Slam and Collision Rules: Any good house rules?

[sir_pudding]

Why wouldn't the bull a) use the rules for slamming with impaling weapons and b) All-Out Attack?

[Refplace]

I am just waiting to see what the new Slam rules are in the DFRPG.

[kdtipa]

Originally Posted by trooper6

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.

My interest though is about the oddity of the damage in the rules as written. The 3000+ HP block of something moved through the air at 1.33 yards per second bumping into a person would do 41d damage.

I think the bull example was not used to suggest that a slam/collision is what you'd choose in order to do damage, but was used to point out again the oddity of the damage rules.

[kdtipa]

Originally Posted by Refplace

I am just waiting to see what the new Slam rules are in the DFRPG.

This makes me think it might be time a fourth edition compendium... a collection of the rules that tweak the basic set rules in good ways, and collects all sorts of interesting things from the wide variety of sources into one place.

[Bruno]

Originally Posted by kdtipa

This makes me think it might be time a fourth edition compendium

We already have a 4e compendium; the Power-Ups line is where it's being published, unit by unit. Kromm's spoken about this before, in general they're having better luck selling it by "chapter" than they expect to sell a big compendium.

[ErhnamDJ]

I prefer to use kinetic energy, as though the slammer was a bullet. So all you would do is figure out the kinetic energy of the slam and use the square root divided by two-and-a-half to determine the damage. What we really need here are good rules for determining what the armor divisor should be.

The slam rules give nonsensical results since they base the damage on hit points rather than the mass of the object. The default rules are pretty much just unusable for anything out of the human scale.

Someone in the IRC chat had come up with a formula to do relativistic damage (which GURPS just ignores) a few years ago. That should be easy enough to recreate. Otherwise you end up with problems when your speedster or spaceship slams into things at relativistic speeds. This comes up fairly often in my games.

[onetrikpony]

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.

I had not considered that.
The way I was reading the damage is based only on the HP and velocity of the attacking character. It does work better if you calculate damage for both because the Slam attacker would usually have more velocity.

My primary concern was that a small 5hp character had a pretty good chance of knocking the larger slam attacker off it's feet. It also transfers better into the knockdown rules.

Thanks.

Why wouldn't the bull a) use the rules for slamming with impaling weapons

WHERE??? I havn't seen these rules.

In order to distinguish between kinetic attacks (bullets) and Slam attack it would be good to determine the difference. There should be some point at which the difference in SM, Velocity, and HP mean the slam is no longer applicable.

[Curmudgeon]

Originally Posted by 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?

I think it makes quite a lot of sense. Part of the problem is that you seem to be assuming the collision is a little bump that stops after an inch or so, if that much. 1.33 yards per second is about 2.66 mph. Now consider a car that approaches you from behind, slams into you at 2.66 mph and doesn’t come to a complete stop until a second later. Oblivious you didn’t see or hear it coming until it struck you. It knocked you to the ground and came to rest with it’s front bumper roughly even with the bottom of your rib cage. People do get seriously injured in cases like that.

Consider the case of your moving tank or block of carbon steel in a zero-gravity situation. If you can’t get out of the way 41d of damage seems perfectly appropriate as it crushes you slowly against the wall, the wall being a bit less yielding than you are.

Collisions with massive (high HP) objects usually don’t do that much damage because people get out of the way.

[Anthony]

Realistically, collision with a massive object will never be worse than collision with a wall at the same closing velocity, unless you are being pinched between two objects. There isn't really a significant difference, for a human, between being struck by a 1 ton car car moving at 1 m/s and a 10,000 ton freighter -- unless you've got a wall behind you, in which case the first is bad and the second probably leaves an unrecognizable smear.

In a simple inelastic collision between a 70 kg human and a 1,000 kg car, when the car is moving at 1 m/s, before collision the total kinetic energy of the system is 500J; after the collision both are moving at 0.935m/s and their total kinetic energy is 467J, so 33J has been converted into other forms of energy, such as damage to human and car.

In a collision between a 70 kg and a 10 kT freighter, with the freighter moving at 1m/s, before collision the total kinetic energy of the system is 5 MJ. After the collision, both are moving at 0.999993m/s and the total energy is 4,999,965J, so 35J has been converted into other forms of energy, such as damage to human and freighter.

This isn't quite no difference, but it's not likely to be distinguishable on the scale of GURPS damage.

[kdtipa]

Originally Posted by Curmudgeon

Consider the case of your moving tank or block of carbon steel in a zero-gravity situation. If you can’t get out of the way 41d of damage seems perfectly appropriate as it crushes you slowly against the wall, the wall being a bit less yielding than you are.

No wall. Just two freely moving objects. I agree that if there is a solid object behind you, 41d makes sense. You're getting crushed.

But with a car at a walking pace, the damage from the initial bump shouldn't be huge. If the car manages to knock you down and start rolling over you... then yeah... more damage, but that's not from the slam.