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    Hard physics question

    Asked Jun 16, 2011, 08:19 AM — 31 Answers
    Could you answer this question for me please? It is the hardest question one may get about young modulus, elasticity ecc.

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    ebaines's Avatar
    ebaines Posts: 10,925, Reputation: 1182
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    #2

    Jun 16, 2011, 08:38 AM

    Roddilla - we are not going to take your exam for you! I suggest you work through each step of the problem, and if you get stuck along the way please show us what you tried and how you got stuck - then we can help point the way.
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    jcaron2's Avatar
    jcaron2 Posts: 983, Reputation: 203
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    #3

    Jun 16, 2011, 09:42 AM
    We can help, but have you worked out ANY of the answers yourself? Parts a and b, for example, are simple trigonometry.
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    Roddilla's Avatar
    Roddilla Posts: 145, Reputation: 3
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    #4

    Jun 16, 2011, 10:11 AM
    Comment on jcaron2's post
    Quote Originally Posted by jcaron2 View Post
    We can help, but have you worked out ANY of the answers yourself? Parts a and b, for example, are simple trigonometry.
    Yes I worked it all out and in fact got a value of 2.36 x 10^-11 for Young's Modulus but I don't know if I worked it out correctly
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    jcaron2's Avatar
    jcaron2 Posts: 983, Reputation: 203
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    #5

    Jun 16, 2011, 10:53 AM
    Your value for Young's Modulus should not have a negative sign in the exponent. It should be on the order of the reciprocal of what you got. What did you calculate for the stress and elongation?
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    Roddilla's Avatar
    Roddilla Posts: 145, Reputation: 3
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    #6

    Jun 16, 2011, 10:57 AM
    Comment on jcaron2's post
    Quote Originally Posted by jcaron2 View Post
    Your value for Young's Modulus should not have a negative sign in the exponent. It should be on the order of the reciprocal of what you got. What did you calculate for the stress and elongation?
    my mistake 2.36 x 10^11 not -11

    could you check if it is good
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    Unknown008's Avatar
    Unknown008 Posts: 8,076, Reputation: 723
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    #7

    Jun 16, 2011, 11:03 AM

    Hm... may I ask you to post the stress and extension you got?

    I'm not getting what you got.
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    jcaron2's Avatar
    jcaron2 Posts: 983, Reputation: 203
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    #8

    Jun 16, 2011, 11:15 AM
    That seems just about right. I get the same answer if I round the extension up to 0.08, but I would suggest you keep at least one more significant digit.

    Jerry, how different was your answer? I think I'm right, but I must admit I'm not entirely sure if I'm off by a factor of two. You've probably done this sort of problem much more recently than I have!
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    ebaines's Avatar
    ebaines Posts: 10,925, Reputation: 1182
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    #9

    Jun 16, 2011, 11:43 AM

    JC: I Think you're correct. I'm getting E=2.45 x 10^11 Pa, or 245 GPa. By the way, Young's Modulus for steel is on the order of 200 GPa (depending on the particular alloy), so this seems reasonable.

    I calculate a stress of 18.8 GPa and strain of 7.7%.
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    Unknown008's Avatar
    Unknown008 Posts: 8,076, Reputation: 723
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    #10

    Jun 16, 2011, 11:49 AM

    Oh, you had more posts going in the meantime

    Yes, the last post I saw was the negative power

    Yes, now it's good
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