Determining yield stress via a bar subjected to tensile force

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Al_Pa_Cone
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Homework Statement



upload_2017-1-6_13-50-58.png

Homework Equations


All Equations Below

The Attempt at a Solution


upload_2017-1-6_13-52-20.png
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Do I use the area of a circle as the force is acting on the circular end? pi r^2
 
Al_Pa_Cone said:
Do I use the area of a circle as the force is acting on the circular end? pi r^2
Of course. The force is equal to the stress times the cross sectional area.
 
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Once again, 1 m2 is NOT 1000 mm2! And GN m is not a unit of force.
 
I should have noticed that given the fact I only did it last week! As for the units, I think GPa is the correct unit to use in my answer? I am getting mixed up by force measured in Newtons and Newton meters as a measurment. I have reworked my answer again?
upload_2017-1-9_12-55-36.png
 
Al_Pa_Cone said:
I should have noticed that given the fact I only did it last week! As for the units, I think GPa is the correct unit to use in my answer? I am getting mixed up by force measured in Newtons and Newton meters as a measurment. I have reworked my answer again?
View attachment 111305
Looks OK, except for the units. GPa is a unit of stress, not force. It would be better to just give the answer in N.
 
Last edited:
No it wouldn't. You have a stress in GPa and you multiply it by an area in m2. What are the units of the answer?
 
Sorry I did not read the last comment properly,
Chestermiller said:
Looks OK, except for the units. GPa is a unit of stress, not force. It would be better to just give the answer in N.
I only caught half of it in my emails (Looks OK, except for the units. GPa) and assumed it was ok.

As my undertanding goes with these units:

A Pascal is a unit which can be used for internal pressure acting in all directions equally within a material
A Newton is a unit which can represent force which acts upon a structure or an object
meters and millimeters is obviously a measurement of distance
meters^2 is a measurement of 2 dimensional area
meters^3 is a measurement of 3 dimensional area

The part where I get mixed up is when I have a GN m^-2

a Giga Newton is type of unit and a m^-2 is another but when do you choose to drop the distance/ area measurement out of the final answer giving a Force answer in simply Newtons or Giga Newtons rather than Newton meters ect

So... I am unsure whether my answer should be in Newtons or Newton meters
As the calculations go I presume they are ok so my attempted answer is:
upload_2017-1-9_13-57-21.png
 
You don't "choose to drop the distance/area measurement", as if this was optional. It disappears as a result of being multiplied by a distance/area measurement. N m-2 * m2 = N.
Remember, quantity equals number plus unit. Until you get the hang of this I would strongly advise you to write your units in your calculations, e.g.
Force = 6 x 10-3 GN m-2 * 1.96 x 10-5 m2 = 1.176 x 10-7 GN
And you should just learn the basic units for common quantities, e.g:
Force: N = kg m s-2
Energy: J = N m = kg m2 s-2
Pressure and Stress: Pa = N m-2
Strain: m/m = dimensionless (sometimes we use "units" like με = 10-6 m/m)
etc.
 
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I think I need to read up some more on common units and measurements, It was introduced into the earlier assignments without covering in the maths bridging course I completed before starting this HNC,. Once again I thank you!