Isaac Newton's first law of motion
said that anything that is moving will want to keep moving in a straight line unless another force acts against it. This is called inertia.
The coin travels around the outside of the balloon because of inertia, i.e. it's trying to travel in a straight line but the curved surface of the balloon swings it around the corner. The coin continues to travel around the balloon until gravity and friction slows it down.
The same principle applies to the motor bike stunts in circular cages
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