Answer:
Net force = 0
Explanation:
Short answer: if the middle book is not acceleration in any direction, the net force is zero.
Long answer: refer to the attached free-body diagram (FBD) to see why the net force is zero. Skills to draw FBD are essential in solving problems in statics.
Top book exerts -mg (downwards) on the middle book.
Middle book exerts -mg (downwards) on the bottom book.
Total downward force is -mg-mg = -2mg
By Newton's third law, when an object is not in motion, reaction equals action of -2mg in the opposite direction, therefore the reaction (upwards) is +2mg.
This makes the net force of -2mg (downwards) + 2mg (upwards) =0
