# static/dynamic friction

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Structure    240
Hi, my last post was a bit of a long shot so I’ll ask again in simpler terms, Does anybody have any information or can point me to anything about calculating static and dynamic friction impulses for a 2d rigid body system. I need it in the form of: Calculate static friction impulse, If its in the friction cone use it, else its on surface of the friction cone Calculate dynamic friction impulse, Thanks.

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Motorherp    613
Don't know of any online resources of the top of my head for this but I can at least tell you the type of stuff you'll need to be doing. Basically you have to apply conservation of momentum at the contact points of the two bodies with an extra force added which only acts in the plane tangential to the collision normal and opposite to the relative motion of the contact points. The magnitude of this force is usually taken to be proportional to the impulse magnitude. For the static friction case the two contact points on each body should not be moving relative to each other in the plane which is tangential to the collision normal after said collision. To determine exactly how each body is affected to maintain this constraint you'll have to apply conservation of momentum at the contact points with the final tangential velocities of the contact points set equal. To do this properly you'll need to convert your maths into a space defined by a set of orthogonal axes aligned with the collision normal and tangential plane axes. Finally, if you have multiple contact point pairs between the bodies or more than two bodies in simultaneous contact you'll have to put all the equations into matrix format and solve them with one of the many matrix solver methods out there.

Sorry to be brief but this should at least get you googling in the right direction. Failing that, if you crack out your high school physics and math books you should be able to derive all this from basic principles.

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JeffLander    660
I wrote some basics on friction for Game Developer some years ago. May be too basic for your needs but has sources for the math and techniques for static and dynamic that would work fine (and does :) on rigid bodies.

www.darwin3d.com/gdm1999.htm#gdm0899

Beyond that you would need to describe more of what your problem with the friction models are.

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Structure    240
@Motorherp

Thanks for the info, im understanding better what needs to be done now, my last post http://www.gamedev.net/community/forums/topic.asp?topic_id=298119 was an attempt to get someone to derive the equations for me, but well ill have another shot at it mysef,

@JeffLander

Thanks for the link, ive read a number of your articles on gamasutra, but I'll take another look at this one over the coming week and see if I can use it.

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Structure    240
Im understanding better what needs to happen now but still not getting anywhere with this,

The friction impulse bit, is what I need except these calculations are for 3 dimensions,

Can anyone help me simplify them to 2d?