# Swords, impacts and elasticity for a noob

So I'm a game developer and I'm trying to understand some (extremely) basic facts of impact mechanics. I had read something entitled Dynamics of Hand-Held Impact Weapons, but it was a bit too complicated for my feeble mind, and really more than I needed to understand for a game.

I'm assuming as average a hit as possible, envisioning the problem as little more than a flat block hitting another flat block; bludgeoning at first, and cutting later after I understand that.

From what I understand, elasticity is a range from 0.0 to 1.0, yes? Assuming the sword (basic flat block for now) has zero elasticity for simple gameplay purposes, how does elasticity affect the Time variable for acceleration? And what force is it that causes the target (a variably squishy flat block) to resist further deformation and retake its shape?

If it sounds like I'm just misguided, guidance in what I ought to know would be fine too. Thanks!

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Elasticity is not simply a range from 0 to 1. My guess is that is that is a parameter in your software that describes how much momentum is transferred from one block to the other, and may determine the model velocities of the blocks after impact. Momentum is distributed differently before and after, but the total remains the same. Without more explanation of the model formula its not possible to say what the effect of that parameter will be. – Mark Rovetta Jan 10 '13 at 16:56
Ah, I had read something about 0-to-1 being a range from perfectly inelastic collisions to perfectly elastic ones. Guess I really have a ton to learn. As many static, controlled variables as possible would be useful to understand what's going on. Two perfect cubes of equal mass colliding perfectly perpendicularly, no external forces involved, no elasticity in the "sword" cube, variable elasticity in the target cube 'n' such. Any other variables would be as simplistic and normative as possible for a peon like myself to understand. – Nemox Jan 10 '13 at 17:19
Yes that makes sense. Inelastic basically means they stick together and elastic means they bounce off each other like ideal billiard balls. So specify the elasticity parameter and initial velocities (before collision) and your model will give the final velocities (after.) i.e. It sets whether you are modeling the batting a hard baseball (~1) or a doughball (<1) etc. – Mark Rovetta Jan 10 '13 at 17:50
I think it's starting to click a bit. Sounds like that shape-reformation force variable really is a huge component that can't be separated. Is there a particular name for that? – Nemox Jan 10 '13 at 18:46

From the comments we exchanged above I think I now understand your question.

I think the parameter in your software is actually what's called the Coefficient of restitution. Its value will alter how your model sword and target behave when they collide.

If it equals 1, the collision is perfectly elastic - the total momentum and the total kinetic energy is conserved (that is the same before and after the collision.) Sword and target bounce off each other and move off with different velocities.

If it equals 0, the collision is perfectly inelastic - the total momentum is conserved, but the maximum kinetic energy is lost from the system. Sword and target stick and move off together with the same velocity.

If it is between 0 and 1 you get an intermediate effect, which is what makes it a useful parameter for model simulations.

Partially inelastic collisions are the most common type of collisions in the real world. For example, the height of a bouncing ball decreases with successive bounces because the coefficient of restitution is less than 1 and the system loses energy.

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Ah, I think I see where I need to go from this point. I saw that link to Resilience at the bottom of that page, which would be the primary aspect of calculating damage. Also, directly related to but not on that page would be determining what the deformation-resistance force is called and how to apply it to the equation. – Nemox Jan 10 '13 at 20:46