Control systems question: Magnitude of a plant to determine Nyquist plot?

shizz

From looking through my notes I can't see anything that is showing me that my thought in this is wrong, but it obviously is.

The plant is G(s) = 20/(s2 - 25).

I have taken the magnitude of this to be: |G(jw)| = 20/sqrt(w4 + 252 )
Taking w at 0,5 and infinity I get 0.8 for 0, 0.566 for 5 and 0 for infinity.

The right answer for w = 5 is 0.4 and I can't see how to get this.

Also, my values are supposed to come out as a minus but obviously mine are positive.

Is it just a case of knowing that its coming from the negative side?

Any help would be greatly appreciated. I'm assuming that this is basic stuff which I've missed from the start

shizz

Never mind. Just got it. You have to treat the real and imaginary part separately in the Pythagoras equation. So the magnitude for this is just the original equation considering the imaginary part is zero.