The Idea Factory
Page 33
CHAPTER 14
Toy Bird. The bird toy is called the "Drinking Happy Bird," made in Taiwan (MIT) by ARORA. At this writing it is available at the Museum Store of the Boston Museum of Science, and at the Children's Story Toystore, 434 Harvard St., Brookline, MA (617-2326182).
Perpetual Motion Machines (general). For further reading, consult the article: "Perpetual Motion Machines," by S. W. Angrist, Scientific American (1968), 218:114-22; and the article entitled "Perpetual Motion Machines" in Encyclopaedia Britannica.
Friction Calculation. From a knowledge of how long it takes the wheel to come to a stop once the energy source that keeps it going is off, it is possible to calculate approximately the power required to keep the wheel moving against the friction that makes it stop.
The wheel rotated at about 1 revolution every 4 seconds. Thus its frequency, f, was 0.25 sect, and its angular velocity, omega, was:
The bicycle wheel can be considered as a hoop rotating about its axle, and its moment of inertia (I) is simply:
M x R2
M X R2, where M is the mass of the rim, and R is the radius of the wheel.
A wheel might have a rim mass of about 1 kg. A 27-inch diameter wheel has a radius of 13.5 inches, or 0.343 meters. Its moment of inertia is then:
It took about 30 seconds for the wheel to come to a stop. An estimate of the average power to keep it going can be made by dividing the energy stored in the motion of the wheel by the time it takes to dissipate that energy. The energy stored in the wheel was:
For the thing to run three weeks, as Dr. Jones claimed, it would need a battery capable of storing:
According to Encyclopaedia Americana, a typical D-cell battery (also known as a Leclanche dry cell) carries about 0.045 kilowatthours per pound. Three D-cell batteries weigh about a pound, so one battery should carry about 0.015 kWh, so one D-cell battery would be more than adequate to power the machine.
The battery was discovered by Luigi Galvani (as in galvanize, galvanic action), professor of anatomy at the University of Bologna, in 1791, when he accidentally brought two dissimilar metals into contact with a moist substance. Allesandro Volta (as in voltage), professor of natural philosophy at the neighboring University of Pavia, developed the first storage battery, the "voltaic pile," in 1800. Professor Volta spent much time in Como, the lovely town on Lake Como, in the foothills of the Alps.
For more on system dynamics of rotational systems, consult Introduction to System Dynamics, by Shearer, Murphy and Richardson (Reading, Mass.: Addison-Wesley, 1971).
Wind Force on Wheel. If the copper tubes had air going through them, to shear along the side of the rim, power would be delivered to the rim, as calculated below.
or about 2 x 10-4 watts in total. The friction power we just calculated above as about 0.005 watts, so the jets would not be able to deliver enough power to overcome the friction. If there were jets, they wouldn't turn the wheel.
Moving Plate Capacitors. For a discussion of these, consult Dynamics of Mechanical and Electromechanical Systems, by Crandall, Karnopp, Kurtz, and Pridmore-Brown (Malabar, Fla.: Krieger Publishing [originally McGraw-Hill, 1968]), Ch. 6.
Motor principle. For more on how electric motors work, consult The Way Things Work, pp. 300-301.
CHAPTER 15
Hacking (general). For more on this subject, see The Journal of the Institute for Hacks, TomFoolery and Pranks, by Brian Liebowitz (Cambridge, Mass.: MIT Museum, 1990). The live cow story may be apocryphal.
Resonant Frequency. For more about this, consult Shearer, Murphy, and Richardson, Introduction to System Dynamics, or any elementary physics textbook. In Halliday and Resnick, Fundamentals of Physics, resonance is defined as follows: "Whenever a system capable of oscillating is acted on by a periodic series of impulses having a frequency equal or nearly equal to one of the natural frequencies of oscillation of the system, the system is set into oscillation with a relatively large amplitude." This has happened with bridges occasionally, and there is a famous film clip of the Takoma Narrows (Washington state) bridge resonating and ultimately collapsing as a result of wind-induced oscillations.
