**javierarantegui**

**Tue, 15 Nov 2016 16:10:11 GMT**
## Hello,
I was trying to do a calculation when I found the following problem. It happens using the latest versions of Calca for Mac and iOS. First, I define some constants:
---
D = 5 µm in m => 5e-6 m
r = 8 cm in m => 0.08 m
v = -8 cm/2 s in m/s => -0.04 m/s
ρp = 900 kg/m^3
ρf = 1000 kg/m^3
mu = 1e-3 kg*m/s^2/m^2*s
---
Then, I try to do this calculation:
---
(v*18*mu/(D^2*r*(ρp-ρf)))^.5 => NaN*i/s
---
and I get this error. But, if I do the same calculation by parts, it works:
---
a = v*18*mu => -0.0007 kg/s^2
b = D^2*r*(ρp-ρf) => -2e-10 kg
c = a/b => 3,600,000/s^2
c^.5 => 1,897.3666/s
---
If I try to take a shortcut, the same error:
---
(a/b)^.5 => NaN*i/s
---
Maybe it isn't a bug and it's me making a silly mistake I can't find.
Javier