When is Laplace variable $s =j\omega$?

Question

Having an exam next week!
I've searched a lot, couldn't find anything I could understand.

When is the Laplace variable $s$ equal to $j\omega$? Because I know that, by definition, $s = \sigma +j\omega$

Thank you!

Answer 1

$s=\sigma+j\omega $ means that $s$ is a complex variable with real part $\sigma$ and imaginary part $\omega$. When the real part is equal to zero, we have $s=j\omega$.