MIT Physics: Spinning Bike Wheel and Conservation of Angular Momentum


[Music] when I hold the spinning wheel nothing crazy happens when I flip it I spin it back and I stop with it I spent it’s the question is why am i spinning so I have to force the we were to turn over now Newton’s third law says that for every force is an equal and opposite force acting as well so when I force the wheel the wheel applying force equal and opposite onto me causing me to turn so the same thing applies on them on the ground put the friction between my shoes and the ground stop me from spinning so another way to explain what’s going on is to conservation of angular momentum now angular momentum is something that tells you how hard it is either start or stop something from spinning and depends on two things one how fast it’s spinning we call that the angular infancy to its shape or what we call the moment of inertia so this wheel is rotating counterclockwise I’m going to teach you a trick so get your right hand I want you to curl your fingers in the counterclockwise direction anything your thumb is pointing upwards we’ll call that deposit the register so see this wheel has positive angular momentum the magnitude about angular momentum is conserved that means the songs on our outside forces the total angular momentum of the system always stays the same so when you flip this thing over and do the right-hand trick your thumb points down on the negative direction so the annual mention of this wheel is now negative so the whole system me the turntable and the wheel starts out at negative L when you flip the wheel the wheel nine positive L so the whole system needs to stay at negative L because of conservation of angular momentum so I need to spend with angular momentum of negative 2l that’s why it’s been the other way [Music]

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