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Physics in Action: A Tug-of-War


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About this Lesson

  • Type: Video Tutorial
  • Length: 2:56
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 31 MB
  • Posted: 07/01/2009

This lesson is part of the following series:

Physics (147 lessons, $198.00)
Physics: Dynamics (15 lessons, $24.75)
Physics: Newton's Three Laws (7 lessons, $8.91)

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 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”.

About this Author

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Here is a really neat demonstration of Newton's third law. On your mark, get set, go. Why did that happen? Well Crystal over here and Amy over here are exerting forces on each other through this rope. Let's stretch the rope a little bit taut. Now, by Newton's third law, the force that Crystal exerts on Amy is exactly equal in magnitude to the force that Amy exerts on Crystal, but obviously opposite in direction. So when they pull on the rope they're obviously drawn toward each other with equal forces, which obviously act in the opposite directions.
Let's see that again one more time. On your mark, get set, go. Now, you may think that Amy and Crystal have to actually pull, make a determined effort to pull, in order for Newton's law to produce the effect that you've just seen. But that's not really the case. Let's see that in a slightly different way in this demonstration. Amy is now going to hold the rope very, very limply. She's going to let the rope drop. She's just holding with two fingers. And Crystal is going to do all of the pulling.
Let's see what happens this time. On your mark, get set, go. Notice you've achieved exactly the same effect. The point is, by Newton's third law, the force that Amy exerts on Crystal is exactly the same in magnitude as the force that Crystal exerts on Amy. And in fact, Amy is going to exert a force on Crystal whether she wants to or not. All she has to do is hold onto the rope. That's a consequence of Newton's third law.
Now let's see this demonstration in another way. Now let's try to make this tug-of-war between Amy and Crystal a little bit more quantitative. Notice they pull on the scale, the readings on both scales are the same. The scales are slightly off, but you get the idea. As the force increases on Crystal's scale, it also increases on Amy's scale. It looks like Amy is getting a little bit tired. But notice, whether she cares to or not, the force on her scale is exactly the same as on Crystal's scale. Crystal is trying much harder. You know Amy is exerting exactly the same force in magnitude on Crystal as Crystal is exerting on Amy. That's the content of Newton's third law. The forces that they exert on each other, through this rope, are equal in magnitude and opposite in direction.
Newton's Three Laws
Physics in Action: A Tug of War Page [1 of 1]

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