( Thrust ( ( ( is just ) ( ( the force ) ( ( the engines ) ( are ( providing ( to ( accelerate ( a rocket ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Thrust)) (VP (VBZ is) (ADVP (RB just)) (NP (NP (DT the) (NN force)) (SBAR (S (NP (DT the) (NNS engines)) (VP (VBP are) (VP (VBG providing) (S (VP (TO to) (VP (VB accelerate) (NP (DT a) (NN rocket))))))))))) (. .)))
Thrust is just the force the engines are providing to accelerate a rocket.
(ROOT (S (NP (NNP Thrust)) (VP (VBZ is) (NP (NP (DT the) (NN force)) (VP (VBG pushing) (NP (DT a) (NN rocket))))) (. .)))