What is the reason for the difference between the wording "ya qawmi" (O my people) in verse 2 and "qawmi" (my people) in verse 5 of Surat Nuh, and what is the difference between them?

The word (ya qawm - O my people) originates from (qawmi - my people), and the yaa’ of the first person singular possessive was omitted due to the entrance of the vocative particle yaa’ upon it. This is the well-known usage when calling out to a noun that is annexed to the yaa’ of the first person singular. As for the word (qawmi - my people) in the fifth verse, (qawm - people) is annexed to the yaa’ of the first person singular, and nothing occurred to necessitate the omission of the yaa’.