## What is the mSMAC SNAG option and why do i need it?

General questions on the SMAC, msmac3D and other Collision Simulation programs
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### What is the mSMAC SNAG option and why do i need it?

Q: What is the mSMAC SNAG option and why do i need it?
A: Intervehicle friction, as simulated by original SMAC and EDSMAC, is in the form of simple Coulomb friction. It requires the existence of a force perpendicular to the contact region and it is proportional to that force.
In actual collisions, tensile forces which are independent of a normal force, frequently act along the vehicle-to-vehicle interface. Also, significant resistance to rotations relative to the collision partner is sometimes generated by interlocking of the damaged structures.
• The SNAG Option inputs on cards 16 and 18 are in the form of impulsive linear and angular constraints that resist relative motions of the two vehicles. They are modeled as reaction constraints that act only to resist relative motions. The mSMAC program conserves both the linear and the angular momentum of the two-body system in the absence of external (i.e., tire/pavement) or fixed object collision forces. Thus, any interacting forces and/or moments that are applied to resist relative motions and, thereby, to improve the predicted position and heading at rest of Vehicle #1 will also affect the corresponding position and heading at rest of Vehicle #2. The analytical approach to input selection consists of iterative adjustments to achieve acceptable matches of the positions and headings at rest of both vehicles. The rationale is the concept that inappropriate inputs can not achieve acceptable responses of both vehicles.
It should be noted that fields 3 and 4 of card 18 permit the input of viscous damping of the linear constraint for the case of long duration linear constraints where oscillations may occur.
Please see the full write up on SNAG in the msmac Input manual:
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