▁compar atively ▁cheap ), ▁water ▁is ▁p ump ed ▁up ▁to ▁the ▁higher ▁lake , ▁thus ▁converting ▁the ▁elect rical ▁energy ▁( running ▁the ▁p ump ) ▁to ▁gravit ational ▁potential ▁energy . ▁At ▁times ▁of ▁peak ▁demand ▁for ▁electric ity , ▁the ▁water ▁flows ▁back ▁down ▁through ▁elect rical ▁generator ▁tur b ines , ▁converting ▁the ▁potential ▁energy ▁into ▁kin etic ▁energy ▁and ▁then ▁back ▁into ▁electric ity . ▁The ▁process ▁is ▁not ▁completely ▁efficient ▁and ▁some ▁of ▁the ▁original ▁energy ▁from ▁the ▁sur plus ▁electric ity ▁is ▁in ▁fact ▁lost ▁to ▁fr iction . ▁ ▁Gra v it ational ▁potential ▁energy ▁is ▁also ▁used ▁to ▁power ▁clock s ▁in ▁which ▁falling ▁weights ▁operate ▁the ▁mechanism . ▁ ▁It ' s ▁also ▁used ▁by ▁counter we ights ▁for ▁lif ting ▁up ▁an ▁elev ator , ▁c rane , ▁or ▁s ash ▁window . ▁Rol ler ▁co aster s ▁are ▁an ▁entertain ing ▁way ▁to ▁util ize ▁potential ▁energy ▁– ▁ch ains ▁are ▁used ▁to ▁move ▁a ▁car ▁up ▁an ▁incl ine ▁( building ▁up ▁gravit ational ▁potential ▁energy ), ▁to ▁then ▁have ▁that ▁energy ▁converted ▁into ▁kin etic ▁energy ▁as ▁it ▁falls . ▁ ▁Another ▁practical ▁use ▁is ▁util izing ▁gravit ational ▁potential ▁energy ▁to ▁descend ▁( per haps ▁coast ) ▁down hill ▁in ▁transport ation ▁such ▁as ▁the ▁descent ▁of ▁an ▁autom obile , ▁tr uck , ▁rail road ▁train , ▁b icy cle , ▁air plane , ▁or ▁fluid ▁in ▁a ▁pipeline . ▁ ▁In ▁some ▁cases ▁the ▁kin etic