1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Largest mass for Ultimate Tensile Strength

  1. Jun 1, 2013 #1
    Vibrations and Waves, A.P. French, questions 3-9
    Please /do not/ provide full solutions. Just suggestions.

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

    A solid steel ball is hung at the bottom of a steel wire of length 2m and radius 1mm, the ultimate strength of steel is 1.1E9 N/m2. What are the radius and mass of the biggest ball the wire can bear?

    2. Relevant equations

    [itex]Y = \frac{\sigma}{\epsilon}[/itex] (line)

    [itex]U = \frac{\sigma_u}{\epsilon_u}[/itex] (point below line)

    [itex]m = \frac{4}{3}\pi*r^3 \rho[/itex]

    [itex]\rho \approx 7850 Kg/m^3[/itex] (Internet)

    3. The attempt at a solution

    For the sake of sanity, I arrange eq.2 by knowns and unknowns.

    [itex]\frac{U*A}{g*l_0} = \frac{m}{\Delta l} [/itex]

    I must find Δ l

    I consider two points on the Y line:

    [itex]Y = \frac{\sigma_u}{\epsilon_1}[/itex]

    [itex]Y = \frac{\sigma_2}{\epsilon_u}[/itex]

    both can be written as

    [itex]Y = C\frac{\sigma_u}{\epsilon_u} = C U[/itex]

    [itex]C = Y/U[/itex]

    Then I stop and think about how this will lead me in circles.


    I have a suspicion that the solution to my problem requires differential equations. Anyone second that notion?
    Last edited: Jun 1, 2013
  2. jcsd
  3. Jun 1, 2013 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    DEs not required. Do you know what Y represents?
  4. Jun 1, 2013 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    You know the ultimate stress the wire is capable of sustaining. You also know the size of the wire. Can you use this information to work back and figure out the amount of mass required to produce this stress? Draw a free body diagram if it helps you to visualize the situation.
  5. Jun 1, 2013 #4
    Y is available in the text.

    [strike]The ultimate stress σu. Do I really know this without knowing the ultimate strain εu?[/strike]

    Have I been wrong this entire time? Is ultimate strength just the stress associated with failure?
    Suggesting the following is incorrect:

    [itex]U = \frac{\sigma_u}{\epsilon_u}[/itex]
    Last edited: Jun 1, 2013
  6. Jun 1, 2013 #5
    Holy hell.

    Yes. I was incorrect about ultimate strength!
    Break through in understanding here folks.

    Ultimate strength is the stress of failure.

    Question solved correctly.

    Our discussion in the other forum makes more sense now SteamKing - cheers :).
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted

Similar Discussions: Largest mass for Ultimate Tensile Strength
  1. Ultimate strength (Replies: 1)