A book on a table
Weight and the normal force.
Why the book does not sink
A book lies on a table. The Earth pulls it down, but the book does not fall through the table.
What do you think the table does to the book?
What this lesson covers
The idea
An object resting on a surface is acted on by its weight, downward, and by the normal force the surface exerts, upward and perpendicular to the surface; for an object on a horizontal floor these two forces balance.
Why the book does not sink
A book lies on a table. The Earth pulls it down, but the book does not fall through the table.
What do you think the table does to the book?
- Pushes it upward
- Nothing at all
- Pulls it down as well
Add books
Put books on the table and look at the two arrows.
Two forces that balance
The Earth pulls the book down with its weight. The table pushes the book up with the normal force, at right angles to its surface. The arrows have equal lengths, so on a horizontal surface the two forces balance.
An object resting on a surface is acted on by its weight, downward, and by the normal force the surface exerts, upward and perpendicular to the surface; for an object on a horizontal floor these two forces balance.
Notes
A resting object has its weight (down) and the normal force (up, perpendicular to the surface). On a horizontal floor they balance.
Check yourself
In which direction does the weight of an object act?
What is the normal force?
An object rests on a horizontal floor. How do its weight and the normal force compare?
Each book weighs 4 N (an example). A stack of 3 books rests on a table. What is the normal force the table exerts on the stack?
Answer: 12 N
The stack weighs 3 × 4 N = 12 N, so the normal force is 12 N upward.
- Downward — correct. Yes!
- Upward. The upward force on an object resting on a surface is the normal force.
- Sideways, along the surface. Weight is the gravitational force. It acts downward.
- The downward pull of the Earth. That is the weight.
- The upward force the surface exerts, perpendicular to the surface — correct. Yes!
- The force that acts opposite to the applied force. That is the force of friction.
- The weight is larger, so the object sinks. The two forces balance, so there is no net force from them and the object stays on the floor.
- The normal force is larger, so the object rises. The two forces balance, so the object does not rise.
- They balance each other — correct. Yes!