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Friction is an interactive sim. It changes as you play with it. It has a Play Area and a Control Area.
Chemistry book has far fewer jiggling atoms as many have broken away. Chemistry book rests lightly on top of a Physics book, and is ready to be rubbed against it. In zoomed-in view of where books meet, surface temperature is cool, and atoms jiggle a tiny bit.
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Test device iPad Air 2 Operating System iPadOS 15.4.1 Browser Safari Problem description For https://github.com/phetsims/qa/issues/791 The first time using Mobile VO on the phet-brand sim, when dragging the books, alerts played that described heat, particle motion, location of the book, etc. However, when trying this on the phet-io brand sim, the alerts didn't play. I went back to the phet-brand sim and wasn't able to get the alerts to play there either. I think that the alerts stop working if the sim is already loaded when you enter VO and come to the sim. If you follow a link with VO, the alerts seem to play.