Is strong a word in the Scrabble Dictionary

Yes, strong is playable

...and is worth 9 points.

Found in the following dictionaries

Enable (Words with friends)
SOWPODS (Collins Scrabble Words)
OTCWL2014 (NASPA Word List 2014)
TWL06 (NASPA List 2006)

"Strong" Definition

adjective

Capable of the effective exercise of authority: a strong leader.
Capable of withstanding force or wear; solid, tough, or firm: a strong building; a strong fabric.
Characterized by a high degree of saturation.
Clear and loud: a strong voice.
Containing a considerable percentage of alcohol: strong punch.

adverb

In a strong, powerful, or vigorous manner; forcefully: a salesperson who comes on too strong.