Simple maths question

[peteT16]

It's all got a bit scrambled

[Dave_ZS]

tbh I got totally fried by that thread

[peteT16]

Yes, easy to tell who the forum eggheads are

[bjrespect]

its a good job im thick shelled

[Punx0r]

I know you've all been desperately waiting for closure on this, so here you go

Knowing only the strain (0.0002) and the final dimension (484.3mm), find the original dimension (D):

E (strain) = X / L

E = (484.3 - D) / D

0.0002 = (484.3 - D) / D

0.0002 x D = 484.3 - D

D + 0.0002 x D = 484.3

1.0002 x D = 484.3

484.3 / 1.0002 = 484.203mm

-------------------------

I can't say I understand how D + 0.0002 goes to 1.0002, but apparently that is a recommended method.

In answer to why a tensile load would make the diameter increase, apparently the load is applied across the diameter (distorting it out of round), rather than along the object's length. Badly worded IMO.

You can resume the egg jokes now

[Dave_ZS]

I think its safe to say the yolks over. That is some maths though.

[Null_Byte]

I can't say I understand how D + 0.0002 goes to 1.0002, but apparently that is a recommended method.

The workings you show there are not correct, or rather they do not follow. You would have to assume a value for D to arrive at that conclusion. You also do not start assigning variables to letters and suddenly switch halfway to substitution. The formula should be worked through fully, with substitution only at the final stage.

I also do not buy that about the diameter either.

It looks suspiciously like somebody is directly trying to apply the formula without thinking about the application and making up an excuse as to why the figures don't add up!

If this is, as I suspect, your lecturers "solution" to the problem, I think you need to find another lecturer!

I suspect that value is for compressive strain, in which case the formula would works in reverse and that working out is correct (sloppy, but correct) down to the magic substitution of 1.0002 for D+E x D Where it looks as if they have lost the plot a bit.

The correct working out for compressive would be:

Code: Select all

S =  F - O
      ____
        O

Multiply both sides by O

Code: Select all

SO = F-O

Add O to both sides

Code: Select all

SO+O = F

Reverse factoring

Code: Select all

O(S+1) = F

Divide both by (S+1)

Code: Select all

O = F
  ____
  S+1

Now you can substitute some values:

O = 484.3 / (0.0002 + 1) = 484.203

However, as you have correctly identified, this is incorrect if the material is under tensile strain, in which case, James (sorry dude I missed yours first time) or my slightly rambling solution are correct.

In short, full marks for effort, but give whoever palmed you off with that rubbish a D minus for comprehension and workings from the Rovertech crew. You are totally right to question it.

[Punx0r]

To fess up, I what I posted above was scribbled down (not by me) along with a verbal explanation. I suspect if the technique is sloppy it might be because it was put that way to help me understand it...

Anyway, I thought you worked in IT, Jon? Is maths a hobby, or did you study it?

[618ireland]

...

In short, full marks for effort, but give whoever palmed you off with that rubbish a D minus for comprehension and workings from the Rovertech crew. You are totally right to question it.

+1