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Physics in Action: Angular Momentum Conservation

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About this Lesson

  • Type: Video Tutorial
  • Length: 4:21
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 46 MB
  • Posted: 07/01/2009

This lesson is part of the following series:

Physics (147 lessons, $198.00)
Physics: The Physics of Extended Objects (25 lessons, $35.64)
Physics: Conservation of Angular Momentum (3 lessons, $2.97)

This lesson was selected from a broader, comprehensive course, Physics I. This course and others are available from Thinkwell, Inc. The full course can be found at http://www.thinkwell.com/student/product/physics. The full course covers kinematics, dynamics, energy, momentum, the physics of extended objects, gravity, fluids, relativity, oscillatory motion, waves, and more. The course features two renowned professors: Steven Pollock, an associate professor of Physics at he University of Colorado at Boulder and Ephraim Fischbach, a professor of physics at Purdue University.

Steven Pollock earned a Bachelor of Science in physics from the Massachusetts Institute of Technology and a Ph.D. from Stanford University. Prof. Pollock wears two research hats: he studies theoretical nuclear physics, and does physics education research. Currently, his research activities focus on questions of replication and sustainability of reformed teaching techniques in (very) large introductory courses. He received an Alfred P. Sloan Research Fellowship in 1994 and a Boulder Faculty Assembly (CU campus-wide) Teaching Excellence Award in 1998. He is the author of two Teaching Company video courses: “Particle Physics for Non-Physicists: a Tour of the Microcosmos” and “The Great Ideas of Classical Physics”. Prof. Pollock regularly gives public presentations in which he brings physics alive at conferences, seminars, colloquia, and for community audiences.

Ephraim Fischbach earned a B.A. in physics from Columbia University and a Ph.D. from the University of Pennsylvania. In Thinkwell Physics I, he delivers the "Physics in Action" video lectures and demonstrates numerous laboratory techniques and real-world applications. As part of his mission to encourage an interest in physics wherever he goes, Prof. Fischbach coordinates Physics on the Road, an Outreach/Funfest program. He is the author or coauthor of more than 180 publications including a recent book, “The Search for Non-Newtonian Gravity”, and was made a Fellow of the American Physical Society in 2001. He also serves as a referee for a number of journals including “Physical Review” and “Physical Review Letters”.

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Let's have some fun now with angular momentum and angular momentum conservation. Andrew here, has agreed to be a victim in this demonstration of angular momentum conservation. He is standing on an almost frictionless turn table. So, once we give him a twist and he starts spinning around, he acquires angular momentum and that's all the angular momentum he's going to have for a long time. Now notice what happens when his arms are drawn in, he spins rapidly. When his arms are spread out, he slows down. Now, why does that happen?
The total angular momentum that he has is a product of the momentum inertia which is determined by how far out his arms are extended and his angle of velocity. How fast he's turning around. When his arms are extended way out, his momentum inertia is big, so his angle of velocity has to be small. When he draws his arms in, his momentum inertia is small, so his angle of velocity has to increase to compensate for it. Notice: he doesn't do anything to control it. The system takes care of itself, all by itself. Again, his arms go out, he slows down, his arms come in, he speeds up. This is exactly what a ballerina does when she draws her arms in to execute a pirouette. You may notice, of course, that Andrew really is slowing down. There really is some friction in the system. But the point is made clear. He brings his arms out, he slows down, he draws his arms in, he speeds up; and this is the demonstration of angular momentum conservation.
Now one of the important things to remember about angular momentum is that it is a vector. Suppose I take this wheel and start it spinning in this direction. The vector in this case, points up. If I turn the wheel upside down, the vector in this case points down. It's important to remember that the total angular momentum of the system like Andrew plus the bicycle wheel is going to remain constant. So, if he take this bicycle wheel, and turns it down, or up, the rest of his body is going to have to compensate for that change of the angular momentum of the wheel-in this case, the direction that the angular momentum is pointing-by having the direction of his body change in a compensating way. We're going to demonstrate that right now. Warren is going to give Andrew a rapidly spinning bicycle wheel. Let's see what happens.
This is the initial angular momentum that the system-the system being the bicycle wheel and Andrew-has and it's going to stay that way for all time; let's go. Notice what happens when he changes the sign of the angular momentum of the bicycle wheel by flipping it upside down; that is compensated for by changing the angular momentum of the rest of the system namely, his body. The total angular momentum of Andrew and the bicycle wheel remains the same, neglecting the small losses due to friction of the turn table. If Andrew starts to get dizzy, of course, you can always turn the bicycle wheel upside down so as to cause himself to almost stop. As now.
Baseball provides another interesting example of angular momentum conservation. We've given Andrew a bat with a weighted ring at the end. Let's see what happens when he swings. Note at the beginning, he has no angular momentum at all: nothing is moving. Let's see what happens as Andrew starts to take a swing. Bat moves in one direction, notice his body twisting in the other. Let's do that again. He moves arm back, forward. Notice the total angular momentum of the system remains the same.
If we were able to calculate-which we could if we put in the effort-how much angular momentum the bat acquires as he swings it around, and how angular momentum his body acquires as it twists in the opposite direction; we would see that the net sum of them is exactly zero. He starts out with no angular momentum, he ends up with no angular momentum. Notice the angular momentum of his bat is a vector pointing in one direction, but the anglement of his body is a vector pointing in the opposite direction and the two exactly cancel.
The Physics of Extended Objects
Conservation of Angular Momentum
Physics in Action: Conservation of Angular Momentum Page [1 of 1]

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