HigherEd 2.0

9 responses so far ↓

• 1 SMBguyAlmighty // Mar 6, 2007 at 11:08 am

  In relations to unsymmetrical bending, the problems have the axis out at an angle with a moment rotating about it…. Why does it not matter sign-wise if the moment about the axis is rotating one way or the other? Does the equation and book account for it somehow - maybe by having it twist in the same direction every problem?

• 2 Augustus Gloop // Mar 12, 2007 at 11:14 pm

  I think the convention is to use the right hand rule for the moments. That way the sign of the moment doesn’t matter. If the moment is the opposite direction then the angle would just be 180 degrees plus that angle. That, at least, seems to be the way the book is presenting the problems.

• 3 berger // Mar 15, 2007 at 8:48 am

  For these problems, our book uses a very specific sign convention, as we have discussed in class. That is, the orientation of the (y,z) axes coincides with the principal axes of inertia, and in fact the z-direction corresponds to the maximum moment of inertia and the y-direction is the minimum moment of inertia (see also the bottom of p. 315).

  In addition, we measure the orientation of the moment using the angle theta, which is measured from the positive z-axis toward the positive y-axis. This is again a specific convention that our book uses in deriving the relevant bending equations.

  So actually the sign of the moment does matter, but because the equations in our book have been derived based upon this specific sign convention, it sort of looks like the result is independent of the moment’s sign.

  So Augustus has it right…you can think of the sign of the moment as being contained inside the angle theta, which is the orientation of the moment vector.

  The description on p. 315 is helpful, although my presentation in class was even better .

• 4 SidewinderX // Mar 15, 2007 at 10:16 am

  Couple of questions… for 6-51, do you actually want up to sketch the results as a volume element, or are having the answers (and work of course) fine?

  My other question is on 6-94… The way my statics worked out, the support strut (BC) takes the full load of the distributed lift load, and that makes it tough for me to figure out the max moment… Did I do the statics wrong, or is there some trick that I’m not noticing to figure that out?

  Thanks!

• 5 berger // Mar 15, 2007 at 10:39 am

  For 6-51, all I am looking for is the little cubic element along the lines of Example Problem 6.15 on page 302. Nothing elaborate.

  For 6-94, yeah it sounds like you’ve got bad statics. Without seeing your work I can’t really guess what’s going on, but you might look at how you handled the linear load (i.e., how did you write the resultant, and where?), and also consult Table 6-1 (p. 275). There is definitely some bending going on in the wing!

• 6 jlb3mb // Mar 15, 2007 at 5:23 pm

  For #75 since the problem statement says A and B only support vertical loads do we assume they have no moment or is it statically indeterminant?

• 7 Whitney // Mar 15, 2007 at 6:18 pm

  jlb - I just assumed no moments.

  For #94, I am confused as to what the value for c should be…

• 8 taf2f // Mar 16, 2007 at 6:04 am

  For 6-94 I too got that By was equal to the distributed load and therefore Ay was 0. I cant seem the find the max moment for it and its driving me crazy.

• 9 berger // Mar 16, 2007 at 8:02 am

  For 6-75, you can model the bearings as a pin and a roller, since there are no forces along the long axis of the shaft.

  For 6-94, there is some confusion about the way the question is worded. What they want you to do is use the “depth = 3 in.” information and use c = 1.5 in. But I agree that it’s a bit unclear. But this is not the important part of the problem–the moment determination is actually the most important, so definitely focus on that.

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