topic Re: Why is RK fixed not working in Mathcad

Why is RK fixed not working

FDS — Thu, 10 Jan 2019 06:14:27 GMT

I have a worksheet that perfectly worked without units, but when I try to insert units RKfixed says that there are still units although I believe I stripped them all. Would someone be so kind to check what I did overlook?

Re: Why is RK fixed not working

-MFra- — Thu, 10 Jan 2019 08:04:29 GMT

Hi,

in the function D(x,--) the variable x isn't present .

Re: Why is RK fixed not working

FDS — Thu, 10 Jan 2019 10:41:26 GMT

Thank you for your reply. The x represents the start and end value for rkfixed and is in my opinion correct. It works when I use it without units

Re: Why is RK fixed not working

terryhendicott — Thu, 10 Jan 2019 10:45:31 GMT

Hi,

Approx 89 is the result as predicted

You do not need when finding the ratio of two values with the same units to divide by the units.

Not sure why you have vectorised the equations?

Cheers

Terry

Re: Why is RK fixed not working

FDS — Thu, 10 Jan 2019 11:05:40 GMT

Thank you for your reply. The vectorization is done because in the final version the inputs are vectors but you are right it is not necessary for the example file. It works indeed perfect when removing the vectorization. However when I use a vector it is no longer working as shown in the attached worksheet. Any further suggestions?

Re: Why is RK fixed not working

terryhendicott — Thu, 10 Jan 2019 11:16:30 GMT

Hi,

Working File.

In the definition of D the parameters need to be unit less. PvO2.c is just a variable for the definition. You could call it "y". The units are applied to get λ.p and not used in the second equation of the function "D".

Re: Why is RK fixed not working

FDS — Thu, 10 Jan 2019 11:33:38 GMT

Thank you very much for your support. Your explanation was very helpful