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. For example, Earth pulls the book and the book pulls Earth. The normal
force and the book’s weight act on the same object.