Chris,

Ned: I could post further responses to what you've said, but apart from
a few minor points I agree with you. Were we face-to-face we could
discuss the merits of different models and likely reach an agreement on
a good one fairly quickly, but it's rather hard via Usenet. Usenet does
tend to be a little combative.

Bill: I am none the wiser. If you believe my FBD to be incorrect, please
explain clearly where you believe I should add or remove forces and why.
Better still, draw your own FBD and post it. Then I'll be glad to
discuss it with you.

Please note that I did describe the problems in each case. However, I
will try this from another angle.

Going all the way back to Newton's Laws and how to apply them, a free
body diagram is a diagram of a free body. A free body is one that has
been isolated from its surroundings, with all interacting bodies
represented by the forces and moments they exert _on the free body_.
Some people treat force fields (magnetic, electric, gravitational)
separately, but it's the same deal: didn't draw the earth? Put in the
weight of the object facing toward the center of the earth (down for
this type of work), and add support reactions.

Never show internal forces on an FBD.

One of the most insightful things I have ever heard on this front came
the late John Wesley Hoover: "take it to pieces". Sadly you can't hear
him bellow that, but do your best to imagine it (add a slight southern
US accent for character). Pick any one or more bodies of a system, draw
them and represent each missing item by the forces/moments it exerts on
the system. Again, never show forces or moments internal to the system.
Feel free to cut any member; when you do, add the forces/moments the
part you removed exerts on the part you kept. I've repeated myself a
bit, but it deserves repeating.

You cannot correctly reason about mechanics w/o a full understanding of
this stuff. Sometimes you will get lucky and miss something that does
not completely change the behavior; there is also risk of missing
something that leads to dangerously incorrect results.

For drill, grab any statics book or relevant Schaum's outline and tackle
the example problems; follow along on one or two, then compare your
independent solution with theirs. In particular, look for problems
suitable to the method of sections; that tends to pound on the rules,
and is very useful for getting at specific information and/or reducing
the coupling of equations one would suffer with a naive decomposition
into the simplest possible FBDs.

From there, review technical beam theory. The FBD of a section of bent
beam and the cut used to get at shear stresses across a vertical section
through a beam will further reinforce how this works. Likewise,
computer shear and bending moment by cutting the beam is good
reinforcement. On the latter, pay particular attention to distributed
loads: they can be reduced to resultants, but only _after_ cutting the
beam. Doing otherwise leaves the support reactions unchanged but alters
the internal shear and bending moment.

Re the top/left diagram we discussed, the "tension member" must be
represented by its tension in the direction of the member. You can
treat that as horizontal and vertical compontents, or as magnitude and
direction. I prefer the latter because it helps in scanning for errors.

I suspect you drew an FBD of the pin supporting the load. Either way,
start there, summing forces to zero vertically and then horizontally.
Out of that you will get the tension in the "tension member" and the
horizontally directed compressive force in the horizontal member.

Then draw an FBD of the beam itself, representing the members above by
the forces they exert on the beam. It is the diagonal tension member
where you have problems. Remember, for a weightless member with smooth
pins, the tension/compression acts along the line between the pins. If
its weight is significant, then draw a free body, including its weight
and unknown horizontal and vertical components of reactions the pins
apply to the member. To start, ignore the weight of the tension member
and treat it as a two-force member.

Bill