▁force ▁works ▁against ▁the ▁gravit ational ▁force . ▁If ▁the ▁book ▁falls ▁back ▁to ▁the ▁floor , ▁the ▁" fall ing " ▁energy ▁the ▁book ▁receives ▁is ▁provided ▁by ▁the ▁gravit ational ▁force . ▁Thus , ▁if ▁the ▁book ▁falls ▁off ▁the ▁table , ▁this ▁potential ▁energy ▁goes ▁to ▁acceler ate ▁the ▁mass ▁of ▁the ▁book ▁and ▁is ▁converted ▁into ▁kin etic ▁energy . ▁When ▁the ▁book ▁hits ▁the ▁floor ▁this ▁kin etic ▁energy ▁is ▁converted ▁into ▁heat , ▁de formation , ▁and ▁sound ▁by ▁the ▁impact . ▁ ▁The ▁factors ▁that ▁affect ▁an ▁object ' s ▁gravit ational ▁potential ▁energy ▁are ▁its ▁height ▁relative ▁to ▁some ▁reference ▁point , ▁its ▁mass , ▁and ▁the ▁strength ▁of ▁the ▁gravit ational ▁field ▁it ▁is ▁in . ▁Thus , ▁a ▁book ▁lying ▁on ▁a ▁table ▁has ▁less ▁gravit ational ▁potential ▁energy ▁than ▁the ▁same ▁book ▁on ▁top ▁of ▁a ▁t aller ▁cup board ▁and ▁less ▁gravit ational ▁potential ▁energy ▁than ▁a ▁heav ier ▁book ▁lying ▁on ▁the ▁same ▁table . ▁An ▁object ▁at ▁a ▁certain ▁height ▁above ▁the ▁Moon ' s ▁surface ▁has ▁less ▁gravit ational ▁potential ▁energy ▁than ▁at ▁the ▁same ▁height ▁above ▁the ▁Earth ' s ▁surface ▁because ▁the ▁Moon ' s ▁gravity ▁is ▁we aker . ▁" Height " ▁in ▁the ▁common ▁sense ▁of ▁the ▁term ▁cannot ▁be ▁used ▁for ▁gravit ational ▁potential ▁energy ▁calculations ▁when ▁gravity ▁is ▁not ▁assumed ▁to ▁be ▁a ▁constant . ▁The ▁following ▁sections ▁provide ▁more ▁detail . ▁ ▁Local ▁approximation ▁ ▁The ▁strength ▁of ▁a ▁gravit ational