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Physics in Action: The Three Balls Demo


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About this Lesson

  • Type: Video Tutorial
  • Length: 3:18
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 35 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”.

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And now you know that, for an object moving in a circle, its instantaneous velocity is tangent to the circle. Well, we're going to use this interesting piece of equipment to demonstrate that point. What we've done is to attach three balls on the edge of a disc. And we've attached this disc to a motor. When I turn on this motor, it's going to cause the disc to turn around. And the balls are going to fly out and continue moving in a circle. Now, as long as there's tension on the string holding the balls in place, the balls will continue to move in a circle. But if we were to cut the string, there would be no longer any force keeping the balls moving in a circle. And by Newton's first law, whatever direction they were moving in at that point, they would continue moving in that direction in a straight line. Of course that straight line is just a line tangent to the circle that they were moving in before. That's the point that we're trying to demonstrate.
Now how exactly are we going to demonstrate this? Well, as I said, we're going to cause this disc to start turning. And once it gets turning, we're going to do the following. This is a very interesting piece of equipment, because it allows us to insert a razor blade like this, at will, into the path of the string. Now when we do that, it's going to cut the string, eliminate the tension keeping the balls moving in a circle. And if what we said is right, these balls are going to shoot off in a straight line and end up in that box over there. Again, we're going to insert the razor blade. And when the razor blade is inserted, it's going to cut the string. The balls will no longer be constrained by any force. And they should therefore continue moving in a straight-line tangent to the circle that they were moving in before.
Let's see how that works in real life. I'm going to turn on the switch now and get the balls spinning. Notice you can see the balls. They're obviously spinning in a circle. And what's keeping them in a circle is the tension of the string, which you can't quite see right now, but which you saw before. Now when I say ready set go, I'm going to press this button over here. This button is going to activate the razor blade which you saw a few seconds ago. It's going to cut all the strings. And we're going to see whether the balls end up moving tangent to their circular path, and end up in that box.
Are we ready? On your mark, ready, set go. Well let's see if the balls are in here. Did they end up in the right place? I see one ball. I see two balls. I see three balls. Perfect success, in fact all three balls traveled from here in a straight line, ended up in the box exactly as you predict on the basis of Newton's first law. It demonstrates again that the instantaneous velocity is tangent to the circle that the balls were traveling in.
Newton's Three Laws
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