D&D 5E Mage Hand and Trap Avoidance

MarkB said:

I don't understand. How does its speed affect being able to drop a rock on a pressure plate? It's not exactly a moving target.

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CleverNickName said:

What about impact force? Like, how hard could a Mage Hand slap a pressure plate? What if you have the Mage Hand ball itself up into a fist and pound on a trap's pressure plate at full speed? Surely that would generate the most force, right? After all, jumping up and down on a bathroom scale makes the needle go much higher than your own body weight...

Well, let's calculate it!

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kigmatzomat said:

Oh, that's actually covered by the rules. Mage Hand can't make attacks so it can't make a fist and pound on anything.

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Chaosmancer said:

So you cannot use mage hand to knock on a door, or knead bread, or cut meat with a knife, or....

We need to be careful with limiting mage hand non-attack actions based on it not being able to attack, because we essentially have to say that Mage Hand can no longer do anything at all to prevent it from being able to attack.

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kigmatzomat said:

You are not going to like the answer. It's why simulationists get headaches. You see the answer is greater than 11.6lbs but less than infinity.

I won't make everyone look at math.

Spoiler

So why 11.6 - infinity? Force is mass x acceleration. So the amount of Force It depends on how much acceleration it can impart. And how fast that thing was moving when it started, as it will need to accelerate/decelerate it appropriately.

Pounds are actually a unit of force, not mass. An object that, at rest, exerts 10lbs of force on a scale masses 0.31 slugs (imperial unit of mass) or 4.5kg (metric unit of mass)

For simplicity we'll assume continuous acceleration of an object that started at rest. The spherical cow of basic physics. May Newton guide my hand.

from d = 0.5(a)t^2 we get 30ft=0.5a(6s^2) => 1.67ft/s^2
::: 0.31 slug x (32.2ft/s^2 / + 1.67ft/s^2 ) = 11.6lbs

But wait! This is a floor! The actual force can be more! What if you catch a 0.31slug/4.5kg object that is falling? What if it got dropped last round and this round you are going to catch it with Mage Hand and then lift it 30ft?

the initial velocity will be v = at where a is gravity so 32.2ft/s^2 x 6s => -193ft/s
d=0.5at^2 ‐vt
30ft =a18s^2 -1158ft
66ft/s^2 = a
:::0.31slug x a=21lbs

Repeat up to v= terminal velocity.

but...this assumes a bit of "bounce" as the hand goes down on impact, then rises up. what if the hand is already at 30ft and can't move away? There's nothing saying anything can "push" a mage hand. More massive things just pass through. Less massive things...??

In that case you can convert the kinetic falling energy to force, that is e=F d, where f is the force and d is the distance the force is applied.

Except....if the hand isn't directed to move.....is the distance ~0?
Because if so, for e>0, F would approach infinity.
so...between 11.6lbs and infinity.

Wheee!

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kigmatzomat said:

My issue with traps is they are usually out if proportion to the treasure. The dinkiest d4 poison costs 100gp/dose. Yes, yes, not an economic simulator, blah, blah, but as a player I am more likely to make more money stealing the trap than whatever it protects.

For the love of sanity people, have your trap be no more than 50% the value of the treasure! Why protect 100gp with a 4,000gp trap?

You want an ogre to protect 100gp from thieving goblins? Have a ceiling beam slam down when one end becomes unpinned. A crude crossbow fires a bolt. A bear trap built into the treasure chest.

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Horwath said:

I have been to apartments with security doors, and only thing that was worth the effort to steal from those apartments was the door..

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el-remmen said:

I am having a hard time parsing this response.

So you are saying it not fair to say if a mage hand is limited to lifting 10bs, it can only apply 10 lbs of force - ok, so how many lbs of force can it apply? I was literally asking that.

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