Module $2$: Southwest's industry dynamics

Read the case study on Southwest Airlines $($same case study as for Module $1).$

Porter's Five Forces model of industry profitability describes how these forces $-$ rivalry, power of buyers, power of suppliers, threat of entrants, and threat of substitutes $-$ influence industry profitability. Yet, these forces vary in strength and change over time.

Pick one of the Five Forces $($rivalry$,$ buyer power, supplier power, new entrants, or substitutes$)$ and describe:

How does this force influence industry profits?

Is it a strong or a weak force and why?

How has this force changed over time $($as described in the case$)?$