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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: 10,136, Reputation: 5589
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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: 1034
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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: 10
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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: 1034
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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: 10
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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: 8,147, Reputation: 3745
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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: 1034
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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: 10,136, Reputation: 5589
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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: 8,147, Reputation: 3745
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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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