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mbigras
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Homework Statement
An object of mass 0.5 kg is hung from the end of a steel wire of length 2 m and of diameter 0.5 mm. (Young's modulus = 2 x 1011 N/m2). The object is lifted through a distance h (thus allowing the wire to become slack) and is then dropped so that the wire receives a sudden jerk. The ultimate strength of steel is 1.1 x 109 N/m2. What is the largest possible value of h if the wire is not to break?
Homework Equations
[tex]stress = \delta = F/A[/tex]
[tex]strain = \epsilon = \Delta l/l_{0}[/tex]
[tex]\delta / \epsilon = Y[/tex]
Average impact force * distance traveled = [tex]\Delta KE[/tex]
ultimate strength = [tex]\delta_{UTS}[/tex]
The Attempt at a Solution
Initially I thought that I couldn't relate ultimate strength to distance traveled once the mass starts pulling on the wire using Young's modulus because the relation between stress and strain at that point is no longer linear. Then I saw a rephrasing of this exact question that included the statement "Assume stress remains proportional to strain throughout the motion". However, in my textbook this statement isn't mentioned, which could be why our answers aren't matching.
But assuming that stress does remain proportional to strain, I calculate the distance the mass traveled using Young's modulus. Using the ultimate strength I found the max force. Relating the two with the work-energy principle I was able to find the change in kinetic energy which is the same as the change in the original gravitational potential energy. Knowing how much poential energy it needs tells me h.
[tex]\delta_{UTS} = F/A = 1.1*10^{9} N/m^{2}[/tex]
[tex]\Delta l = \delta_{UTS}*l_{0} / Y = .011 m[/tex]
[tex]F = \delta_{UTS}*A = 215.985 N[/tex]
[tex]F*\Delta l = mgh[/tex]
[tex]h = .48 m[/tex]
This is not the answer the book has which is h = .23 m. Using the work-energy princple seems to me where I might be making some assumptions that aren't realistic. Also the wording "sudden jerk" is throwing me for a loop, maybe assuming the wire stretches all the way isn't realistic. It feels like I'm missing something here.
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