Tensile Stress in a Rod

[rad10k]

Homework Statement

Calculate the tensile stress in a rod 40 mm in diameter with a load 200 KN ?

Homework Equations

pi*r^2

The Attempt at a Solution

3.14 * (20*20) = 1256 mm^2

200 * 1000 = 200 000 N

200 000 / 1256 = 159.24 N/mm^2

Tensile stress = 159.24 N/mm^2

Could anybody tell if this is correct and offer some guidance if its not? Thanks

 

[rad10k]

just to add I am using tensile stress = f/a

 

[p21bass]

Looks good to me!

 

[rad10k]

Thansk )

 

[pongo38]

Comment: ridiculously "accurate" for practical purposes. Even 159 N/mm2 is slightly incredulous.

 

[rad10k]

Not sure what you mean by that but its for course work not pratical purposes.

 

[pongo38]

I just think that you should contemplate the meaning of what you have calculated. If you asked me what was the temperature in my room right now, and I replied that it was 15.36 degrees celsius, you would perhaps consider that the 6 was unnecessary, and possibly also the 3. There are a number of things that could be said about stress. One of them is that, under tensile stress the cross-sectional area gets smaller due to the Poisson effect (Like you see when you stretch a rubber band, and it gets thinner as well as longer). So the actual stress is bigger than you have calculated (although, admittedly, the people who define standards don't ask that people do recalculate the stress). But if you did, you would see how at least the last figure would change. I would challenge you also to say why you thought that 5 significant figures was the "correct" truncation of your result. Why not 7 or 3 or some other number. You can't answer that without asking other questions, such as "why do you want to know?". My plea is that you should not regard coursework as something to be finished, but rather some spur to getting more meaning out of it. Please think about the meaning of any calculation you do, and then choose an appropriate accuracy.

 

[rad10k]

I can see your point I shall think about that when answering question in future . Though I don't want to hand some work in and then get the answer wrong because I rounded it too far .