Draw force diagrams

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.

Draw force diagrams

Choose one object

Find the interactions

Use the motion to compare forces

A useful distinction

Your turn.

A book rests on a horizontal table with no other pushes or pulls. Name the two forces on the book and their directions.

Should the downward force the book exerts on the table appear in the book's free-body diagram?

Now make it your course.