Aforce of gravity pulls downward on a book on a table. what force prevents the book from accelerating downward?

The Normal Force

Explanation:

According to Third Newton's Law,  also konwn as The principle of action and reaction :

"If a body A exerts an action on another body B, it performs on A another action that is the same and in the opposite direction."

In this case this is what happens with the book on a table,  gravity force pulls it downward, but the Normal Force (perpendicular to the surface of the table) acts in the opposite direction with the same magnitude, preventing the book from accelerating downward.

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double the distance = divide strength by 4