Mathematical Error or Units

[mohammed43]

Hello,

can someone point out the mistake in the attached worksheet. I highlighted the mistakes in yeloow(on the right half of worksheet). I have very simple equations but the mathcad is giving me a different answer to that of manual calcualtion..
Also, the mathcad is rounding off 1.953X10^3 to 2.00x10^3, how do we stop rounding off ??

Thanks for the help..

 

[fel3]

Here are my thoughts. I just took a quick look, so there may be more to this than I am seeing.

M[p] is not being rounded off. Instead, it is currently formatted to just one decimal place. If you reformat to three decimal places (via the menu: Format|Result), you will get what you are looking for.

In the equation for I[k}, the piece (T[p]*(h^3)/12) will have units of L^4 based on the previously defined values and units for T[p] and h. So will the long piece that is added to it, which basically reduces to another T[p]*(h^3). Thus, the result should have units of L^4, not L^3. It occurs to me that the "12" in (T[p]*(h^3)/12) might actually be 12 inches and thus a badly handled unit conversion from a non-Mathcad environment such as a text book. However, if this is the case, a similar division by inches is needed for the second half of the equation.

I hope this helps.

Fred

==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[jayrod12]

I think the "12" is actually just from the geometry no? isn't the base formula for I for a rectangle bh^3/12?

 

[fel3]

jayrod12…

That makes sense, more sense than "12" being a conversion factor. I just didn't recognize the obvious. That's what happens when you stay up past 2:30 a.m. photographing a total lunar eclipse.

Fred

==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[mohammed43]

>12 is just a constant and the units of I{k} must be in^4 or ft^4 (In the attached worksheet, it was my mistake I should have written in^4 (highlighted in green))

I should get I.k or I.c value 2264 in^4 or 0.109 ft^4 not 3.5*10^4 in^4 or 1.7 ft^4 as calculated by mathcad


Thanks..

 

[IRstuff]

You either have an input data error, or an equation error. Mathcad does not make calculation errors. The fact that you wrote in^3 in your sheet shows how fallible human data entry is. One other thing to check is whether all your multiplications are actually multiplications and subscripts are subscripts.

The fact that your Ic and Ik produce identical results suggests that your hand calculation is using a different equation for some reason. Additionally, your Iy calculation has both of the first two terms returning zero, which is highly suspect, so is G.tl supposed to be zero, because that's what you have in your sheet. Since that's the only term that's subtracted, I would look there first.

TTFN
faq731-376

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[bkal]

Interstingly, my hand calculation (using a calculator) agrees with your Mathcad yellow results, i.e. 34606in^4. The first term T.p*h^3/12 = 2662 in^4.

 

[rmix22]

Interstingly, my hand calculation (using a calculator) agrees with your Mathcad yellow results

Interestingly - really? Thats exactly what I would have supposed. As IRSTuff already wrote, Mathcad will not make that kind of calculation errors. We don't know why mohammed43 thinks that the result should be different. Either he is referring to a result given to him by somebody else or he calculated it himself manually. In each case there is either an error in the manual calculation, or different equations and/or numbers were used. Chances are that some formula in the sheet was typed in wrong, maybe a wrong sign or the like. BTW, if we change the sign of the summand h/2, the result will not match the one mohammad is expecting, but it comes close.

@mohammed43: as you were confused because Mathcad is rounding every result to one decimal and is also showing trailing zeros (which can be very annoying, especially when showing integer constants like ORIGIN or in plot labels). Thats not the default setting of Mathcad. Somebody obviously had set it that way, either on purpose or unintentional. If you didn't do that, you either are using a sheet which was prepared by someone else or you based your sheet on a template someone else made.
You can change the setting either for a single regions or for all regions in that worksheet by using the menu Format\Result. If the cursor is inside a region/result when you enter the menu, the changes will only apply to that region, if the cursor is somewhere at an empty space of the sheet, the changes apply to the whole sheet. This principle also applies to other changes like plot colors, etc.