Damage
For slams and collisions use delta V and the object with the smallest hit points to calculate damage die.

Vs = the smallest object's speed in hexes per second
VL = the Largest object's speed in hexes per second
Vs - VL = dV = delta V
HPs = the smallest object's hit points
HPL = the largest object's hit points

(dV)*(HPs)/100 = # of dice of unmodified damage.  For rounding see B430.

Due this damage to each object. Each object rolls for the damage inflected on the other.

Knock back / Knockdown
{inelastic collision}
    If both objects are moving in the same direction use:
[(HPs*Vs + HPL*VL) / (HPs + HPL) = V system

  If both objects are moving in opposite directions use:
[(HPs*Vs - HPL*VL) / (HPs + HPL) = V system
The sign indicates direction.

    General rule. Each object roll against the appropriate skill or DX to remain standing. Apply shock, Active Defense Modifiers, Melee Attack Modifiers (for exceptions see Slam B???) to your DX roll to keep "standing" (have an action next turn). The GM may have additional modifications due to extreme system speed changes or may simply tell the players what happens without rolling.
    Rules of thumb if V > HP then damage (energy) dominates, not much kockback. If V < HP then knock back (momentum) dominates, not much damage.

Strikers
    Creatures and objects that are frequently running into things have a striker with DR for collisions only. See strikers B88.  Example: a mountain sheep may have horns that have DR4 for bullets, arrows, spears, etc. but have a DR12 for collisions with the horns.
    The GM may rule that a creature has a collision DR for skills such as a football tackle or a Bear's shoulder slam (brawling). The justification is that these skills train you to use structurally sound parts of your body to strike with. No more then DR2 on top of any natural DR for skill based collision DR.

Shields & Blocking
    When slamming with a shields use the shield's DR for collisions. If you block the slam with a shield then use the shield's DR for the collision. You may also block a collision with a striker if the GM OK's it. The mountain sheep would be a good example of this.

Example #1:
    Daring Dan jumps into the water to save Fluffy the cat (along with its diamond studded collar) and is slammed by a barge moving at 2 hexes per turn.
Dan has 10 HP + 1 for holding the cat for a total of 11 HP moving at 0 hexs per turn.
The barge has 500 HP moving at a speed of 2 hexes per turn.
Damage:
    (2)*(11) / 100 = 0.2d or 1d-3
Dan rolls against the barge and the barge rolls against Dan. Not much happens.
Knock back & Knockdown:
The barge and Dan are now moving at 2 hexes per turn.
The barge does 0 damage to Dan. Who then rolls against his swimming skill to avoid knockdown (has an action next turn in this case).
Example #1b:
The barge then slams into  the 10,000 HP dock.
Damage:
    (2)*(500) / 100 = 10d
Dan better not be caught between the dock and the barge! (Too bad about the cat).
Knock back:
 [10,000*0 + 500*2] / (10,000 + 500) = 0.1 hexes per turn. There is no knock back or knockdown in this case.

Example #2:
    Ruth in falcon form slams into a galloping rider from behind.
Ruth is moving at 60 and the rider is moving away at 20 so dV = 60-20 = 40
Ruth has 3 HP and the rider has 10 HP
Damage:
    (40)*(3) /100 = 1.2d or 1d
1d crushing to the falcon, 1d cutting to the rider due to the falcon's cutting striker. See falcon.
Knock back & Knockdown:
[(3*60 + 10*20)  / (3+10)] = V system = [29]
The GM may have additional modifiers for surprise. The rider will roll against his riding skill, a failure knocks off the rider! Remember that the horse is not part of this system.
Example #2b:
If this was from the front (head on) then:
Damage:
    (60+20)*3 / 100 = 2.4d or 2d damage.
Knock back:
[(3*60 - 10*20)  / (3+10)]  = V system = [-1.5] This implies that the system will be moving in the rider's direction, only a lot slower!
Again the GM may have additional modifiers for knock down due to the extreme system speed change.

Example #3:
    Fred is holding the line is slammed by Derf.
Fred has 10 HP, moving at 0, full defensive.
Derf has 11 HP, moving at 4, all-out-attack for damage.
Damage:
(10) * (4+0) / 100 = 0.4d or 1d-2 damage, + 2 for Derf's all-out-attack for 1d.
Derf is using a shield and gets its DR against collisions.
If Fred is sucessful is using his shield to block he will also get the shield DR. 

Knock back & Knockdown:
[(10*0) + (11*4) / (11 + 10) = 2 hexes per turn
Fred gets the full defencive bonus of +2 to his knockdown roll wether he blocks or not.

  
Dave's World GURPS


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