# How to calculate friction coefficient between a cube and ground?

I have been struggling with how Unity’s physics engine works. I am trying to predict the movement of cube on a ground.

I have set ground’s dynamic friction and static friction to 0 and it’s friction combine to average. For the “moving cube” the dynamic friction and static friction are 0.6 and friction combine is average. So the effective dynamic and static friction should be 0.3.

Now when I set the velocity of static cube to 10 it travels a distance of 8.4 . Mass of cube = 1 and gravity = 9.81. Reverse calculation shows that static and dynamic friction of 0.6 were used instead of the expected 0.3. In other words I expect the cube to move a distance of 16.8 (2 * 8.4) instead of 8.4.

After experimenting with different values I am pretty sure that this is a bug. When friction combine for both the interacting bodies is set to “Average” then the friction that is used is not the “Average” but the “Sum” of the two friction values.