▁of ▁motion . ▁This ▁law ▁of ▁motion ▁is ▁most ▁commonly ▁par ap hr ased ▁as : ▁" For ▁every ▁action ▁force ▁there ▁is ▁an ▁equal , ▁but ▁opposite , ▁reaction ▁force ." ▁ ▁Ex amples ▁include ▁jet ▁engines , ▁ro cket ▁engines , ▁p ump - jet , ▁and ▁more ▁un common ▁variations ▁such ▁as ▁Hall ▁effect ▁thrust ers , ▁ion ▁drives , ▁mass ▁drivers , ▁and ▁nuclear ▁pul se ▁prop ulsion . ▁ ▁Energy ▁use ▁ ▁Prop uls ive ▁efficiency ▁ ▁For ▁all ▁reaction ▁engines ▁that ▁carry ▁on - board ▁prop ell ant ▁( such ▁as ▁ro cket ▁engines ▁and ▁electric ▁prop ulsion ▁drives ) ▁some ▁energy ▁must ▁go ▁into ▁acceler ating ▁the ▁reaction ▁mass . ▁Every ▁engine ▁was tes ▁some ▁energy , ▁but ▁even ▁assuming ▁ 1 0 0 % ▁efficiency , ▁the ▁engine ▁needs ▁energy ▁amount ing ▁to ▁ ▁( where ▁M ▁is ▁the ▁mass ▁of ▁prop ell ent ▁exp ended ▁and ▁ ▁is ▁the ▁exhaust ▁velocity ), ▁which ▁is ▁simply ▁the ▁energy ▁to ▁acceler ate ▁the ▁exhaust . ▁ ▁Compar ing ▁the ▁ro cket ▁equation ▁( which ▁shows ▁how ▁much ▁energy ▁ends ▁up ▁in ▁the ▁final ▁vehicle ) ▁and ▁the ▁above ▁equation ▁( which ▁shows ▁the ▁total ▁energy ▁required ) ▁shows ▁that ▁even ▁with ▁ 1 0 0 % ▁engine ▁efficiency , ▁certainly ▁not ▁all ▁energy ▁supplied ▁ends ▁up ▁in ▁the ▁vehicle ▁– ▁some ▁of ▁it , ▁indeed ▁usually ▁most ▁of ▁it , ▁ends ▁up ▁as ▁kin etic ▁energy ▁of ▁the ▁exhaust . ▁ ▁If ▁the ▁specific ▁imp ulse