WolframAlpha seems to correctly accept this
Plot[ReplaceAll[Sqrt[g Sqrt[s + x^2]],{s -> 100.0, g -> 1000}],{x,0,1000} ]
and this
Sorry, Fred. I didn't read carefully enough. I was using Mathematica.
Best, David
The rule needs to be applied before Plot evaluates:
Plot[Sqrt[g Sqrt[s + x^2]] /. {s -> 100.0, g -> 1000}, {x, 0, 1000}]