Why are the symbolic units not evaluated by a square root?

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Lucas Clark
Lucas Clark 2020-1-12
评论： Paul 2024-3-15
clear, clc
u = symunit;
d = .025*u.m; %Automatically convert to meters
l = .6*u.m;
A = pi*d^2/4;
g = 9.81*u.m/u.s^2;
E = 207e9*u.Pa;
I = pi*d^4/64;
lambda = 76.5e3*u.N/u.m^3;
omega_n = vpa((pi/l)^2*(g*E*I/(A*lambda))^(1/2),4)
When I plug this in, the units stay the same inside the square root.
Why aren't they evaluated?

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采纳的回答

Star Strider
Star Strider 2020-1-12
It appears that the square root is still expressed as a square root, so the units remain the same within it:
omega_n =
27.42*(1037.0*(([Pa]*[m]^6)/([N]*[s]^2)))^(1/2)*(1/[m]^2)
Doing this:
[omega_n,omega_n_u] = separateUnits(omega_n)
produces this:
omega_n =
882.80792016869907152094706300452
omega_n_u =
1*((1/[N]^(1/2)*[Pa]^(1/2)*[m])/[s])
So the units convert correctly. It is necessary to specifically request the conversion.
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Lucas Clark
Lucas Clark 2020-1-12
So, I tried a few other things.
When I broke down the units of E from:
E = 207e9*u.Pa;
to
E = 207e9*u.N/u.m^2;
Then everything simplified very easily and I ended up with a value of 1/seconds.
I'm not sure what this means. Let me know if you understand why it didn't work the first time.
Thank you.
Star Strider
Star Strider 2020-1-12
My pleasure.
The reason it did not work the first time is that the units will not convert until you request that they be converted. This likely makes things easier for the Symbolic Math Toolbos, so that it does not have to convert and then re-convert interim results, converting them only when requested at the end.

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更多回答（1 个）

David Goodmanson
David Goodmanson 2020-1-13
编辑：David Goodmanson 2020-1-13
Hi Lucas,
try your original code but with the last two lines
omega_n = (pi/l)^2*(g*E*I/(A*lambda))^(1/2)
omega_n = vpa(simplify(omega_n),4)
omega_n = 882.8*(1/[s])
Then you don't have to do any conversions yourself, e.g Pa to SI.
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Breyonna
Breyonna 2024-3-14
I am hoping that there is a better way. I'm off to video tutorials.
Paul
Paul 2024-3-15
Ran in:
Maybe I'm missing something, but I don't unerstand what the issue is. One extra call to simplify seems to solve the problem. What is the downside of that?
u = symunit;
d = .025*u.m; %Automatically convert to meters
l = .6*u.m;
A = pi*d^2/4;
g = 9.81*u.m/u.s^2;
E = 207e9*u.Pa;
I = pi*d^4/64;
lambda = 76.5e3*u.N/u.m^3;
omega_n = vpa((pi/l)^2*(g*E*I/(A*lambda))^(1/2),4)
omega_n = 
omega_n = simplify(omega_n)
omega_n = 

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