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I cannot find my copy. I guess I threw it away, but my recollection isOn one or the
early pages the statement is made that the second condition of
equilibrium means that an object may not be in equilibrium under the
action of three or more forces unless the forces are concurrent. On the
next page is a "see-saw" in equilibrim but the forces are surely not
concurrent unless you say that parallel forces are concurrent at infinity
or some such.
I understand the word "may" in the statement to imply that one
must impose more tests when the forces are not concurrent. It
does not say that if the forces are not concurrent the system
*will* not be in equilibrium. It is the case of three parallel
forces which first illustrates that non-concurrent forces hold
a body in equilibrium. Two (equal and opposite) forces cannot
do so. The author is saying that (as you apparently prefer)
parallel forces are *not* concurrent, but the body *may* still
be in equilibrium, e.g. the see-saw. In other words, it appears
that you agree with the author.
I think there is no error here. The statement is carefully
thought out. It is correct in the strictest sense, but it is
not terribly illuminating, as it evidently confused you, and
you already know the answer. It is clear that from a student's
point of view this statement would likely have negative value.
Leigh
(It has always been my interpretation that parallel line are
concurrent in infinity. Is there something terribly wrong with
that? Mathematicians have often objected to the way physicists
do mathematics (e.g. Dirac's delta function) but usually have
recanted later and "legitimized" established physical practices
as bona fide mathematics. I think having parallel lines meeting
at infinity is reasonable; methematicians may not. It doesn't
bother me a lot.)