HCR - Static and Kinematic Friction

Each material interaction has 2 friction coefficients:

• One static (${\mu }_s$).
• and one kinematic (${\mu }_d$),

The static friction coefficient $({\mu }_s)$ is used to break the stacity (so when the object is still), while the second one $({\mu }_d)$ is used to determine the friction force that slows down the motion.

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• ~Example:
  • Wood on wood: ${\mu }_s=0.25\sim 0.5$, ${\mu }_d=0.2$
  • Glass on glass: ${\mu }_s=0.94$, ${\mu }_d=0.4$
  • Ski on dry snow: ${\mu }_s\simeq 0$, ${\mu }_d=0.04$
  • When the object is moving and ${\mu }_s\gg {\mu }_s$ (usually true) we approximate the static friction coefficient ${\mu }_s$ to 0, so we do not need to calculate it.
  • Basically when we define the friction cone, the angle is equal to:$\theta ={\tan }^{-1}({\mu }_s)\text{or}\theta ={\tan }^{-1}({\mu }_d)$And it depends if the grasp is static or if the object is sliding in the hand.