( ( ( ( ( ( ( A ball ) ( ( rolling down ) ( a slope ) ) ) ( ( is also ) ( affected ( by friction ) ) ) ) , ) but ) ( ( ( the force ) ( of ( gravity ( that ( causes ( it ( to move ) ) ) ) ) ) ) ( is stronger ) ) ) . )
(ROOT (S (S (NP (NP (DT A) (NN ball)) (VP (VBG rolling) (PRT (RP down)) (NP (DT a) (NN slope)))) (VP (VBZ is) (ADVP (RB also)) (VP (VBN affected) (PP (IN by) (NP (NN friction)))))) (, ,) (CC but) (S (NP (NP (DT the) (NN force)) (PP (IN of) (NP (NP (NN gravity)) (SBAR (WHNP (WDT that)) (S (VP (VBZ causes) (S (NP (PRP it)) (VP (TO to) (VP (VB move)))))))))) (VP (VBZ is) (ADJP (JJR stronger)))) (. .)))
A ball rolling down a slope is also affected by friction, but the force of gravity that causes it to move is stronger.
(ROOT (S (NP (NNP Gravity)) (VP (VBZ causes) (S (NP (NNS rocks)) (VP (TO to) (VP (VB roll) (ADVP (RB downhill)))))) (. .)))