Why do we need normal and tangential energy?

Topics related to collision & Trajectory analysis formerly on our 'Registrants only' area however which we get asked about frequently so believe shoud be in the open forum too
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brian
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Joined: Tue Jul 14, 2009 10:52 am

Why do we need normal and tangential energy?

Post by brian » Sun May 23, 2010 8:59 pm

Q: May 20, 2010: In some analysis methods, it is said that we can find the energy loss distribution in normal and tangential direction. What we need is total energy loss right? Why do we need to distribute energy loss to n and t direction?

A: In collisions in order to determine the magnitude and direction of the Impact Speed Change, forces/energy must be resolved into normal and tangential directions. The principal force is generally normal to the collision surface and so it is referred to as the ‘normal’ force. In addition, friction forces may act to resist relative movement of the colliding surfaces. Some are proportional to the normal force and are referred to as coulomb forces. Some are tensile forces and are independent of a normal force. The tensile forces frequently act along the vehicle-to-vehicle interface.
Coulomb friction forces and tensile resistance forces can be lumped together and referred to simply as ‘tangential’ forces.
In planar impact mechanics total energy loss is broken into the normal and tangential directions to approximate the Principal Direction of Force (PDOF) and for calculation of Impact Speed Changes and directions.
In CRASH3 and other techniques, the PDOF is simply approximated.
Brach, et al, do not like the simplicity of the PDOF.
From “Residual Crush Energy Partitioning, Normal and Tangential Energy Losses”
  • “the use of the work of the normal impulse to represent crush energy and the work of the tangential impulse to represent energy loss from tangential effects over the intervehicular contact surface eliminates the need for visual estimation of the PDOF and the use of the CRASH3 tangential correction factor.
But of course they also mention
  • “The energy partitioning process is intuitive, and does not follow directly from any principle of mechanics.”
Brach, et al apparently prefer 'visual and intuitive energy partitioning' rather than something simple and straightforward like the Principal Direction of Force (PDOF).
For additional information please also see:
1) SMAC inter-vehicle friction coefficient
2) SNAG – The impulsive constraint option
3) Planar Impact Mechanics assumptions
4) What is the CRASH3 Energy Correction Factor (ECF)?
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