Muddy Card Responses Lecture M7
Can a section line be curved, or must it be straight.
The shape does not matter. You just
need to ensure that all forces (cut bars, and applied loads) that cross the section boundary are
included. This is just the same as the concept of a control volume in thermodynamics or
fluids.
Why is the moment sum always the perpendicular distance from the point about which
the moment was taken. Why is it not just the distance?
This goes back to the definition of a
moment as
r x F
. The crossproduct operation gives us the perpendicular component of
r
with respect to the line of action of
F
.
Another way to look at this is that
F
can be resolved
into a component parallel to
r
and a component perpendicular to
r
. The component parallel
to r passes through the origin, the point we are taking moments about, and therefore does not
have a moment arm about it.
If the bar looks like this:
where does the direction change?
It doesn’t.
You can subdivide the bar into any section
you want by “cutting” it perpendicular to the axial direction. Each such section will have
forces of equal magnitude and opposite direction pulling out from each end. This is
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 Winter '12
 CharlesColeman
 Force, Aeronautics, Astronautics, two bars

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