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A Wikipedia entry defines physics, broadly, as "... the general analysis of nature, conducted in order to understand how the universe behaves." Infants seem to have some built-in understanding of the world - they won't crawl over what appears to be an edge, even when they are on a sheet of glass that would protect them from falling.

Other concepts of how things behave seem to develop over time. An understanding of object permanence - "the understanding that objects continue to exist even when they cannot be seen, heard, or touched", only develops a bit before the age of 2.

As a child, I was fascinated by the following two objects (which I still have):

A 16mm projector and its speaker

The handle gives you some idea of the size, but to be more precise, the box on the left is 47 cm wide (18.5 inches). What surprised me was that the much larger box on the left weighs only 12.7 kg (about 28 pounds), while the smaller box on the right weighs 21.5 kg (about 47 pounds), almost 70 percent more. The difference is large enough to be easily felt just by picking them up. It seemed odd to me that the smaller box weighed more than the larger box.

Here's what you see if you open the boxes to find out what's inside:

A 16mm projector and its speaker

The right-hand box is a heavy 16-mm sound motion picture projector, complete with vacuum-tube amplifier circuitry. You can click on the photo if you want to see an enlarged picture of the projector (return here with your "Back" button).

The left-hand box is mostly empty, containing only a speaker. It would be even lighter if I took out the spooled electrical cord used to connect the speaker, usually placed at the front of a room, to the projector at the back.

I was being introduced to the concept of "density".

I was similarly surprised when, in some science class, I first heard Newton's first law of motion. The first part of it was not surprising, saying, "A body at rest remains at rest". Sure, something that is sitting there doesn't suddenly start to move for no reason.

But the second part, which says in essence, "a body in motion remains in motion", came as something of a surprise. Surely, if you rolled a ball on a flat surface, it would eventually come to a stop on its own. It took a bit of thought to realize that the ball stopped only because various forces resisting its motion were acting on it, such as air resistance and friction against the surface. That same ball thrown into interstellar space would continue traveling in a straight line forever.

I enjoyed my high school physics course, even though it was not very well taught (see my earlier blog entry on Mr. Lusch). I particularly liked the experimental laboratory sessions. But it was in my classical physics courses at MIT that I had a series of particularly imaginative and memorable lab experiences. Some of the most memorable experiences involved things that went wrong.

One of the most interesting experiments was entitled "Air Suspension Gyroscope". I think it became something of a classic. The main piece of equipment, the gyroscope itself, was a 5 cm diameter steel sphere (actually, I think the exact dimension was 2 inches). It had a small flat circle machined on one side, with a diameter of about a centimeter, and a small dot etched into it, off-center. It was magnetized to saturation along an axis perpendicular to the axis of the flat part.

The sphere was set into a base with a concave depression, somewhat less than hemispherical, with compressed air pumped up through a hole in the bottom. Thus, the heavy steel sphere was suspended on a cushion of air, so it could rotate freely. The support also had a magnetic coil in it, driven by 110 V, 60 Hz, from a wall socket. We caused the sphere to spin at 3,600 RPM (revolutions per minute), after which the magnetic field from the coil in the base kept it spinning indefinitely. Note 1 

I've described this experiment to lead up to an event which happened one day in the lab. It didn't happen to me - it was described to me by somebody else who had been in a different lab session. A friend of his, walking by in the hall, had seen him in the lab working on this experiment. Knowing nothing about what was going on, he nevertheless walked into the room to strike up a conversation. He glanced down at the shiny steel 2-inch ball on the table in front of him.

The strobe light used in the experiment was still on, illuminating the sphere in a bright light. The strobe light was doing what strobe lights do: it was freezing the motion. Thus although the sphere was spinning at 3,600 RPM, it appeared stationary - individual minute scratches on its surface could be seen. Note 2 

So the wiseguy who had come in from the hall looked at the sphere, said, "What's this?", and before anyone could stop him, he picked it up! He now was holding in his hand a 2 inch steel sphere which was spinning at 3,600 RPM. Needless to say, it didn't stay in his hand for long. It shot out and started bouncing around the room, careening off walls and chair and table legs, as everybody jumped in the air to avoid being struck.

