15-Moonstone

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wrong results with (In) natural logarithm

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Hi there

Why I'm having the wrong answer when calculating Re using (In)

When Mathcad calculates it I get Re= 1.65 , however in my calculator I got Re=0.67

Thanks

S

StuartBruffAnswer

23-Emerald V

Log is ln in both Mathematica and Matlab.

I prefer Mathcad's notation, as it accords with the ISO 80000. This is also the notation used by my HP 32S RPN calculator, which also gives -1.6286.

The ISO 80000 standard specifies the following notations for different logarithm bases:

ln 𝑥 for the natural logarithm (base 𝑒)
lg x for the common logarithm (base 10)
lb 𝑥 for the binary logarithm (base 2)
log 𝑥 when the base does not need to be specified, such as in general logarithmic identities that are true for any base.

Stuart

Werner_E

25-Diamond I

When Mathcad calculates it I get Re= 1.65 , however in my calculator I got Re=0.67

1.65 ??? I see -1.63 in your picture. A negative value!!

And are you sure about the 0.67? Chances are that you mean -0.71 !?? At least you write Re:=0.71 in the screenshot.

So what do you actually mean and be careful as of the sign!

Which logarithm are you using on your calculator?

If you use log (base 10 logarithm) instead of ln (base e logarithm) you get -0.71. But its still a negative result.

You may note that some authors (and also some math programs) use log for the base 10 logarithm (like Mathcad and most pocket calculators) while others use log for the base e logarithm (like MatLab and Mathematica with a capital "L"). Both a wrong according to the standards as log should not be used at all. The correct names are lg and ln.

EDIT: Another explanation could be that you forgot to type a pair of parentheses around the two factors (71 and sigma) in the denominator and so you actually calculated

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YA_10963798

Author

15-Moonstone

The value in my calculator is Re=0.67..but I round it to Re=0.7 in the screen shoot .... I want to use (In) not ( log ) and it shouldn't be negative . I didn't get a negative value in my calculator

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Werner_E

25-Diamond I

@YA_10963798 wrote:

The value in my calculator is Re=0.67..but I round it to Re=0.7 in the screen shoot .... I want to use (In) not ( log ) and it shouldn't be negative . I didn't get a negative value in my calculator

But you wrote 0.71, not just 0.7 !??

And the difference in sign? the result in the screenshot is -1..63 but in your text you claim that Prime would come up with +1.65 ????

What about my theory as of the missing parentheses which I added in my replay above?

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StuartBruff

23-Emerald V

Mathematica agrees with Mathcad, as does Matlab.

In[30]:= 0.34*Log[15/(61*118^0.71)]

Out[30]= -1.6286

>> 0.34*log(15/(61*118^0.71))

ans =
-1.6286

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YA_10963798

Author

15-Moonstone

I want to use ( In) not ( log)

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S

StuartBruff

Answer

23-Emerald V

Log is ln in both Mathematica and Matlab.

I prefer Mathcad's notation, as it accords with the ISO 80000. This is also the notation used by my HP 32S RPN calculator, which also gives -1.6286.

The ISO 80000 standard specifies the following notations for different logarithm bases:

ln 𝑥 for the natural logarithm (base 𝑒)
lg x for the common logarithm (base 10)
lb 𝑥 for the binary logarithm (base 2)
log 𝑥 when the base does not need to be specified, such as in general logarithmic identities that are true for any base.

Stuart

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Fred_Kohlhepp

23-Emerald I

Usually (in Mathcad) equations with units will balance. If you expect an answer to be a force and Mathcad claims it's a moment you should look for a mistake in the equation. In older engineering texts where equations were empirically developed this can create a problem. (Dividing out units as you've done is one solution.) So I would first challenge your formula for Re.