D&D 5E Giant Boulder vs. Immovable Rod

[Imaculata]

You miss the point - scaling the boulder up to that grandiose size is what makes it escapable, and thus foolish. Adding the size makes it a worse trap, not a better one.

So what I should do instead is add a steep slope into a deadly pit, with thousands of tiny marbles...

 

[Paul Farquhar]

I get the urge to design a dungeon full of elaborate traps that fail to function when the players trigger them.

 

[MarkB]

I get the urge to design a dungeon full of elaborate traps that fail to function when the players trigger them.

Have them learn halfway through that a rival, larger group of adventurers is about to raid the dungeon, so that the challenge becomes to repair and improve the traps in time to unleash them against the rival team.

 

[Bedrockgames]

In the gaming I'm DMing, the heroes are exploring a trap filled dungeon, and have correctly guessed that there is a giant boulder trap at the end of a certain hallway (based on Rolling Sphere from the DMG, but upgrade because it's a 15-foot sphere instead of a 10-foot). They have an immovable rod, and plan to use it to block the giant boulder.

I'm considering letting it just work because its called an immovable rod, and because I like to reward smart play.

The thing is though, that according to the book, the immovable rod actually has a weight limit of 8,000 pounds. I did some quick calculations, and a 15 foot diameter sphere of granite should weigh nearly 300,000 pounds.

That's way more than the rod can hold, but I also figure the stone wouldn't be putting its full weight limit on the rod. I'm pretty sure there's a way to calculate the force of the boulder on the rod, but my physics skills are rusty. (Also intuitively I feel like the current speed should factor into it, not just the acceleration).

Both the rod and the sphere can be thwarted by Strength checks, so I'm thinking about having them make some kind of opposed check against each other or something like that.

I don't just want to do a gotcha on my players. My goals are to have fun encounters and encourage clever gaming. With that in mind what would you do?

This is one of the reasons I moved away from quantifying this kind of thing in my own games. I used to get really into how much people could lift exactly according to their strength scores, and this would key to items and such. But I realized I am much more cinematic in my logic when it comes to how I want the world to function.

That said, I think it is really up to you how much you want to impose the real world weights here. It is your judgement as GM. You took the extra step of calculating the boulder weight (which I am assuming is a level of detail the designers of the rod probably never really considered). So I think it is definitely a factor to consider, it just something you can easily hand wave. I think the key issue is whether the players are likely to share your perception of the weight and whether they believe they are playing in a game world where those kinds of physiques operate exactly as they do in our own.

 

[Ruin Explorer]

I get the urge to design a dungeon full of elaborate traps that fail to function when the players trigger them.

I dunno about you but I used to get a big kick out of designing traps which had once been terrifying but age, wear, and misuse or the like had made relatively non-threatening, or putting in traps which had gone off and not reset or the like, when I used to design dungeons. For whatever weird reason it seems to scare players slightly more than "real" traps do. Especially if some of the decrepit traps are still functional.

 

[J-H]

Great math here... here's a slightly different approach.
The giant 300k lb boulder hits the immovable rod, which resists with a force of up 8,000 lbs at 1G (32 ft/sec ish) before breaking. I'm assuming gravitational acceleration since it's not specified.

The question then, in my mind, becomes "What does an 8,000 lb impact at 32 ft/sec spread over a 16"x1" surface due to a rock?"

I did a minute of searching and didn't find a solid answer, but if it was my game, I'd treat it like the rock just got hit by a big sledgehammer. I'd probably have the rock crack substantially, slow down some, and then break itself into two big and several smaller pieces all jammed together over the next 40-50'. The party now has 300,000lb of rock blocking the passage. I hope they didn't need to get past the rock.

 

[MarkB]

Great math here... here's a slightly different approach.
The giant 300k lb boulder hits the immovable rod, which resists with a force of up 8,000 lbs at 1G (32 ft/sec ish) before breaking. I'm assuming gravitational acceleration since it's not specified.

The question then, in my mind, becomes "What does an 8,000 lb impact at 32 ft/sec spread over a 16"x1" surface due to a rock?"

I did a minute of searching and didn't find a solid answer, but if it was my game, I'd treat it like the rock just got hit by a big sledgehammer. I'd probably have the rock crack substantially, slow down some, and then break itself into two big and several smaller pieces all jammed together over the next 40-50'. The party now has 300,000lb of rock blocking the passage. I hope they didn't need to get past the rock.

If you set the rod head-on to the boulder, you can bring that impact surface area right down. At that point it's like the rock's being shot with a bullet.

 

[NaturalZero]

One of my favorite spells is Wall of Stone because A. It creates a load of permanent stone and B. the text says you can make it any shape you want, which leave a bunch of interpretive space. The material component is a bit of granite, so at least in my mind, the wall is made of granite (30 hp PER INCH of thickness!). It does say that you make 10 foot panels, but since you can make it any shape you want, you could make the panels into a layered jawbreaker that is effectively a solid sphere.

 

[jasper]

If you set the rod head-on to the boulder, you can bring that impact surface area right down. At that point it's like the rock's being shot with a bullet.

Which instantly shatters. Rocks fly everywhere and the DC is double and damage is tripled. BABHAHAHA

 

[Stormonu]

You said Strength checks can stop the boulder? Give the Rod a Strength score for this action, say 30. If it holds, it will stop the boulder. Fails, either it overloads the magic in some way or pushes it further along where it makes another Strength check, possibly stopping the boulder further down the hall than anticipated.

I wouldn't try to apply real physics. I've seen how bad that goes wrong in 3E with falling damage (my tale of the "off the ropes" shoulder drop from a Dire Bear that broke the games rules in half, and the Fighter who could hit harder than their galleon could ram so that he strapped himself to the prow as a ramming figurehead...).