What is Young Modulus and Why Does it Matter?

[XuFyaN]

in young modulus of a metal bar , we have a horizontal bar on one side we attach some weight so there is some change in angle but no change in length then why it is called young modulus?

 

[Stonebridge]

If you mean that the bar is bent, then this involves the upper surface stretching and the lower surface compressing. If so, then Young's modulus is involved in the mathematics, as it deals with compression and stretching of materials.

 

[XuFyaN]

no..there is no bent ..just a change in angle

 

[Stonebridge]

What angle is changing?

 

[XuFyaN]

i mean if there is a horizontal bar and we attach some weight on one of its end, then the bar will moves down from that end, this is called shear modulus because the angle is changed, but in certain conditions it is called young modulus, i wanted to know in which conditions we call it young modulus instead of shear modulus??

(actually in our young modulus practical we've used steel rod , hanged it horizontally & fixed it from the middle then we attached the weight to its one end so the rod move down for this end but there is no change in its length (only change in angle becoz rod moves down), then why we call it young modulus since there is no change in length ? )

 

[Stonebridge]

I have already answered the question.
If you hang a weight from the end of a metal bar and that end "moves down", then the bar has bent. You cannot bend a bar without causing some part of it to expand and/or another part to compress.
There is no change in the length of the whole bar, but there is stretching and compression of the material.
Shear strain involves the two surfaces moving as a result of force parallel to the surface.

With shear, surfaces AB and CD move as shown. θ is the shear strain angle.
With bending, AB expands and CD is compressed.
If you hang a weight on the end of a rod that is held at the other end, the result is bending. If so, Young's modulus is involved to describe the resultant strain.
If the strain is like the top diagram, shear strain, then the shear modulus is involved.
In most cases, you don't get pure shear, so the analysis is more complex.

 

[XuFyaN]

hey thanks a lot , I've tried to understand and understand some of it part...
will you please check this diagram and tell me that the condition which you've specified will be applicable for this diagram or not ?

 

[Stonebridge]

Is there anything holding the left end of the rod so that it is fixed?
Or is the only support at the centre in the form of a pivot?
It's impossible to say what is happening because the diagram is incomplete.
You need to either:
a) show another weight on the left side that balances the system
or
b) show what is happening at the centre pivot to balance the system
or
c) explain how the mass is supported

If the rod is straight there is no bending and no shear. It is just a question of balance and "moments".
Can you say exactly what you are doing in this experiment?

 

[XuFyaN]

hey..i was wr0ng & you were right i had some misconception that was cleared by your explanation..thanks a lot