Force of gravity

[Starman 2112]

Why is it that a item can overhang an edge and stay motionless for a while, then suddenly the item will topple over the edge? I'm referring to such items as books where there isn't a shifting load. Why does it not topple over the edge immediately.

[ian.miller]

i believe its because of the turning force, if the book is *just* over the edge from centre - the force between the bit of the book overhanging and that which is not (trying to push it back down on the shelf) is split (for example) 51/49 with the 51 winning and starting the books fall.

However, as that difference is so small and the "book" is heavy the book doesn't accelerate into it's fall very fast but as the book starts its fall the balance is further tipped in favour of the part that's over the edge so it goes

51/49
60/40
80/20

until 100% of the book is unsupported and it continues on it's way to earth at 9.8m/s (if my memory serves)

[bernardo]

This sort of thing would also be subject to microscopic changes in the shape and size of things - e.g. the book shrinking or growing according to moisture retention / absorption / temperature changes, the shelf creaking, puffs of wind, etc.