What is being measured when Contact Force is measured in IP?
When two bodies collide, they deform and remain in contact for a short period of time until they rebound and push each other away. The contact force of such a collision is spread over this short period of time (typically resembling a spike shaped bell curve). The calculation of this force-time profile is extremely difficult as it requires an analysis of the structural deformation of the colliding bodies. Because IP simulates motion under the assumption of perfectly rigid bodies, it cannot calculate the magnitude of the contact force. It instead calculates the momentum exchange of the collision (the impulse of the collision, equal to the area under the Force vs. Time curve), which is enough to accurately predict the motion of the colliding bodies. When the “Contact Force” between two bodies is measured, IP reports the impulse of the collision divided by the animation time step (summed with the normal force between the two bodies in contact). Appendix A.7 of the User’s Manual provides fu
