Pseudowork and real work

I have a story to tell about pseudowork, the integral of a force along the displacement of the center of mass, which is different from the true work done by a force on a system, which must be calculated as the integral of the force along the displacement of the point of application of that force. If the system deforms or rotates, the work done by a force may be different from the pseudowork done by that force. For example, stretch a spring by pulling to the left on the left end and to the right on the right end. The center of mass of the spring does not move, so the pseudowork done by each force is zero, whereas the real work done by each force is positive. Because the total pseudowork is zero (which can also be thought of as the integral of the net force through the displacement of the center of mass), the translational kinetic energy of the spring does not change (more generally, the work-energy theorem for a point particle shows that the change in translational kinetic energy is equal to the total pseudowork). Because the total work done on the spring is positive, the internal energy of the spring increases.

In 1971 in the context of the big PLATO computer-based education project at UIUC I had several physics grad students working with me to develop a PLATO-based mechanics course. They and I each picked an important/difficult mechanics topic and started writing tutorials on the topics. Lynell Cannell was assigned energy and I became concerned that she was the only member of the group not making progress. I was about to have a talk with her about this when she came to me to say that she was hung up on a simple case.

She said, “Suppose you push a block across the floor at constant speed. The net force (your push and the opposing friction force) is zero, so choosing the block as the system no work is done, yet the block’s temperature rises, so the internal energy is increasing. I’m very confused.” I said, “Oh, I can explain this. You just, uh, well, you see, uh…..I have no idea.”

We went and talked to Jim Smith, an older physicist very interested in education, very smart, and a good mentor for my then-young self. Jim had thought it through and explained the facts of life to us, with a micro/meso model of the deformations that occur at the contact points on the underside of the block, such that the work done on the block is different from the pseudowork done on the block.

I got very interested in the matter and fleshed out Jim’s insight in more and more detail, but when I showed my analyses to physics colleagues they weren’t having any. Finally I decided to send my paper to AJP (the American Journal of Physics), and the reviewers rejected it. One reviewer said, “Sherwood applies Newton’s 2nd law to a car, which is illegitimate, because a car isn’t a point particle.” I sent it to The Physics Teacher, and the editor replied that he wouldn’t even send it out to reviewers because the physics was so obviously completely wrong.

I asked AJP for an editorial review, and the reluctant response by an associate editor was, “Well, I guess Sherwood is right….but that’s not how we teach this subject!” Finally, in 1983, AJP did reluctantly print the paper “Pseudowork and real work” which you’ll find on my website. This was the first half of the original paper. The second half, applying the theory to the case of friction, “Work and heat transfer in the presence of sliding friction” (also available on my web site), was published jointly in 1984 with William Bernard, because AJP had received a related paper from Bernard and put the two of us in contact with each other.

At that time there had been some short articles in AJP on the topic, but there hadn’t been a longer article on all the aspects. In fact, given physicist resistance to the truth, Bernard was engaged in a war of attrition, sending short articles to AJP on various aspects of the problem, trying to build up to the full story. Nor had there been any article on friction.

The grand old man of PER, Arnold Arons, was a fan of my first paper and summarized it in his book on how to teach intro physics. Even he however was quite skittish about the friction analysis, in large part because he was strenuously opposed to mentioning atoms in the intro physics course, for philosophical reasons. Arons tried to explain the pseudowork issue to his friend Cliff Schwartz, the editor of The Physics Teacher, but he never succeeded; Schwartz remained forever convinced that this was all massively wrong.

After the papers were published I wrote to Halliday and Resnick about the matter and got a nice letter back from Halliday (which alas I seem to have lost) which said essentially, “I see that you’re right, but to do this right would mean a very major revision throughout our book”, which is true. This adds to the irony of Ruth Chabay and me being given the 2014 AAPT Halliday and Resnick Award for Excellence in Undergraduate Teaching (here is a video of our talk on the occasion, dealing with thinking iteratively).

John Jewett of Serway and Jewett did do it right, and theirs might be the only other intro textbook that does full justice to the topic; most textbooks make major errors in the energetics of deformable systems (some textbooks have brief sections on the momentum aspects of multiparticle systems but typically say little about the energetics). In 2008 Jewett published a solid five-part tutorial on the subject in The Physics Teacher.

In my original articles the analysis is couched in terms of the two different integrals, for work and for pseudowork. We found that even strong Carnegie Mellon students had difficulty distinguishing between these two very similar-looking integrals. So eventually we changed our textbook to emphasize two different systems (point-particle and extended) instead of two different integrals; the distinction between the two systems is more vivid than the subtle distinction between the two integrals.

The point-particle model of a system has the mass of the system that is modeled as an extended system and that moves along the same path as the center of mass of the extended system. The change in kinetic energy of the point-particle model is given by the integral of the net force acting at the location of the point particle, and this is equal to the change in the translational kinetic energy of the extended system. The change in the total energy of the extended system is equal to the sum of the integrals of each force along the path of its point of attachment to the system.

Here is a video of an apparatus that shows the effects. Two pucks are pulled with the same net force, but one is pulled from the center and doesn’t rotate, whereas the other puck has the string wound around the disk, and it rotates. Somewhat surprisingly, the two pucks move together, but in fact the Momentum Principle guarantees that the centers of mass of the two pucks must move in the same way if the same net force is applied. Here is a computer visualization of the situation.

Bruce Sherwood

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2 Responses to Pseudowork and real work

  1. Andy "SuperFly" Rundquist says:

    Thank you, Bruce, for your tenacity on this subject! I remember just last year struggling with this topic and you helped me out tremendously.

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