A book at rest
Nothing is moving. That doesn’t mean nothing is acting.
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Add to my journalThe question
A book rests on a horizontal table. No one is touching it. Take .
- Draw the forces acting on the book and name the source of each.
- Find the magnitude of the normal force.
- Explain why the normal force and weight are not a Newton’s-third-law pair.
Try all three parts before revealing the solution.
A hint to get started
Choose only the book as your system. Which other objects interact with it? Then use the fact that its vertical acceleration is zero.
Show the worked solution
The Earth exerts weight downward. The table exerts a normal force upward. There is no need to add a horizontal force in this situation.
With upward positive:
Therefore
Both arrows act on the book. A third-law pair acts on different objects: the table’s upward force on the book pairs with the book’s downward contact force on the table.
Check your understanding
If the book were moving horizontally at constant velocity on a frictionless table, would the vertical equation change?
Check the extension
No. Its vertical acceleration would still be zero, so the normal force would still balance the weight. Constant horizontal velocity does not itself require a horizontal net force.