How would you rule the impact force of weapon/tool into the weight limit of items like Tenser's Floating disk or an Immovable Rod? [closed]

Question

As a DM, how would you rule the interaction between spells that specify how much weight they can support and the force of impact of a hammer or a sword? While it's obvious what happens if you apply a constant force (such as an item that weighs a certain amount), what about other forces? I get that RAW, we don't have enough information to answer this, so I'm asking how a DM would approach this.

As a sidenote - so no one panics - I'm not planning on forcing my DM to confront this issue unless I've both found a decent answer to the question and discussed it with him beforehand.

Let's look at some examples:

If I place a 499 lb anvil onto Tenser's Floating Disk, the initial force applied to Tenser's Floating Disk is going to be at least somewhat higher than 499 lb, as when I set down the anvil, the disk will need to apply enough force to the anvil to halt its motion. Even if the anvil is set down extremely slowly, it will still have to decelerate as it comes in contact with the anvil. Does this mean that its functional weight limit is significantly lower? Or is 500 lb its functional weight limit, while its actual capacity to resist force is actually quite a bit higher?

Tenser's Floating Disk

This spell creates a circular, horizontal plane of force, 3 feet in diameter and 1 inch thick, that floats 3 feet above the ground in an unoccupied space of your choice that you can see within range. The disk remains for the duration, and can hold up to 500 pounds. If more weight is placed on it, the spell ends, and everything on the disk falls to the ground.

What about something that isn't moving slowly? If my character (a warforged weighing around 300-400 lb when you include armour and equipment) were to drop down even just a couple of feet before landing on the disk, would the force of impact (several times the 300-400 lb he weighs) end Tenser's Floating Disk?

Or - looking at my original problem - if I wanted to use Tenser's Floating Disk as a field anvil, would the force of my hammer (2 to 5 lb) hitting the disk at relatively high speeds (30-40 ft/s) dispel it? Looking around, it looks like the impact force of a hammer hitting an anvil can easily get to several tons.

If the hammer blow would dispel Tenser's Floating Disk, would it similarly - given enough mass and velocity - cause an Immovable Rod to fall? After all, the immovable rod can only hold up to 8000 lb (4 tons) of weight.

Immovable Rod

This flat iron rod has a button on one end. You can use an action to press the button, which causes the rod to become magically fixed in place. Until you or another creature uses an action to push the button again, the rod doesn't move, even if it is defying gravity. The rod can hold up to 8,000 pounds of weight. More weight causes the rod to deactivate and fall. A creature can use an action to make a DC 30 Strength check, moving the fixed rod up to 10 feet on a success.

While it might be simple to rule that the force of impact is magically absorbed by the spell and it only concerns itself with forces applied consistently over a period of time, what ramifications would this have in regard to objects that are only dangerous due to their kinetic energy? Take, for example, most any blow or projectile. Blows from creatures less than gargantuan* would simply bounce off Tenser's Floating Disk (a 1st level spell) allowing you to essentially have complete cover from melee attacks by hiding underneath it. Or worse, wielding an immovable rod like a club then pressing the button right as you block to allow you to either block or shatter up to gargantuan weapons*.

Things to consider:

• When a hammer and an anvil connect, the impact (while short) isn't instantaneous. They both bend/dent over an extremely small period of time while the applied force reverses the hammer's movement. Were the hammer to change velocity instantaneously, its acceleration at that moment (and thus the force applied to it as per F = ma) would be infinite, shattering them both. However, part of what makes an anvil/hammer so effective is the relatively low bend/dent of steel, which results in extreme forces applied to anything between the two.
• As such, how much do Tenser's Floating Disk (described as a plane of force) and immovable rods bend/dent on impact? The longer the period of time the impact is absorbed over, the less force is applied at any one moment. As such, saying that there is no bend doesn't work, as in that case, anything that came in contact with either of them would shatter, regardless of how slowly they went.
• Is it appropriate to look at the force applied to the spell as a whole, or would it be better to look at the pressure applied over the contact area? Is there a difference between 500 lb applied over a very small portion of Tenser's Floating Disk vs spread out over the entire disk? Would a pick (less contact area, higher pressure) or a hammer (less pressure, but more mass/inertia/force) be more likely to break the spell?

Notes:
* I had a hard time finding the weight of gargantuan weapons.
Basing it off of strength/carry capacity, a medium creature with strength of 10+ will effectively wield weapons up to 10-15 lb, and has a heavy load of up to 400 lb. A Warforged Colossus (gargantuan construct) has a strength score of 30 and an expected heavy load (based on size and strength) of approx 20,000 lb (50x as much), which would lead me to expect its heavy weapons to be about 500 to 750 lb, barely more than Tenser's Floating Disk's lb limit.
Basing it off of similar proportions, gargantuan creatures are approx 8x. taller than medium creatures, and would thus have approx. 512x as much mass (8^3). Again, starting from heavy weapons that are 10-15 lbs for medium creatures, we'd expect a similarly-proportioned equivalent gargantuan weapon to be between 5000 and 7500 lb, just shy of the immovable rod's 8000 lb limit.

Answer 1

D&D isn’t a physics simulation.

Spells and magic items have the game effects that they say they do, nothing more. To address the specific concerns raised:

Blows from creatures less than gargantuan* would simply bounce off Tenser's Floating Disk

Nothing in the spell description says that it affords any protection in combat. As a DM I would say that at best it’s as good defensively as hiding under a table — maybe partial cover, maybe disadvantage to the attacker, maybe advantage to the attacker.

wielding an immovable rod like a club then pressing the button right as you block to allow you to either block or shatter up to gargantuan weapons.

First, pressing the button on the rod uses an action, so it can’t really be done reactively in this way.

More importantly, nothing in the item description allows it to be used in this way. There are no general rules for blocking or parrying with weapons that account for the relative masses of the weapons, or damage to them. Without a feature that specifically allows it, characters can’t “block” in any way at all that has game effects (remembering that a round of melee combat represents potentially several exchanges of strikes, parries, evasions, etc).

More broadly, most of the time in the D&D rules, words are understood to have their conventional, non-technical English meanings. So while pounds of weight are technically a measure of force, most general usage treats pounds as a measure of mass, the same as kilograms. If you treat Tenser’s Floating Disk as being able to “hold up to 230 kilograms”, and the Immovable Rod “up to 4,000 kilograms”, then maybe some of your concerns will be allayed.

Overall as a DM, beyond following the spell/item descriptions, the principles I would bear in mind are:

• Magic is magical; it doesn’t need to be consistent, and it doesn’t function within the framework of physics as understood in the 21st Century.
• This is a game for telling fun fantasy stories. If a player wants to get creative with a spell or item in a way that is consistent with the tone of the game I’m running, then I’m fine giving them one-off benefits. I’m also fine with such things not being reliably repeatable.

As a DM, how would you rule the interaction between spells that specify how much weight they can support and the force of impact of a hammer or a sword?

There is no such interaction as far as I’m concerned, so I wouldn’t really need to rule.

Answer 2

The only force in D&D 5e is force damage

In the real world there are physical concepts like mass, force, impulse, work, energy, pressure, and torque. In D&D there is only weight.

Tenser’s Disk can hold up to 500 pounds and it doesn’t matter if this is placed gently or dropped from space. If it’s less than or equal to 500 pounds the spell holds it up, if it’s more the spell fails.

The same is true of the Immovable Rod’s 8,000 pound limit.

D&D is a game, it works this way because those are the rules. Just like bishops can only move diagonally in chess, we accept limitations to the simulation because it makes the game playable.