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Homework Help: A multiple concept problem thermal expansion/stress

  1. Mar 12, 2012 #1
    Hi guys!

    I think I need to explain myself before I ask for the advice :) I'm 27 (so been in grade 12 ages ago) - I didn't have science, I wrote it years later. Now I'm enrolled for a Bsc degree in the hope of getting into medical school (very tough in South Africa). I'm putting a lot of time into studying and started physics now - but I'm clueless - so have to start from the bottom and hit a bit of a roadblock!

    I got a multiple concept problem - I managed to follow along in the book to where they reached the section just before the answer... but how they get from the final part of the equation to the answer is a mystery - and I see many more like these in the book but no explanation so I'm guessing this part was sort of "assumed knowledge from school".

    1. The problem statement, all variables and given/known data

    The problem involves steam beams with concrete on the sides. So with the increase in temp the beams have no space to expand but the concrete is exerting pressure to avoid expansion of the beams. They want to know what pressure the concrete is exerting.

    2. Relevant equations

    3. The attempt at a solution

    So in the book they mention and explain Youngs Modulus

    After some equations have been substituted into others (this i understand which is why I haven't included this) you end up with

    Stress = Y (alpha) (T-T0)

    and we also know that
    y=2.0 x 10^11 N/m^2 and (alpha) = 12 x 10^-6 (Celsius degrees)^-1

    Now if you put that into the above stress equation you get :

    (2.0 x 10^11 N/m^2) {12 x 10^-6(Celsuis degrees)^-1}(42-23)

    And now I have no clue further....

    the books answer is 4.6 x 10^7 N/m^2

    I have no idea how they even get close to this answer. If anyone can help me please explain as if I am a little kiddie :) Doing this after SO many years is really difficult but with many hours of studying I'm really motivated to learn!

    Thank you!
    Last edited: Mar 12, 2012
  2. jcsd
  3. Mar 12, 2012 #2


    User Avatar

    Staff: Mentor

    Hi Janinever! http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    The books answer differs from your expression by a factor x19.2

    Could this hint at somethinghttps://www.physicsforums.com/images/icons/icon5.gif [Broken]
    Last edited by a moderator: May 5, 2017
  4. Mar 12, 2012 #3
    Not sure about the 19.2?

    I find the answers using the calculator - but how to write that answer in the terms of for example (4.6 x 10^7) is confusing...

    I sort of get how it could get to there but further on in the book there are answers such as 5.6 10^-2 (for these I also get a normal decimal on the calculator but have no clue how they convert it to those answers).

    Sorry for sounding so confusing!
  5. Mar 12, 2012 #4


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    Staff: Mentor

    Now that you've gone back and fixed it, I think you agree with the book. Yes? :smile:
    You need some work on converting from 0.0002 to 2.0x10-4 ? Maybe someone can point to a good site. That's converting from decimal notation to scientific notation. Maybe try a google search for a tutorial on that topic.
  6. Mar 12, 2012 #5
    Wonderful, now I know what it's called :) (have a lot ahead of me!!) - I'm doing a maths module as well, I should probably get a bit further with that before tackling the physics - scientific notation seems to be one of the next sections. Thanks so much! :)
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