The term resonance has also been stolen by the literary community, as that feeling of goose bumps when everything in the book comes together and you finally figure out what the author is trying to say.
CHAPTER 16
For more on the strobe light and how to use it, consult Electronic Flash, Strobe, by Harold E. Edgerton (Cambridge, Mass.: MIT Press, 1979), 2nd ed.
Second Amusing Anecdote. An, the Israeli army officer, and I attended an open mechanical engineering department meeting in the spring of 1983. Professor Rohsenow conducted the meeting, and the topic steered toward how the government was cutting back on research funding.
Rohsenow said, "Well, if you assume that private industry will take up the slack with the money that they don't pay the government in taxes, we should be fine."
Professor Weare asked, "Excuse me, but how can you make that assumption?"
"I'm a Republican," Rohsenow answered.
CHAPTER 17
For more on LISP, consult Structure and Interpretation of Computer Programs, by Abelson and Sussman (McGraw-Hill and MIT Press, 1985), and LISP, by Patrick H. Winston, Berthold Klaus, and Paul Horn (Reading, Mass.: Addison-Wesley, 1981). For reading on artificial intelligence, consult Artificial Intelligence, by Patrick Winston (Addison-Wesley, 1984). Also Godel, Escher, Bach, by Douglas R. Hofstadter (New York: Vintage Books, 1989), is supposed to be really good along these lines, although I haven't read it. Which is not to imply that I've read any of the other books referred to herein. Bicycles. If you're ever near Dinant, Belgium (incidentally the birthplace of Adolph Sax, inventor of the saxophone), go to Musr a de la Petite Reine, in the village of Falmignoul. It was there in 1980, anyway. You might want to call ahead (Belgium: 082-74.44.05) or write. They have samples of the many different types of bicycles invented during the nineteenth century. The address is: Musee de la Petite Reine; Falmignoul, Province de Namur; Belgium. The museum was created by Mr. Ernest Wouters.
CHAPTER 18
Please note, the thesis results presented in this chapter are meant to illustrate the process of analyzing and discussing experimental data. The work I did fed into later work by E. Balles and M. Theobald. That work may be located by referencing Society of Automotive Engineers papers published by Professor J. B. Heywood.
The Harvard Bridge. The MIT Museum Shop has a file on this. It's not called the Technology Bridge because when it was built in the late 1800s, MIT was still in Boston. Technically it was named after John Harvard, not Harvard College, and since it had structural problems at various points in its history, culminating in its replacement in the late 1980s, MIT was not all that eager to have the bridge's name changed to the Technology Bridge.
Wheatstone Bridge. For more detail on how these work, see instrumentation by Kirk and Rimboi (Alsip, Ill.: American Technical Publishers, 1975), p. 117.
In a hot-wire circuit, the imbalance in resistances causes a voltage difference between two points of the bridge. That voltage difference then goes into an amplifier, which makes the voltage across the total bridge go up or down, to maintain the hot-wire anemometer at a constant temperature, and hence a constant resistance. So if the flow slows down, the hot wire gets hotter, and the amplified voltage goes down. Conversely, if the flow speeds up, the wire cools and the voltage quickly goes up to keep the wire hot. The amplified voltage is what you read on the oscilloscope screen. Thanks to Alfred Weil for correcting my explanation. Spark Plug. See The Way Things Work, pp. 308-309. Also see Internal Combustion Engines, by Edward F. Obert (Scranton, Pa.: International Textbook, 1968), p. 532ff. The diagram is from that book.
CHAPTER 19
See Psalm 91.
CHAPTER 20
Final quotation:
Let knowledge grow from more to more. But more of reverence in us dwell. That mind and soul, according well May make one music as before But vaster.
-Fro
m in Memoriam, by Alfred Lord Tennyson
Index