( ( A ( bow wave ) ) ( ( is ( produced ( when ( ( ( the speed ) ( of ( a source ) ) ) ( is ( greater ( than ( ( the speed ) ( of ( ( the waves ) themselves ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (NN bow) (NN wave)) (VP (VBZ is) (VP (VBN produced) (SBAR (WHADVP (WRB when)) (S (NP (NP (DT the) (NN speed)) (PP (IN of) (NP (DT a) (NN source)))) (VP (VBZ is) (ADJP (ADJP (JJR greater)) (PP (IN than) (NP (NP (DT the) (NN speed)) (PP (IN of) (NP (NP (DT the) (NNS waves)) (ADVP (PRP themselves)))))))))))) (. .)))
A bow wave is produced when the speed of a source is greater than the speed of the waves themselves.
(ROOT (S (NP (DT A) (NN bow) (NN wake)) (VP (VBZ is) (VP (VBN created) (SBAR (WHADVP (WRB when)) (S (NP (DT the) (NN wave) (NN source)) (VP (VBZ moves) (ADVP (RBR faster)) (PP (IN than) (NP (DT the) (NN wave) (NN propagation) (NN speed)))))))) (. .)))