Abook weighing 5 n rests on top of a table. a downward force of magnitude 5 n is exerted on the book by the force of do the downward force in part a and the upward force in part b constitute a 3rd law pair? the reaction to the force in part a is a force of magnitude exerted on the by the its direction is .

A book weighing 5 N rests on top of a table. A downward force of magnitude 5 N is exerted on the book by the force of gravity.

Do the downward force in Part A and the upward force in Part B constitute a 3rd law pair? Yes

The reaction to the force in Part A is a force of magnitude 5 N, exerted on the book by the table. Its direction is upward .

Explanation:

According to Newton's third law of motion, when two bodies interact between them, appear equal forces and opposite senses in each of them.  

To understand it better:  

Each time a body or object exerts a force on a second body or object, it (the second body) will exert a force of equal magnitude and direction but in the opposite direction on the first.

In the case of the book in the figure, the 5 N force exerted downwards due gravity is equal to the 5 N force exerted upwards by the table (Normal force).