( ( Fluid motion ) ( ( caused ( by ( ( an ( external force ) ) ( such ( as gravity ) ) ) ) ) . ) )
(ROOT (S (NP (JJ Fluid) (NN motion)) (VP (VBD caused) (PP (IN by) (NP (NP (DT an) (JJ external) (NN force)) (PP (JJ such) (IN as) (NP (NN gravity)))))) (. .)))
Fluid motion caused by an external force such as gravity.
(ROOT (S (NP (NNP Gravity)) (VP (VBZ causes) (S (NP (NNS rocks)) (VP (TO to) (VP (VB roll) (ADVP (RB downhill)))))) (. .)))