third law of motion

NOUN
19 LETTERS · STARTS WITH T

third law of motion is a valid 19-letter word worth 28 points in Scrabble and 30 points in Words with Friends.

SCRABBLE WORDS WITH FRIENDS
T1H4I1R1D2L1A1W4O1F4M3O1T1I1O1N1
1 + 4 + 1 + 1 + 2 + 0 + 1 + 1 + 4 + 0 + 1 + 4 + 0 + 3 + 1 + 1 + 1 + 1 + 1 =28 PTS
DOUBLE WORD → 56TRIPLE WORD → 84
MEANING

What does third law of motion mean?