Eventually, due to these collisions and to friction (remember Newton's law), it came to a stop. It was rather badly damaged, covered with dents, because some of the collisions it had suffered had been at a rather high speed. The sphere had cost MIT a couple of hundred dollars, in the dollars of the early 60s. I never found out whether the student got charged for the damage.

Compressed air suspension was frequently used in our experiments, whenever we wanted to come as close as possible to eliminating friction (of course, it can never be eliminated completely). We did a lot of experiments illustrating the conservation of momentum, using metal disc shaped "pucks" which were suspended on a layer of compressed air. This was done by bringing air down into the puck via a tube that came down from the ceiling. The air exited through a hole in the center of the puck, radially, in all directions. On a flat surface, given a push, the pucks would glide smoothly along with very little loss of velocity.

We needed to be able to determine the direction and speed that the pucks were traveling. This was done by covering the table with a special paper called "Teledeltos paper" (here's an article about some of its uses by the late, great Bob Pease). The paper is impregnated with carbon in order to make it electrically conductive, and our version was additionally coated with a very thin layer of wax.

In the center of each puck, actually in the center of the hole the compressed air came out of, was a pointed metal electrode floating barely above the surface of the paper. A motor driven apparatus produced an electrical spark 30 times per second, capable of melting a small hole in the wax coating on the paper. Thus if you gave a puck a push, and turned on the sparker as it left your hands, the result would be a line of dots on the paper. The line showed you the path the puck had taken, and the spacing between the dots, known to have occurred a 30th of a second apart, gave you its velocity.

You could thus slide two of the pucks towards each other, and measure the direction and velocity of each puck both before and after their collision. Such collisions can then be shown to preserve the overall "momentum" of the two pucks taken together. "Conservation of Momentum" is one of the important principals of classical physics, along with "Conservation of Energy".

A classmate, Mike Bertin, described sliding the pucks towards each other for a collision in the center of the table. The idea was to count off "1-2-3", releasing the pucks on 3, on their way to the desired collision. On 3, Mike's lab partner was to turn on the electricity to the spark electrodes.

Unfortunately, two mistakes were made. The first was that Mike was holding the pucks not by their bases, which he should have done, but rather by the upper part of their electrodes, sticking out on the top. The second mistake was his partner's - he switched on the electricity very slightly before the count of 3, while Mike still had his hands on the electrodes. In any event, Mike's count, which should have gone "1-2-3", was more like "1-2-AARGH!!!", as he received a nasty (but not dangerous) electrical shock.

You may have noticed that both experiments I've described so far involved the use of compressed air. This air was supplied by a pump in the corner of the lab, which pressurized a large tank. The air coming out of this tank went through a reducing valve, which could be adjusted to set the desired pressure delivered to the lab tables. From that valve, tubes snaked all over the room so that each lab table had its own compressed air supply.

The next laboratory experience I'll describe, unlike the two above, actually did happen to me personally. Again, compressed air was in use for the experiment. In this case, the air at each table exited through a long plastic tube, probably only about 3/8 of an inch in diameter. At the end of the tube, the stream of air drove a tiny propeller, the sole purpose of the air being to spin that propeller. Note 3 

We were required to hand in our laboratory reports at the end of the lab session. We did not go off to write them up to be handed in later. This was to prevent the students from putting a great deal of effort into creating a beautiful write-up, as opposed to fundamentally understanding the material on the spot. These were the days before computers, so in fact we turned in hand-written reports. Note 4 

Thus at the end of that day's experimentation, we were all hunched over our lab tables, busily writing away in order to complete our reports before the end of the session, when they had to be handed in. In the back of my mind, I could hear the graduate student lab assistant mumbling to himself as he followed a checklist for the lab's shutdown procedure. He said something about shutting off the air pressure to the room, and I glanced up to see him reaching behind the compressor, fumbling for some valve. Apparently, he grabbed the wrong valve, and started turning the reducing valve that controlled the air pressure distributed to the room.

I became aware that something was going wrong when I heard all of the little propellers on the end of their plastic tubes starting to spin at a higher rate. They began to sing loudly, and the hissing of the air became audibly more insistent. The graduate student turning the valve seemed oblivious to this. I looked up to see what was going wrong, looking in the direction of my lab partner across the table. And I was horrified to see a spherical bulge suddenly appear in the rubber air hose coming down not far from his head. Before I could say anything, the bulge went from the size of a lemon, to an orange, to a cantaloupe, and to a watermelon, at which point the hose burst with a loud bang.

