Given that,
Weight w=10gf
(a). Moment of force = magnitude of force X perpendicular distance of the line of action of force from the axis of rotation
=50cm×10gf
=500cmgf
(b). To apply the least force we obviously have to make the perpendicular distance highest.
Hence, the least force should be applied at 100 cm mark
Thus, by principle of moments
500gfcm=100cm×F
F=5gf
Hence, the force should be 5gf upwards at the 100 cm mark