What is the different between force and drag
 

Hellow everyone
I am modelling an oscillating cylinder in still water. I tryed to monitor FORCE minus DRAG in (1 0 0)direction using expression, but it showed a result of zero all the time. So I am wondering how does fluent calculate DRAG and FORCE. Dose DRAG equal viscous drag plus pressure drag? Or, dose DRAG only equal pressure drag and FORCE is DRAG plus viscous drag?
Btw, my cylinder oscillated with a speed of 0.001*cos(t) in the (0 0 1) direction. I used a sliding mesh to let the cylinder move.