Slam and Collision Rules: Any good house rules?

[kdtipa]

I'm running a campaign where characters have access to telekinesis that is different than normal... allows for the possibility of lifting and moving a tank, but slower than you could a bicycle. For purpose of my example for this post, let's say that a character is going to use TK to lift a 31.25 ton tank, and at that weight, they can only move it 1.33 yards per second. This is basically a walking pace.

Now imagine that there's an enemy standing there oblivious, and the tank is slowly brought in to collide with the enemy (obviously dropping it on them would work better, but I'm trying to figure something out with the slam and collision rules). If the collision happens we figure out the damage with...

HP x Velocity (yards per second) / 100

And then do a little adjusting to figure out what that number means in dice. If this hypothetical tank has 3125 hit points (1 HP per 20 pounds equals humans roughly). I see that the basic set has an example of a tank with 300 hit points, so maybe I'll run the numbers again with that, but...

3125 x 1.33 / 100 = 41.67

300 x 1.33 / 100 = 4

I don't agree that a tank would only have 300 hit points, but if it helps... let's going with a 31.25 tons block of carbon steel or something that you can imagine has 3125 hit points. However huge it has to be, if it's moving at 1.33 yards per second, according to the rules as written, it would be 41 dice of damage.

That seems odd to me. Something moving at a walking pace toward me bumping into me shouldn't do 41d of damage. That doesn't make sense.

I did some reading on a physics lesson, and it seems to be because it's about impulse rather than momentum, which is based on the change in velocity instead of the velocity. If a 30 ton block of metal collides with me at 2MPH I'm pretty sure it would just move me. Or I'd jog away from it. The change in speed isn't enough to be a worry.

So I'm wondering if anyone has come up with a way to model this better? What happens if a car in a parking lot accidentally hits a person at 5 miles per hour?

[khorboth]

I hadn't considered this particular problem before, but, perhaps...

make a damage cap.

For "hard" objects (like tanks and baseballs), damage is capped at moveX5

For "soft" objects (like people and volleyballs), damage is capped at moveX3

So your tank will do a pretty reliable 5X1.33 = 7hp of damage. It'll hurt a lot, but not kill. Seems reasonable.

A tank at 10 MPH, move 5, will totally crush someone. I'm still happy with that.

Make sure to only apply this when there's room to be moved. If trapped between gently floating tank and a brick wall, smashing happens.

The numbers may need some tweaking, but I feel like the cap is the right way to go. At a certain point, it doesn't matter how much weight is behind the slam, you're getting moved the same amount if it's a 10-tonn or a 50,000-tonn object.

[Refplace]

Originally Posted by Ransom

If you use a damage cap it should be based on the hp of the smaller object. An iceberg hitting a ship at a walking pace should probably do more than 5x move damage.

Resistance would help there and should be a factor in all collisions/slams
Hittign something on a frictionless surface probably hurts less than on rough ground where its ahrder to shove soething aside.

[Nereidalbel]

I just wouldn't even call something that slow a collision. Treat it more like a Shove, and only worry about damage if somebody gets pinned between the tank and something somewhat more resistant to being pushed around than a human.

[Rysith]

Originally Posted by kdtipa

If this hypothetical tank has 3125 hit points (1 HP per 20 pounds equals humans roughly). I see that the basic set has an example of a tank with 300 hit points, so maybe I'll run the numbers again with that, but...

Part of this is because in GURPS, HP is not a linear function of weight, so the tank doesn't have 3125 HP. That causes it's own problems (which you can find plenty of threads about - large things aren't nearly as durable against scratch damage as they 'should' be). That, coupled with GURPS in general not scaling particularly well well outside of human scale, means that yes, a 3125HP object colliding with someone slowly is going to cause weird things.

My advice would be to just rule it to something reasonable if it ever comes up. A good starting point could be modelling it as a fall, rather than a collision, in that someone falling is essentially colliding with an immensely massive object. Plugging in numbers to calculate a fall at 1.33 yards/second (on the order of 6 inches, unless I failed the math), you get a fall that doesn't even count as a fall in GURPS terms - which seems reasonable for the situation.

[onetrikpony]

The slam rules are pretty silly considering this is GURPS.

The scenario in the OP doesn't even highlight most of the problems.
Consider; Charging bull vs child.

Bull HP 25 x speed 5 /100 = 1D+1 Damage

If the Bull rolls 2 and the child rolls 5 the bull falls down, end of story.

Also the way the slam attack relates to the knockback rules is pretty nebulous.

If the bull just walked calmly up to the child and hooked him with a horn he'd do more damage, wouldn't take any damage, and the child would go flying 3-4 yards.

I'm also interested in a good HR. I havn't gotten around to it yet.

[kdtipa]

Excellent replies so far. Thank you. :)

The damage cap, and/or basing the damage on the lower HP of the two objects? I like that.

Using weight instead of HP because of the odd scaling issues for HP definitely seems like a good way to go.

I definitely agree that being squished between the floating tank and a concrete foundation would be bad for the person. Maybe the existing RAW damage still works for that.

