The Feynman Lectures on Physics has been a valuable resource for physics teachers since it was published in 1963. In 1998 I created a conservation of energy lesson based on chapter 4. This post will describe the lesson, how it was developed, and other interesting things that have grown out of it. I also have a detailed teacher guide for those interested in trying this with their students.
In 1994 the book Six Easy Pieces by Richard Feynman was published. It contains six chapters from The Feynman Lectures on Physics. They were chosen because they were considered to be accessible to a lay audience, hence the easy part. My copy came with audio cassettes containing the original lectures given to Caltech introductory physics students. After listening to them in my car, I started to think how I could use them in my high school physics class. I really liked Feynman’s use of toy blocks as an analogy for energy in the conservation of energy lecture. This happens to be chapter 4 in both books. It has some great demonstrations, including the famous bowling ball pendulum. This lecture had possibility, but its 51 minute length would probably not work for a high school class. I carefully read the text and found I could edit out the part on gravitational potential energy where he discusses a reversible machine. This is a good part but without an animation or other visual it would fall flat in a high school class. I also edited out the general discussion about conservation laws toward the end. This left 31 minutes that would serve as a great introduction to an energy unit.
My next task was to develop demonstrations that I could perform during the lecture. Some were easy to figure out, others not so much. More about that later. Overall it wasn’t too hard to track down some toy blocks and set up some standard physics lab contraptions. I had already installed a mount for the bowling ball pendulum, otherwise that would have been the most trouble. I decided to follow along with his lecture with some writing on the board. Since the lecture came with some great figures, I printed these out on transparency and did some of the lecture on the overhead projector.
The first time I tried it the student response was good enough to motivate me to add to and improve upon the presentation. There was one part I was puzzled about. He talks about how a pendulum swings to the same height regardless of its path. You hear a click and some laughter at the end. There is no mention of this in the text. One year I had an ah-ha moment, Feynman was doing the cut-short pendulum from the Conceptual Physics lab manual. I quickly added that to the list.
I have done the Resurrecting Feynman lecture for over 20 years. I presented an abridged version of it at a Northern California/Nevada American Association of Physics Teachers meeting at Stanford in April, 2000. Also on stage at that meeting were physics Nobelists Steven Chiu and Douglas Osheroff. I like to think Feynman was there too. Feynman did often attend the southern California AAPT meetings.
I have shared the lesson and my teacher guide at many workshops. I finally decided to make a video of it last year. You can keep reading or take some time to watch.
A few weeks after I posted the video on YouTube, I was contacted by Michael Gottlieb from Cal Tech. He is one of the editors for The Feynman Lectures on Physics and co-author of Feynman’s Tips on Physics. He informed me that one of his jobs as editor was investigating copyright violations. Wrote Dr. Gottlieb: ” I’m empowered by Youtube’s Content Verification Program to remove Youtube videos that violate FLP’s copyrights, while serving DMCA takedown notices to the posters. However, I’m not doing that in your case, because I like your video, and I hope it will qualify as “Fair Use.” With his help Caltech did approve the use of the audio and let me keep the video up. I continued to exchange emails with Dr. Gottlieb about this particular Feynman lecture. He did not agree with me about the use of the cut-short pendulum that I had been so happy about figuring out. He sent me the picture below and speculated that Feynman had used his finger to interrupt the pendulum’s swing and that the click I heard was an artifact.
I was confident that I was hearing the pendulum bob hit an obstruction as it wrapped around it. I had done this Paul Hewitt demo enough times to recognize it. Dr. Gottlieb dug further into the Cal Tech archives and found this picture that clearly shows the obstruction below the white line that the pendulum string was wrapping around and the pendulum then hitting. Dr. Gottlieb thinks the sound heard is Feynman returning the chalk to the tray. I do not agree because he is not anywhere near the blackboard when doing this demo as the above picture shows. It is hard to get clear confirmation about a guess you made about something that happened when you were not quite 1 year old.
After writing this I encountered what is perhaps the first use of this demonstration. Galileo describes it in his book, Dialogues Concerning Two New Sciences. Galileo says; ““Imagine this page to represent a vertical wall, with a nail driven into it; and from the nail let there be suspended a lead bullet of one or two ounces by means of a fine vertical thread, AB, say from four to six feet long, on this wall draw a horizontal line, DC, at right angles to the vertical thread AB, which hangs about two finger-breadths in front of the wall. Now bring the thread AB with the attached ball into the position AC and set it free; first it will be observed to descend along the arc CBD, to pass the point B, and to travel along the arc BD, till it almost reaches the horizontal CD, a slight shortage being caused by the resistance of the air and the string; from this we may rightly infer that the ball in its descent through the arc CB acquired a momentum [impeto] on reaching B, which was just sufficient to carry it through a similar arc BD to the same height.
