Calculation of force on a cylinder

Hi, I intend to calculate the force on a cylinder in a flow. Using computeforces module, should I:
1) just enter the mk id of the cylinder for force calculation, or
2) I should specify a separate mk id for the side of the cylinder facing the flow, and then enter the new mk id for the force calculation? If this is the case, how I should do it? Can I say "left" or "Right" for a cylinder?
Thanks
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