In the inertial frame of reference (upper part of the picture), the black object moves in a straight line, without significant friction with the disc. However, the observer (red dot) who is standing in the rotating (non-inertial) frame of reference (lower part of the picture) sees the object as following a curved path due to the Coriolis and centrifugal forces present in this frame.

Fonte: http://en.wikipedia.org/wiki/File:Corioliskraftanimation.gif