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Moments and free body diagrams help

  1. Apr 18, 2006 #1
    Hello all,

    Since I have run into the problem of regularly leaving moments out of my free body diagrams (and vice versa), I thought I'd pose a question on the topic. In trying to wrap my head around the concept, I've gone back to the beginning of the semester's notes and that didn't really help, nor did the tutor I asked. I guess I never fully grasped moments must have managed to coast by because of my superficial understanding :redface:. But now that we've started covering internal forces, I see that I need to understand it better.

    So my question is, how do you look at something at tell whether or not there is a moment acting on it? Are there any specific cues? I don't know, I guess I need a checklist or something. I've found something like that to identify zero-force members, and even something similar to help with analyzing friction problems. But it would appear that moments are assumed to be... self explanitory or something.

    In order to have something more specific to wok with, I've drawn up a one of the things I've struggled with recently - that's the file attached.

    So, say you have a frame like that, with pin connections at the base (By the way, for anyone with a Hibbeler 10th edition on hand, it's the diagram for Prob. 7-34). If the member AC were removed (making CB a cantilever), there wouldn't be a moment about that point. A tutor told me that but didn't do a good job of explaining why. I thought a downward force applied at the end of a cantilevered beam would always produce a moment, because it's only supported at one end. Or is it to do with the type of connection. Or maybe because I'd disassembled the frame and that changed something about the interactions... I'm confused. :uhh: I'd appreciate any input at all. Thanks.


    Attached Files:

  2. jcsd
  3. Apr 18, 2006 #2


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    Your attachment has not been approved yet...

    I'm not sure what to tell you other than, if you are looking at the reactions at a point, if there are forces involved that do not go through that point, you will have a moment most likely. Granted, some types of joints do not support moments. You simply have to learn them and commit them to memory. There's no real magic to it. Like you already are used to looking for the sum of the forces, you have to get used to also looking for the sum of the moments.
  4. Apr 18, 2006 #3
    Thanks for replying Fred,

    I drew out the same diagram using forward and backslashes but it came out all condensed (i.e. - spaces were negated) in the post preview, so that's why I posted a word file instead. It has since been approved.

    But on to what you said. I see what you are saying - there are things that you should just make a habit of when sitting down to solve a problem. First step, figure out what they're asking, second, draw a free body diagram, then so on.

    I'm going to go off on a tangent for a second here. In threads that answer curious high school students' question "Should I become a physicist or an engineer?" what is often time pointed out is that a physicist uses foundational physics knowledge to try and understand the how and why of unknown physics topics, whereas an engineer would use it as a tool in solving a problem. Naturally, in seeing that, you can more easily make a decision about the two.

    I think that is a good point being made. But, I feel that here is a case where it is probably useful to aim to understand the why, actually. From what I hear about strength of material, moments will come up again for me. Also, with my concentration area being aeronautical engineering (which I won't really dive into until my junior/senior years), it would be handy to have an innate ability to deconstruct something in my mind and understand the reactions and results that come into play. After all, isn't an airplane wing just cantilevered beam?

    Statics textbooks give a list of diagrams that break down reactions for supports in 2-D (i.e. – pin connections, rockers, etc.) and 3-D (i.e. - ball and socket, hinge, bearing, etc.), but my point is that I wouldn't naturally look at those and see the same things the diagrams says are going on and that should be doable. And while it is possible to get through by just memorizing “this or that connection reacts this way,” it makes me uncomfortable to have to rely on that (as I have been doing - clearly not always so successfully).

    (‘Nother tangent coming.. watch out :blushing:) That approach is like how you handle language courses. In German, I was told to just memorize nouns along with their "genders". A noun, say, “the tree”, can be masculine, feminine, or Elton John. Just kidding, neuter is the third. Since, unlike with “the boy” or “the girl”, a noun's gender isn't always logical, you simply have to remember what it is in order to properly formulate sentences. But unlike a language, science has laws, things that can be deduced if you build up the ability... to deduce. That's all I'm trying to do.
  5. Apr 18, 2006 #4


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    There are many things that I think you will find out, make more sense or become common sensical when you have a bit more experience under your belt or have done things a bit longer. There are a lot of things in engineering that do not follow common sense, especially if one hasn't been exposed to things before. The fact that you are thinking the way you do is a very good sign. Don't worry about it too much if something doesn't seem natural to you. It will come with time and work.
  6. Apr 18, 2006 #5
    Thank you Fred,

    You're very reassuring. I guess you have a point. As with all things, I'm sure true comfortably comes with time. But, I would have thought that after a semester of regularly working with moments, things would be a bit more obvious to me than they are right now. I will just be a bit more patient with myself :rolleyes: - and continue to seek help along the way of course.

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