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When the marble is viewed from with respect to the ground, it experiences a net inward acceleration, and hence force. However, when the marble is displaced slightly, it moves towards the edge. Why? My first answer would be centrifugal force, but isn't it fictitious?
EDIT: The marble isn't the center of the conversation here. It's the outward force. The marble and the merry-go-round can be substituted with just about anything. A car on a bend, a child being spun around by someone or a centrifuge. My question is simply, why does any of these "things" get pushed outwards. A simple answer would be centrifugal force. But isn't that fictitious? The reason why I asked about the marble and merry-go-round is because of a thought experiment involving the two in Physics by Resnick and Haliday.