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11) A metal wire of diameter 0.68 mm and natural length 1.5 m is fixed firmly to the ceiling at one end. When a 6.8 kg mass is hung from the free end, the wire extends by 2.8 mm.


a) the stress in the wire

B) the Young modulus of the material of the wire.

 Dec 2, 2017

FIRST find the VOLUME of the wire:

pi x r^2 x l = 5.45 x10^-7 m^3


THEN find its diameter given the NEW length of 1.5028 M


pi x r^2 x l = 5.45 x 10^-7    results in r =3.3968 x10^-4 m

then, as before   Stress = F/A      F= ma  = 6.8(9.8) = 66.64 newtons

A= area = pi x r^2    where r is the 'new ' stretched r   

     Let me know what you come up with for an answer and we will continue.....   ~EP

 Dec 2, 2017

If you have the correct answer for stress (I got 1.83838 x 10^8 P )


 then, (I had to look this one up !)

Young's Modulus =  Stress / (\( \Delta \)L/ Lo)         \(\Delta \)L = change in length = .0028 m     Lo=1.5 m   (given)

 Dec 2, 2017

Hi ElectricPavlov,

It is great to see you on the forum again  laugh

 Dec 3, 2017

Thanx, Melody !    

ElectricPavlov  Dec 3, 2017

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