How Do You Calculate the Effective Stiffness of an Iron Bar?

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What is the Spring Stiffness?

1. One mole of iron (6.02x10^23 atoms) has a mass of 56 grams, and its density is 7.87 grams per cubic centimeter. You have a long thin bar of iron, 2.4 m long, with a square cross section, 0.15 cm on a side.

You hang the rod vertically and attach a 234 kg mass to the bottom, and you observe that the bar becomes 1.22 cm longer. Calculate the effective stiffness of the interatomic bond, modeled as a "spring":

Next you remove the 234 kg mass, place the rod horizontally, and strike one end with a hammer. How much time t will elapse before a microphone at the other end of the bar will detect a disturbance?

Homework Equations


Youngs Modulus? or Hookes law?

The Attempt at a Solution


ks = F/x
and i tried using youngs modulus doing it the long way and i got .0046 still wrong
 
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You can solve it by ks=F/x, but what values are you using for F and x? And regarding part 2, at what speed does the vibrating wave travel?