SAT Physics Subject Test

Chapter 3 Newton’s Laws

THE NORMAL FORCE

When an object is in contact with a surface, the surface exerts a contact force on the object. The component of the contact force that’s perpendicular to the surface is called the normal force on the object. (In physics, the word normal means perpendicular.) The normal force is what prevents objects from falling through tabletops or you from falling through the floor.

Normal Force Notation

The normal force is
denoted by FN, or simply
by N. (If you use the latter
notation,
be careful not
to confuse it with N,
the abbreviation for
the newton.)

14. A book whose mass is 2 kg rests on a table. Find the magnitude of the normal force exerted by the table on the book.

Normal vs. Weight

The normal is not always
equal to mg. It is whatever
needs to be in a given
problem to make sure
the object does not break
through the surface. Use
Fnet = ma to calculate it.

Here’s How to Crack It

The book experiences two forces: The downward pull of Earth’s gravity and the upward, supporting force exerted by the table. Since the book is at rest on the table, its acceleration is zero, so the net force on the book must equal zero. Therefore, the magnitude of the support force must equal the magnitude of the book’s weight, which is Fw = mg = (2)(10) = 20 N. This means the normal force must be 20 N as well: FN = 20 N. (Notice that this is a repeat of sample question 6, except that now we have a name for the “upward, supporting force exerted by the table.” It’s called the normal force.)

Also note that weight and the normal force are not an action-reaction pair, even though they are equal. The forces in an action-reaction pair work on different objects (e.g. the Earth pulls the book and the book pulls the Earth). The normal force and the book’s weight act on the same object.