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

The force of the table pushing it upward.

Normal force

The force that prevents the book from accelerating downward is the normal force.

To understand why the normal force is exerted, we need to first consider Newton's third law of motion, which states that for every action, there is an equal and opposite reaction. In this case, the book exerts a force on the table due to its weight, which is the force of gravity. As a result, the table exerts an equal and opposite force upward on the book, known as the normal force.

The normal force is perpendicular to the surface of the table and acts to balance the weight of the book, preventing it from accelerating downward.

To calculate the normal force, you need to understand that it is equal in magnitude and opposite in direction to the force of gravity acting on the book. So, if the weight of the book is given, you can determine the normal force by multiplying the weight by -1, as it acts in the opposite direction. If the force of gravity on the book is given, you can take the positive value of it to determine the magnitude of the normal force.