The difference between 'What's going on?' and 'What's happening?'
These expressions seem to be interchangeable, but I'm wondering if they have exactly the same meaning. Does one sound more polite, or stronger than the other?

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Answer

both can be used in a situation like you walk into a room with friends "hey guys whats happening/ whats going on?"
whats going on can be used if something is going wrong for example, something is on fire in the office " WHATS GOING ON?!?!"  intonation can play a big part in how these phrases sound as well. you can make it condescending, questioning, surprised, angry etc....