Lo mthetho uthi xa uyazi i-mass ye-object okanye yento, usengakwazi ukubala ukuba ingakanani na i-force ekwinto ethile ngenxa ye-gravity okanye uzibuthe. kufuneka ubesemhlabeni ukuze ukwazi ukusebenzisa le-formula. Ukuba usennyangeni okanye ukwenye i-planet, usenokuyisebenzisa i-formula kodwa i-g iyakwahluka.