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Deformation of solids

  1. Feb 12, 2014 #1
    1. The problem statement, all variables and given/known data

    Hey, I have problems solving this question. May I know how to solve this question. Somehow, the answer I got is to the power of negative. can someone show me the step by step solution to this question? (Question 18 from this paper) the link is below.
    http://maxpapers.com/wp-content/uploads/2012/11/9702_s13_qp_13.pdf


    2. Relevant equations
    E = Fl / Ae


    3. The attempt at a solution
     
  2. jcsd
  3. Feb 12, 2014 #2

    BvU

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    Hello Yan, and welcome to PF. There's a few rules here that are universally accepted -- and severely imposed.
    One is to use the template. You did that in a smart way. Laziness can be a good quality in physics.
    But you missed number 1 and number 3. The first error could have been avoided by rendering the problem instead of dumping the whole exam. The other is unforgivable, since it prevents helpers from helping you properly.

    At first I thought I might help you a little by alleviating the stress that can accompany having to struggle at an exam. but the date tells me this is practicing. So we have the time, right? Let's use it effectively, both yours and that of potential helpers.

    This is what I found at number 18 on a first attempt:

    18 The formula for hydrostatic pressure is p = ρ gh.
    Which equation, or principle of physics, is used in the derivation of this formula?
    A density =mass/volume
    B potential energy = mgh
    C atmospheric pressure decreases with height
    D density increases with depth
     
  4. Feb 12, 2014 #3
    I think you are thinking of question 19, correct?

    How do the weights of the masses compare to their full size counterparts?
    How does the cross sectional area of the cable compare to its full size counterpart?
    How do the lengths of the cables compare to their full size counterparts?
     
  5. Feb 13, 2014 #4
    Oops my mistake, it's question 19 actually.

    It is stated that linear dimension of the model is 1/10 of the full size

    So e is directly proportional to Fl/A since E is the same for both
    Hence e model = (1/10)F x (1/ 10) l / (1/10) A
    e full size = F l /A
    e model : e full = 10^-1
    However, the right answer is 10^2
    May I know what is the proper approach to this? Thx
     
  6. Feb 13, 2014 #5
    The force is equal to the weight of the mass. The weight of the mass is proportional to its volume. Volume is proportional to its linear dimension cubed cubed. The area of the cable is proportional to its linear dimension squared. The length of the cable is proportional to its linear dimension to the first power.
     
  7. Feb 13, 2014 #6
    ImageUploadedByPhysics Forums1392296383.288636.jpg

    Okay, now that you have explained this to me , I kinda have a rough idea about the question. So here's my working:) is it correct?
    However, I can't really relate the last proportionality you explained just now to me . In my working, I just times 1/10 to the force (load) , length and also volume. Is it the right way?
     
  8. Feb 13, 2014 #7
    No. 1/1000 the force, 1/100 the area, and 1/10 the length.
     
  9. Feb 13, 2014 #8
    Okay. Here's my new working. Is it correct ? ImageUploadedByPhysics Forums1392300235.893834.jpg
     
  10. Feb 13, 2014 #9
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