Masses  m,2m,3m,……  12m,  are  kept  at  the  locations  of  numbers  of  a  clock  having circular dial.  is gravitational field at the center of dial.

Effectively we can write At 9.30 the hour hand is in direction of g→ .At 3.30 the hour hand is in opposite direction of  g→.There will be no component of g→ along hour hand at 6:30., as they are perpendicular to each other.There will be no component of g→ along hour hand at 12:30., as they are perpendicular to each other.