Does physX SDK follow the physics law?

Hi, physX developers.

I’m a newer developer with physX SDK and recently encounter an odd problem.
We try add force to slider for simulate the vehicle driving behaviour, and create a simple demo to test the friction how to influence the “vehicle” moving, we set the gravity(0,-10,0), slider mass 1.0, slider shape box dim(1,1,1), static frition 0.5(both shape and world, use CombineMode NX_CM_AVERAGE), we compute the force which just can push the slider moving is f=umg=5N, then we addLocalForceAtLocalPos(5.1, NxVec(0,-1,0)), but we can’t get the result we expected.

What’s error with the test? Shall we missed sth? NX_SKIN_WIDTH is 0.0, all other parameter is set to default.
Any reply, prompt or clue is appreciative. :)