The spinning platter of a computer hard disk can act as a sort of gyroscope. Some early portable MP3 players contained a single hard disk, and the angular momentum of the disk could be ?felt? when the MP3 player was turned or rotated. It is theoretically possible to make an MP3 player using two identical hard disks mounted in such a way that the angular momentum of the disks is not felt when the player is rotated. In which arrangement in Figure P9.72 are the spinning disks mounted in this way?

Figure P9.72

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Arrangement 1 Arrangement 2 Arrangement 3