I had never heard a noise like that, and never have since. In fact, I became mostly deaf immediately afterward, so loud was the report. I was terrified to think that I might have permanently lost my hearing, but in fact, it gradually returned to normal over the next few minutes.

I do remember all the physics I learned in those laboratory sessions, but it's interesting that what stands out the most in my mind are the times when something went wrong. Those were the most memorable sessions of all.

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#0145   *CHILDHOOD   *MIT

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© 2012 Lawrence J. Krakauer   Click here to send me e-mail.
Originally posted August 23, 2012

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Footnotes (click [return to text] to go back to the footnote link)

Note 1:   We set the sphere to spinning by blowing compressed air across its surface, with the flat spot on the sphere off to one side. Thus we were spinning the sphere around the axis through the center of the flat spot, so the embossed dot on the flat spot was going around in a circle. By watching the dot with a flashing strobe light, we could see when the sphere had reached the rather high rotational speed of 3,600 RPM.

At that point, we switched on the magnetic field. Since the alternating current line frequency in the United States is 60 Hz, which means 60 cycles per second, and since there are 60 seconds in a minute, the magnetic field from the coil in the base was flipping up and down 3,600 times per minute, the same rate as the spinning of the sphere. Thus the coil captured the magnetized sphere, and supplied just enough energy to keep it spinning.

The experimental procedures then went on to do various experiments on the spinning sphere. For example, the center of support of the sphere (the geometric center) was different from the center of gravity, shifted slightly by the material removed to create the flat spot. This caused the spinning sphere, acting as a gyroscope, to precess, which is to say to rotate slowly around a vertical axis. By sighting the flat spot through a telescope from the far end of the table as the sphere went around, we were able to compute, believe it or not, the latitude of our lab in Cambridge, Massachusetts (due to the rotation of the earth under us in the time the precession went once around). We came pretty close.

You can order instructions for this experiment from the American Journal of physics, or buy them as a book from Amazon, but in either case it will cost you. And after that you'll have to somehow build the equipment, which will cost you even more.   [return to text]

Note 2:   How did the strobe light freeze the motion of the sphere? A strobe generates extremely short pulses of very bright light, the flashes often lasting only about a microsecond (a millionth of a second). In our case, the sphere was rotating at 3,600 RPM, meaning 60 times a second. And the strobe light was also flashing 60 times per second.

A microsecond-long flash would freeze the image of any scratch on the sphere on a viewer's retina. By the time of the next flash, the scratch would have gone completely around the sphere, and returned to exactly the same position. Hence to the viewer, the scratch (and the sphere it was on) would look stationary.

Of course, the eye can't resolve individual flashes at a rate of 60 per second - in fact individual flashes start merging together at a repetition rate of only 10 flashes per second. That's why although your television set is showing you 30 individual stationary image frames per second, they appear to you to merge into smooth motion.   [return to text]

Note 3:   Some of you may be wondering why we needed a little propeller spinning at the end of a plastic tube. Some of you may not be wondering. For those in the former category: the experiment revolved around a rather large "capacitor", comprising two plates about a foot square, separated by half an inch or so of air. This capacitor was charged to a high-voltage.

We moved the little spinning propeller to various points in the electrical field outside the capacitor, the so-called "fringing field". As the propeller aligned itself with the field, and then turned at right angles to the field, it slightly perturbed the voltage seen across the capacitor, a signal we were able to measure. The higher the perturbation, the stronger the field at that location. We were thus able to map out the capacitor's otherwise invisible fringing field, which was the purpose of the experiment.   [return to text]

Note 4:   Years later, an MIT classmate of mine, Martin Schrage, got a job teaching a laboratory course at Wellesley College. In that course, the lab reports weren't turned in until later. They were marked on a scale from 1 to 10, 10 being the best. He always emphasized to the class that their grade was based on their understanding of the material, and that it was not necessary to put a lot of work into the appearance of the report itself.

Nevertheless, he received a great many beautifully produced reports. When a student went over the top on one occasion, handing in a beautifully bound report with carefully drawn multicolor diagrams, Martin marked it down from a 10 to a 9, with the comment "-1 point for unnecessary neatness".

The student came in to see him, livid. I think he reversed the deduction, but felt he had made his point.   [return to text]

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