131
and some theorists even believe that they may be able to reduce it to two or
one. Although the unification of the forces of nature is one of the most
beautiful and important achievements of physics, it makes much more sense
to start this course with the more practical and easy system of classification.
The unified system of four forces will be one of the highlights of the end of
The practical classification scheme which concerns us now can be laid
out in the form of the tree shown below. The most specific types of forces
are shown at the tips of the branches, and it is these types of forces that are
referred to in the POFOSTITO mnemonic. For example, electrical and
magnetic forces belong to the same general group, but Newton’s third law
would never relate an electrical force to a magnetic force.
The broadest distinction is that between contact and noncontact forces,
which has been discussed in the previous chapter. Among the contact forces,
we distinguish between those that involve solids only and those that have to
do with fluids, a term used in physics to include both gases and liquids. The
terms “repulsive,” “attractive,” and “oblique” refer to the directions of the
forces.
Repulsive forces are those that tend to push the two participating
objects away from each other. More specifically, a repulsive contact
force acts perpendicular to the surfaces at which the two objects
touch, and a repulsive noncontact force acts along the line between
the two objects.
• Attractive forces pull the two objects toward one another, i.e. they act
along the same line as repulsive forces, but in the opposite direction.
• Oblique forces are those that act at some other angle.
normal
force
static
frictionkinetic
frictionfluid
frictiongravityelectrical
forces
magnetic
forces
contact
forcesnoncontact
forces
forces
between
solids
forces between
solids and
fluids or fluids
and fluids
repulsive
oblique
not
slipping
slipping
obliquealways
attractive,
depends on
the objects'
masses
depends on
the objects'
charge, an
electrical
property
Section 5.2Classification and Behavior of Forces
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