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It should not be necessary to memorize this diagram by rote. It is better
to reinforce your memory of this system by calling to mind your
commonsense knowledge of certain ordinary phenomena. For instance, we
know that the gravitational attraction between us and the planet earth will
act even if our feet momentarily leave the ground, and that although
magnets have mass and are affected by gravity, most objects that have mass
are nonmagnetic.
This diagram is meant to be as simple as possible while including most
of the forces we deal with in everyday life. If you were an insect, you would
be much more interested in the force of surface tension, which allowed you
to walk on water. I have not included the nuclear forces, which are respon-
sible for holding the nuclei of atoms, because they are not evident in
everyday life.
You should not be afraid to invent your own names for types of forces
that do not fit into the diagram. For instance, the force that holds a piece of
tape to the wall has been left off of the tree, and if you were analyzing a
situation involving scotch tape, you would be absolutely right to refer to it
by some commonsense name such as “sticky force.”
On the other hand, if you are having trouble classifying a certain force,
you should also consider whether it is a force at all. For instance, if someone
asks you to classify the force that the earth has because of its rotation, you
would have great difficulty creating a place for it on the diagram. That’s
because it’s a type of motion, not a type of force!
Normal forces
A normal force, F
N
, is a force that keeps one solid object from passing
through another. “Normal” is simply a fancy word for “perpendicular,”
meaning that the force is perpendicular to the surface of contact. Intuitively,
it seems the normal force magically adjusts itself to provide whatever force is
needed to keep the objects from occupying the same space. If your muscles
press your hands together gently, there is a gentle normal force. Press harder,
and the normal force gets stronger. How does the normal force know how
strong to be. The answer is that the harder you jam your hands together,
the more compressed your flesh becomes. Your flesh is acting like a spring:
more force is required to compress it more. The same is true when you push
on a wall. The wall flexes imperceptibly in proportion to your force on it. If
you exerted enough force, would it be possible for two objects to pass
through each other. No, typically the result is simply to strain the objects so
much that one of them breaks.
Gravitational forces
As we’ll discuss in more detail later in the course, a gravitational force
exists between any two things that have mass. In everyday life, the gravita-
tional force between two cars or two people is negligible, so the only
noticeable gravitational forces are the ones between the earth and various
human-scale objects. We refer to these planet-earth-induced gravitational
forces as weight forces, and as we have already seen, their magnitude is
given by |F
W
|=mg.
Chapter 5Analysis of Forces