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Perpetual Motion

September 24, 2008 Leave a comment Go to comments

In my post last night on Neil Diamond, I wrote about my stay at the Barrington Court Hotel in Leeds in the summer of 1977, describing in particular evenings spent in the lounge watching BBC on the TV. I recalled two highlights: broadcasts of the final round of the Open golf championship and of a Neil Diamond concert. Let me tell you about another televised highlight, the live demonstration of a perpetual motion machine.

I am neither a historian of science nor a physicist, so I am not well positioned to describe the centuries of effort to build perpetual motion machines or the reason that such efforts are doomed to failure. Roughly, such machines can’t exist because of the principle of conservation of energy. A perpetual motion machine, if it were to exist, would be a mechanical device that moves in some ordered way forever. After construction, one would apply an initial force to get it moving, then one would sit back and watch it move in the same way unceasingly, without further application of energy. In normal everyday life, we know that when we start something moving, it eventually stops, due to other forces acting on it, such as friction. A perpetual motion system would somehow be free of the effects of friction, or other forces. It would just keep moving along. This is what’s impossible.

Nonetheless, I saw such a machine on British TV in July 1977. It was demonstrated on some sort of comedy revue, a sketch show. I marvel at the idea that one would have a physics demonstration on a mass appeal television show, a comedy at that. Clearly the producers had great respect for the audience, recognizing that it would understand, at least intuitively, that a perpetual motion machine is silly, and even something worth laughing at.

Here’s how it worked. A picture would help, but I’ll try to describe it with words alone.

One starts with two pulleys, one mounted a couple of feet above the other, with a rope looped around them. Just picture two wheels, perhaps screwed onto a board, one above the other, with a rope looping around to form a simple shape like a belt in your car engine. On the rope are placed two weights, equidistant from each other, so that if the rope were pulled on in order to make it move around the pulleys, the two weights would circulate around the loop, always opposite each other. When one weight his passing over the top pulley, the other would be passing over the bottom pulley. When one weight is halfway between the pulleys, the other weight is halfway between also, but on the other side. If you were prepared to keep pulling on the rope, you could make this system move around the pulleys continuously. Of course, this isn’t a perpetual motion machine, because it is relying on the regular application of force, or energy, on your part. (I don’t want to be careless here. Force and energy are not the same. Basically, energy corresponds to the work done in moving something, and the work is force times distance. But the point is, in pulling on the rope to keep it moving, you are applying force and expending energy.)

Now we get to the key idea. On one block, you write “9” to indicate that it is a 9-pound weight. On the other block, you write “6” to indicate that it is a 6-pound weight. Imagine the 9-pound weight on one side of the pulley system, with the 6-pound weight on the other side. Thanks to gravity, the 9-pound weight will move downward and the 6-pound weight will move upward, until the 6-pound weight rolls over the upper pulley while the 9-pound weight rolls over the lower pulley. Now what happens? What happens, of course, is that the weights turn 180 degrees, so that what once was the 9-pound weight is a 6-pound weight, as one can see by reading the number on it. And the 6-pound weight has become a 9-pound weight. So the system keeps moving, because the newly-9-pound weight will move down, the newly-6-pound weight will move up, and then they will move around the pulleys, rotate yet again, and the process continues. Forever. With no application of an external force once we get it started. Perpetual motion!

I learned a lot at the Barrington Court that summer.

Categories: Culture, Science, Television
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