( ( ( ( ( ( A ( certain amount ) ) ( of force ) ) ( must ( be ( ( applied ( to ( the fluid ) ) ) ( before ( ( any flow ) ( is induced ) ) ) ) ) ) ) ; ) ( ( this force ) ( is ( called ( ( the ( yield stress ) ) ( -LRB- ( ( f ' ) -RRB- ) ) ) ) ) ) ) . )
(ROOT (S (S (NP (NP (DT A) (JJ certain) (NN amount)) (PP (IN of) (NP (NN force)))) (VP (MD must) (VP (VB be) (VP (VBN applied) (PP (TO to) (NP (DT the) (NN fluid))) (SBAR (IN before) (S (NP (DT any) (NN flow)) (VP (VBZ is) (VP (VBN induced))))))))) (: ;) (S (NP (DT this) (NN force)) (VP (VBZ is) (VP (VBN called) (NP (NP (DT the) (NN yield) (NN stress)) (PRN (-LRB- -LRB-) (NP (NNP f) (POS ')) (-RRB- -RRB-)))))) (. .)))
A certain amount of force must be applied to the fluid before any flow is induced; this force is called the yield stress (f').
(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)))) (. .)))