Suppose we have two sticks held in a vertical plane, pivoted at one point (check the diagram). If we let go, why is the initial acceleration of the center of the mass not equal to g? Why does the pivot exert some force at the beginning? By contrast, if we keep the sticks in a horizontal plane, the initial acceleration is g. This is a solved problem in Introduction to Classical Mechanics by Morin (8.11) where he has shown the proof quantitatively ; however I cannot understand the qualitative aspect of it. The stick of length b is massless, and is glued perpendicular to the stick of length l. PS : It is NOT a homework question! As I said, it is a solved question in Morin.