if there is any way to cast the sphere and move it in the same turn on only your one action (without metamagic). Particularly, what happens if the creature enclosed in the sphere has momentum when the sphere is cast. Can a creature run at the prismatic barrier and cast Otiluke's Resilient Sphere to get through it? I presume a creature can cast while moving, and if not I know a creature can cast while concentrating on a spell; At the very least a creature should be able to use a ready action to cast the spell and concentrate on it, run at the wall, and activate the spell before he hits the wall.

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  • \$\begingroup\$ "it would however be fallacious to say this must be an exhaustive list of what ways the sphere can be moved. As it doesn't say it can't be moved by other means" - you may find this xkcd comic interesting reading. \$\endgroup\$
    – Miniman
    Feb 16, 2020 at 2:05
  • \$\begingroup\$ @Miniman Damn you, i read that comment and then continued to go on a 30 min trail of xkcd posts trying to find where i left off...5 or so years ago... \$\endgroup\$
    – Wartowel
    Feb 16, 2020 at 2:55
  • \$\begingroup\$ Why the down vote for this question D: I was fully anticipating down votes for my answer, but this is a legitimate question! Did someone just feel one down vote wasn't enough for my answer? Also @Miniman I did love it! \$\endgroup\$
    – Dezvul
    Feb 16, 2020 at 2:58
  • \$\begingroup\$ Also, I didn't intend to imply that there must be other ways to move the sphere. I was just trying to cover my bases to prevent an answer that said to move the sphere you need to use an action (as this kind of answer would have been unsatisfactory for me). I like the answer that it is up to the DM, this is more or less what I was looking for. \$\endgroup\$
    – Dezvul
    Feb 22, 2020 at 3:16

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The rules of D&D 5e do not model momentum

Movement in D&D 5e is very abstracted:

On your turn, you can move a distance up to your speed. You can use as much or as little of your speed as you like on your turn, following the rules here.

Your movement can include jumping, climbing, and swimming. These different modes of movement can be combined with walking, or they can constitute your entire move. However you're moving, you deduct the distance of each part of your move from your speed until it is used up or until you are done moving.

Basically, you have a speed, and on each of your turns you can move up to your speed in any direction that your mode of locomotion can carry you. There are additional complexities for having multiple speeds, difficult terrain, and other aspects, but nothing in the rules makes any attempt to simulate acceleration, deceleration, turning radius, momentum, or any other physical aspect of movement beyond just the fact that you change locations. There is no physics simulation involved in determining where and how you can move.

So, if you're going to invoke concepts like momentum, that puts you squarely in DM ruling territory, and you're going to have to ask your DM how things will work. If you are the DM and need to come up with a ruling for this, consider a more mundane but otherwise similar scenario: a creature uses their full movement to run out onto slick ice and starts sliding across its surface. What happens?

Personally, I would rule that the creature continues coasting on their next turn, and cannot otherwise move unless they do something that would stop the coasting (e.g. jamming their sword into the ice to stop themself from sliding, or using their action to push against the sphere as described in the spell). Regardless, I probably would not allow the combined distance of coasting plus controlled movement to exceed their allotted movement for the turn. In other words, I would rule that coasting 1 foot costs 1 foot of your movement for the turn (or possibly more than 1 if friction is involved). But keep in mind that all of this is my own ruling–it has no basis whatsoever in the rules for movement, because once again, momentum simply isn't part of the rules.

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