Having repeated this experiment many times, let us now drive a nail into the wall close to the perpendicular AB, say at E or F, so that it projects out some five or six finger-breadths in order that the thread, again carrying the bullet through the arc CB, may strike upon the nail E when the bullet reaches B, and thus compel it to traverse the arc BG, described about E as center. From this we can see what can be done by the same momentum [impeto] which previously starting at the same point B carried the same body through the arc BD to the horizontal CD. Now, gentlemen, you will observe with pleasure that the ball swings to the point G in the horizontal, and you would see the same thing happen if the obstacle were placed at some lower point, say at F, about which the ball would describe the arc BI, the rise of the ball always terminating exactly on the line CD. But when the nail is placed so low that the remainder of the thread below it will not reach to the height CD (which would happen if the nail were placed nearer B than to the intersection of AB with the horizontal CD) then the thread leaps over the nail and twists itself about it.” Here Galileo describes the demonstration that I had been discussing with Dr. Gottlieb. From Galileo, to Feynman, to Hewitt, must be a good demonstration!
The first section I cut out of the recorded lecture describes doing experiments with a reversible machine. These experiments result in developing the concept of energy and work. It is an intriguing discussion but I thought most of my students either couldn’t follow it unless there was an animation or an actual mock-up of the machine. I noticed that in one of the pictures that Dr. Gottlieb sent me there is such a mock-up on the lecture table! It is gratifying to know Feynman had the same instinct. What I lacked was someone who would build it for me.
Our correspondence also discussed the famous pendulum demonstration. He told me that the same bowling ball was still being used in Caltech physics classes but that it was painted pink sometime after 1961. Dr. Gottlieb then went on to describe how it has become part of a minor myth:
“It’s interesting to note that this lecture has generated myths about Feynman, the most prevalent being how Feynman presented the bowling ball demonstration, a complete fabrication that was originated by Michael Scott, who claims to have been present as one of the students, and who later became wealthy and funded the Feynman Chair for Theoretical Physics at Caltech, who’s first occupant was a friend of mine, Kip Thorne. This myth was recently illustrated in a (supposedly biographical) book, “Feynman” by Ottaviani and Myrick.”
The myth is recounted in a Los Angeles Times article. According to the Times, on the first day of class, Scott recalled, “in the hall, there were 183 new freshman and a bowling ball hanging from the three-story ceiling to just above the floor. Feynman walked in, and without a word, grabbed the ball and backed against the wall with the ball touching his nose. He let go, and the ball swung slowly 60 feet across the room and back–stopping naturally, just short of crushing his face. Then he took the ball again, stepped forward, and said: ‘I wanted to show you that I believe in what I’m going to teach you over the next two years.”
Dr. Gottlieb mentioned how this story has been recounted many times including in WikiQuotes. He goes on to write “However, the photos and recordings of the first lecture of FLP do not bear Scott’s story out. There is no bowling ball hanging from the ceiling in the first lecture of the series. The bowling ball was used only in lecture 4, on conservation of energy, but even then, Feynman did not just walk in, without a word, and proceed as described above. You can hear on the commercial CD that he starts talking about the pendulum at minute 35. He starts handling it at 36:20, while continuing to talk about it, and does the actual experiment (followed by applause) at 36:40. (You can see it also in the photos.) Furthermore, after the experiment, he does not say “I wanted to show you that I believe in what I’m going to teach you over the next two years.” So, I am convinced that this story is piece of Feynman Lectures mythology and am very curious to know what its true origin is – whether, for example, Scott saw Feynman demonstrate in the manner described at some other time (for example, as a guest lecturer in a later Freshman class).
In his reply to Dr. Gottlieb about this discrepancy, Michael Scott accepts his memory may be in error. He does remember that he got a 37/100 on the first quiz and it was the second highest score in the class. Perhaps Scott’s false memory even made it into the 2013 TV movie, The Challenger Disaster. Their depiction of the Feynman bowling ball demonstration is closer to Michael Scott’s recollection than it is to the actual lecture.
I have seen many different versions of the bowling ball pendulum demonstration. Walter Lewin first breaks a piece of glass because he pushes it. I find that defeats the purpose of the demonstration. He obviously had someone else cleaning up after the lecture. Others take advantage of large lecture halls to have it swing a large dramatic arc, just missing the floor. That guarantees even more energy is lost, reducing their chance of a close encounter with the bowling ball. I stuck with the way shown in my YouTube video for decades. Here is a video of me demonstrating it to some physics teachers at a PTSOS workshop. I did come up with a twist on Feynman’s demonstration that I think is original. I had two bowling balls out for a discussion about elastic collisions and I had an idea.
Interested in doing the Resurrecting Feynman lesson yourself? See my detailed teacher guide and resource list here: https://the-physics-well.net/?page_id=943