But, for helping the conversation along: two movable objects, one drastically out weighing the other. How do we model damage and what happens? And how do we keep it from turning into calculus?

Here's a simple first stab at damage calculation...

damage = (weight in pounds of the lighter object / 20) x (velocity in yards per second) / 100

The difficulty with that is the baseball scenario... where a baseball is thrown at a person on purpose to do damage. I think rules just use thrust, but technically it's a collision too...

0.3 pounds / 20 = 0.015
100 MPH pitch = approx 49 yards per second
0.015 * 49 / 100 = 0.00735

That's the minimum damage of 1d-3 for a collision. Maybe that's accurate. Hmm. I mean... batters sometimes do get hit by those pitches, and it hurts, but how few of them have needed medical attention afterward?

A person hit by the slow tank would be...

200 pounds / 20 = 10
10 * 2 y/s / 100 = 0.2

That's still the minimum damage of 1d-3 which makes some sense since you're just being bumped by the super heavy object at a walking pace.

Still should probably figure out knockback. I think the tank has a pretty good chance of moving the person while the baseball won't move them by sheer force.

[kdtipa]

Originally Posted by onetrikpony

The slam rules are pretty silly considering this is GURPS.

The scenario in the OP doesn't even highlight most of the problems.
Consider; Charging bull vs child.

Bull HP 25 x speed 5 /100 = 1D+1 Damage

If the Bull rolls 2 and the child rolls 5 the bull falls down, end of story.

Also the way the slam attack relates to the knockback rules is pretty nebulous.

If the bull just walked calmly up to the child and hooked him with a horn he'd do more damage, wouldn't take any damage, and the child would go flying 3-4 yards.

I'm also interested in a good HR. I havn't gotten around to it yet.

This is also excellent. Great points. I hadn't included in my list of problems the damage being done to both objects. Even with the damage being decided by the lower weight (as proposed for part of the house rule), the bull has to choose to take equal damage if they use a slam attack...

maybe if you are doing it on purpose, you are setting yourself up to best handle the impact. As a pilot/driver, you buckle up and brace yourself. As a person colliding with another person you turn your should into it or use your hands to control the impact to yourself... maybe the person initiating the slam attack take half damage or something?

[Dustin]

Originally Posted by onetrikpony

Consider; Charging bull vs child.

Bull HP 25 x speed 5 /100 = 1D+1 Damage

If the Bull rolls 2 and the child rolls 5 the bull falls down, end of story.

I don't think Slam works that way. The bull does 1d+1, but a 5 HP child does
HP 5 x speed 5 / 100 = 1d-3 damage to the bull. There's still a 1 in 36 chance of the child winning, but not so goofy. This might end up resolved as a Trample, anyway, given the difference in sizes.

I'm really hoping that Dungeon Fantasy Roleplaying gives us some simpler Slam and falling damage rules...

[trooper6]

Originally Posted by Dustin

I don't think Slam works that way. The bull does 1d+1, but a 5 HP child does
HP 5 x speed 5 / 100 = 1d-3 damage to the bull. There's still a 1 in 36 chance of the child winning, but not so goofy. This might end up resolved as a Trample, anyway, given the difference in sizes.

I'm really hoping that Dungeon Fantasy Roleplaying gives us some simpler Slam and falling damage rules...

Dustin is correct here. I think the other thing the people miss is that Slams are not the attack to make if you want to do lots of damage to someone, they are attacks you make if you want to knock someone down. If you want to do damage, you move and attack or trample.

So let's do this with actual numbers from the books.

We will use the stats for the Ox on B460.
ST 27, Move 4, SM +2, Enhanced Move 1 (Ground Speed 8), Impaling Striker Horns (+1 per die damage when using striker). Generic Thrust damage: 3d-1

We'll go with a 5 year old as described on B20.
ST 6, SM -2

Option 1: Bull decides to do an Overrun/Trample attack.

The Bull is up to its full move of 8. It is planning to overrun/trample that small child. First is the slam attack. Let's say the bull has moved 7 hexes so that is still has movement to overrun.
The bull does: 27*7/100= 2d cr (minimum 2)
The child does: 6*7/100 = 1d-2 cr (maximum 4)

So there is a low possibility that the bull could fail to knock down the child, and a very remote possibility that the bull falls down instead (which any GM worth their salt could easy describe in a plausible manner). But in all likelihood the bull does decent damage to the child and knocks them over.

Since the bull has 4 more size categories on the child, the bull gets an automatic overrun/trample. This will automatically do another 1d crushing damage. It is also large-area injury.

Option 2: Bull decides to do a Slam and then on the next turn trample.

This is a two turn sequence where the bull slams the child and then on the next turn just starts trampling the child.

Turn one is the slam as above. Turn two, the bull does 3d-1 trampling damage.

Option 3: Bull decides to Gore the child.
The bull is not interested in knocking the child over (slam) but is more interested in doing lots of damage. So it does a move and attack. This has negatives due to skill cap...but the bull doesn't know any better. If it is successful, it would do 3d+2imp damage to the child.

Slams are not for damage, they are for knockdown.