Why are Momentum Calculations and SMAC Results Sometimes Different?

General questions on the SMAC, msmac3D and other Collision Simulation programs
MSI
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Why are Momentum Calculations and SMAC Results Sometimes Different?

Post by MSI »

Q: (from email) I've been reviewing a SMAC simulation and if I do a Momentum DeltaV calculations based on the SMAC Results for speed and direction at contact and separation i get different results for DeltaV. Any ideas?

A: In SMAC the DeltaV or impact speed change is calculated based on integration of the acceleration components during the collision which are calculated every millisecond. So it is analogous to what occurs in full scale tests
Please see the thread How does SMAC Calculate deltaV and PDOF

When you do a simple two step linear momentum calculation based on two points: impact & separation, it may be different than measured/simulated depending on the amount of rotation between impact and rotation.
If you do an angular momentum calculation between impact and separation it may be closer.
However they both have the simplifying assumption of instantaneous exchange of momentum and that the impact and separation points are at the same location and include no external forces.
Whereas in a collision the vehicle move and may rotate between impact and separation and the tire forces are acting which may increase the amount of Deltav (increase in acceleration of MU*distance traveled, etc where MU is amount of frictional drag)
Here’s a topic on it: Why do we need Angular Velocities in Collision Analysis?