Any way to get 'realistic' friction?

[voidpointer]

I originally posted this question over in the 'Game Engine Support and Discussion' forum at BlenderArtists.org because I didn't know this forum existed! http://blenderartists.org/forum/showthread.php?t=132841

setLinearVelocity and friction - trying to move an object in a straight line

I am trying to do a simple simulation of a box moving along a surface. Based on the angle and speed that I want the box to move, I use setLinearVelocity to set the box in motion and let physics take over. I am not using any damping and just using friction on the box and the plane that the box is sliding on. I have that all working, but the problem is that the box is not moving along a straight line like I would expect it to and is instead moving along a curve. It seems to me that the friction is being applied to each component of the velocity separately instead of being applied to the entirety (magnitude) of the velocity vector.

I think the example I have attached should clearly demonstrate what I am trying to describe. In my example file, pressing SPACE sets the cube in motion.

I'd also like to note that to see the behavior I would have expected (and want), you can load my example file, press F8, and change the Physics Engine from "Bullet" to "Sumo". Then when you Start Game and press SPACE, you'll see the cube move in a straight line along the angle that I have specified. However, I don't want to use Sumo because it is deprecated and will cause other issues.

Thanks in advance,
Ryan