1. Dec 31, 2009

### Dell

in a question i am solving, i am asked to plot the stress in a beam as a function of the moment,
but as far as i know, or at least in all the other questions i have solved, i have been asked to plot the strains/stress as a function of my placement (y). could this be a mistake or is this a legitimate question to ask,
when plotting sigma(m) do i take y as a constant, then i will have a linear graph and my slant will be -Y/(I) ??

2. Dec 31, 2009

### PhanthomJay

That question is a bit vague, I'm not sure if they are asking about how the maximum stresses change in a beam as the moment changes along its length; or whether they are asking how the stress varies at a given cross section of the beam where the Moment is constant and the stress varies as function of the y distance from the fibers to the neutral axis (stress = +/- My/I.). Please clarify.

3. Dec 31, 2009

### Dell

stress at a given cross sectioמ as a function of M

4. Dec 31, 2009

### PhanthomJay

At a given cross section, the moment is constant for a given loading condition, and the stress varies as function of y, so I guess they are asking that if M were to change at a given cross section due to a different loading condition, how does the stress change at that cross section; in which case, you are correct that you get a straight line linear relationship between the moment at that cross section and the stress at a certain point in that cross section, where the slope of the line, passing through (0,0), is y/I , where y is the distance from the neutal axis to the point on the cross section in question. Whether the slope is + y/I or - y/I, is a matter of convention (stress generally considered positive in tension, negative in compression). Double the moment, you double the stress, etc.

Last edited: Dec 31, 2009
5. Dec 31, 2009

### Dell

thanks, seemed a bit trivial to me, thought there must be something more to it, but you say not?

6. Dec 31, 2009

### PhanthomJay

I never liked the question in the first place, so I'm just guessing at what it's looking for.