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!

Homework Help: Young's Modulus of iron wire

  1. Feb 24, 2009 #1
    1. The problem statement, all variables and given/known data
    A hanging wire made of an alloy of iron with diameter 0.09 cm is initially 2.2 m long. When a 66 kg mass is hung from it, the wire stretches an amount 1.12 cm. A mole of iron has a mass of 56 grams, and its density is 7.87 g/cm3.

    Based on these experimental measurements, what is Young's modulus for this alloy of iron?

    2. Relevant equations
    Y = [tex]\frac{F/A}{dL/L}[/tex]

    3. The attempt at a solution
    What is F? Is it the mass of one mole times gravity?
    F = 0.056kg x 9.8 m/s2
    A = 2.54e-6
    dL = 0.0112
    L = 2.2
  2. jcsd
  3. Feb 24, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Don't worry about the mole...concern yourself with the weight of the hanging mass.
  4. Feb 25, 2009 #3
    Or alternatively, here's a sample of certain dimensions, here's an applied load, here's the change in length, the sky is blue and I had a sandwich for lunch. A popular technique to distract you and see if you understand what you're doing is to throw in extraneous information.
  5. Feb 25, 2009 #4
    isn't young's modulus not dependent of the total mass? isn't it just based on the type of material such as iron. so it doesn't matter if the total mass is 100 kg or 1 kg, they should have the same young's modulus.
  6. Feb 25, 2009 #5
    The mole is a useless bit of information. Youngs modulus is a material property so therefore not dependant directly on the mass or geometry for ideal cases.

    It is important to note that the mass does have an effect indirectly.

    In the original question: the mass of the bar is about 12g (worked this out in my head so check it). This compared to the 66Kg load is insignificant so the mass of the bar can be ignored.

    However if the bar were very large (100kg) then this is obviously more significant than the load mass (66kg) so then the mass of the bar becomes important.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook