( Stress ( ( is ( ( the force ) ( applied ( per ( unit area ) ) ) ) ) . ) )
(ROOT (S (NP (NN Stress)) (VP (VBZ is) (NP (NP (DT the) (NN force)) (VP (VBN applied) (PP (IN per) (NP (NN unit) (NN area)))))) (. .)))
Stress is the force applied per unit area.
(ROOT (S (NP (NP (DT The) (NN force)) (VP (VBN applied) (PP (TO to) (NP (DT an) (NN object))))) (VP (VBZ is) (VP (VBN called) (NP (NN stress)))) (. .)))