▁field ▁var ies ▁with ▁location . ▁However , ▁when ▁the ▁change ▁of ▁distance ▁is ▁small ▁in ▁relation ▁to ▁the ▁distances ▁from ▁the ▁center ▁of ▁the ▁source ▁of ▁the ▁gravit ational ▁field , ▁this ▁variation ▁in ▁field ▁strength ▁is ▁neg lig ible ▁and ▁we ▁can ▁assume ▁that ▁the ▁force ▁of ▁gravity ▁on ▁a ▁particular ▁object ▁is ▁constant . ▁Near ▁the ▁surface ▁of ▁the ▁Earth , ▁for ▁example , ▁we ▁assume ▁that ▁the ▁acceleration ▁due ▁to ▁gravity ▁is ▁a ▁constant ▁ ▁(" standard ▁gravity "). ▁In ▁this ▁case , ▁a ▁simple ▁expression ▁for ▁gravit ational ▁potential ▁energy ▁can ▁be ▁derived ▁using ▁the ▁W ▁= ▁F d ▁equation ▁for ▁work , ▁and ▁the ▁equation ▁ ▁The ▁amount ▁of ▁gravit ational ▁potential ▁energy ▁held ▁by ▁an ▁elev ated ▁object ▁is ▁equal ▁to ▁the ▁work ▁done ▁against ▁gravity ▁in ▁lif ting ▁it . ▁The ▁work ▁done ▁equals ▁the ▁force ▁required ▁to ▁move ▁it ▁up ward ▁multip lied ▁with ▁the ▁vertical ▁distance ▁it ▁is ▁moved ▁( rem ember ▁W ▁= ▁F d ). ▁The ▁up ward ▁force ▁required ▁while ▁moving ▁at ▁a ▁constant ▁velocity ▁is ▁equal ▁to ▁the ▁weight , ▁m g , ▁of ▁an ▁object , ▁so ▁the ▁work ▁done ▁in ▁lif ting ▁it ▁through ▁a ▁height ▁h ▁is ▁the ▁product ▁m gh . ▁Thus , ▁when ▁account ing ▁only ▁for ▁mass , ▁gravity , ▁and ▁alt itude , ▁the ▁equation ▁is : ▁ ▁where ▁U ▁is ▁the ▁potential ▁energy ▁of ▁the ▁object ▁relative ▁to ▁its ▁being ▁on ▁the ▁Earth ' s ▁surface , ▁m ▁is ▁the ▁mass