HL Special relativity introduction [IB Physics HL] 

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Hello, I have a doubt about the question at the end of the video. Why can't u=velocity of object, measured in S, be the velocity of ship B. I thought according to the question the velocity is being calculated according to the person inside B and for them wouldn't B count as stationary. You know since they are also moving at the same velocity as ship B inside it. I'm finding this quite confusing 😅

tysm

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