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what is the formula for rotational forces?
The formula to calculate the torque generated by a force applied perpendicular to the radius is:
With :

is the torque expressed in newtonsmeters [N.m] 
is the force expressed in newtons [N] 
is the radius expressed in meters [m], note that , the length of the OP segment.
See also : What is the general formula to calculate the torque generated by a given force?