Identify the forces acting on a book resting on a horizontal table.
Choose one object
A free-body diagram represents one object and the external forces acting on it. Start by choosing the book. Draw it as a simple dot or box, then add arrows for forces on the book.
Find the interactions
Earth pulls the book downward through gravity. The table pushes the book upward through contact. We call that contact force the normal force because it acts perpendicular to the table.
↑ Normal force, N
■ Book
↓ Weight, mg
For a book at rest with only these two vertical forces, the arrow lengths are equal.
Do not draw a separate “force of rest.” Being still describes the book's motion; it is not an additional interaction.
Use the motion to compare forces
The book remains at rest, so its acceleration is zero. Newton's second law says that the net force is ma, which is also zero here.
Taking upward as positive gives N − mg = 0. Therefore N = mg. The forces balance; neither force disappears.
A useful distinction
The book's weight and the table's normal force act on the same object, so they are not a Newton's third-law pair. The partner to the table pushing the book upward is the book pushing the table downward. That partner acts on the table and does not belong in the book's diagram.
Your turn
Make the idea stick.
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2 practice questions01
A book rests on a horizontal table with no other pushes or pulls. Name the two forces on the book and their directions.
Try it in your own words
Compare with the explanation
Earth's gravitational force (weight) acts downward. The table's normal force acts upward. With no acceleration and no other vertical forces, their magnitudes are equal.
02
Should the downward force the book exerts on the table appear in the book's free-body diagram?
Try it in your own words
Compare with the explanation
No. That force acts on the table. The book's diagram includes only forces acting on the book, not forces the book exerts on another object.
Keep for later
Your reference sheet
The key ideas, ready when you need a refresher.
Open the reference sheet
Choose the object
Include only external forces acting on that one object.
Weight and normal force
Earth pulls the book down. The horizontal table pushes it up.
At rest in this example
Zero acceleration means zero net force. N − mg = 0, so N = mg.
Third-law pairs
The two partner forces act on different objects.
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