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21 April 2013

Precise language: lose the "potential" in "potential energy"?

Although I hate traveling during the school year, I thoroughly enjoy being around good physics teachers. Yesterday's AP Physics 1 Consultant Training in Chicago provoked interesting discussion, powerful insight, plenty of arguments, and other features of effective collaboration between teachers.

I'll distill the what insight I have about the new exams eventually.  The AP Physics 1 and 2 curriculum framework is 150 pages worth of dense eduspeak.  It's going to take a while to parse the text and expose the intent of the authors, especially when the College Board has released so few exam items.  All I can say for now:  The exams are beyond excellent, and you should teach to them.

The new exams will require significant amounts of quality writing.  Gone are the days when "justify your answer" meant give one sentence with an equation.  Students will have 90 minutes to answer about five free response questions; so the readers will expect adroit use of verbiage in those responses.

The Committee has spent years parsing their own language, so that every statement in the curriculum framework is self-consistent, and consistent with the language on the exam, and consistent with their perception of best physics teaching practice.  Precision of language is, I agree, important in teaching first-year physics; I've written much on the subject.  That said, sometimes pedantic consistency conflicts with common sense... while I'm more than willing to take a stand with students on the difference between "work is done by the gas" and "work increases", I'm not willing to call out someone who says "a 1 kg mass is thrown off of a cliff.* "

*Objects and systems have a property called "mass."  Thus, it is imprecise to refer to a "mass."  No, we should say "an object with a mass of 1 kg is thrown off of a cliff."  While that's entirely correct, and I am glad the exams will be stated precisely, I have no intention of having an argument about such a subtle difference with a first-year student.

Even in the midst of what sounds like heinous baloney, useful nuggets emerge.  On the tail end of the discussion about objects with mass, Jeff Funkhouser pointed out to me that he doesn't use the term "potential energy."  Part of his reasoning I'd categorize as correct but pedantic -- since potential energy only can arise as a result of a conservative force, and since a force requires an interaction between multiple objects, it's not entirely correct to say that an object "has" potential energy.  Rather, a ball at the top of a cliff has energy stored via the interaction between the ball and the earth (or between the ball and the earth's gravitational field); a mass* attached to a spring has energy stored via its interaction with the stretched spring.


The pedagogical point Jeff is making -- the point which I think is entirely correct -- is that this omnibus term "potential energy" gets in the way of understanding.  Focus on the source or storage of the energy, not on the catch-all term "potential."

I'm in the midst of a 9th grade conceptual physics unit on energy.  In any problem, we begin by writing in words the energy conversion:  for example, "work done by a rope is converted to kinetic energy."  That allows us to write the equation "Fd = (1/2)mv2."  Finally, we can use semiquantitative reasoning to determine whether doubling the force of the rope doubles, more than doubles, or less than doubles the object's speed.

So consider a ball dropped from 2 m high onto a vertical spring.  What's the energy conversion?  I've trained my students to write "gravitational potential energy is converted to spring potential energy."  But having read through stacks of papers, I see that Jeff is right.  How often have I read "potential energy is converted to potential energy?"  Or even with a correct statement of energy conversion, how many times have I seen the resulting equation written as "mgh = mgh"?

Enough already.  The term "potential energy" is hereby banish├ęd from my classroom.  We will instead write "gravitational energy is converted to spring energy."  


  1. Interesting idea. I am concerned about my students being able to independently use the internet and textbooks to augment their learning. Thus, I worry that if I ban the term "potential energy" from my classroom, it will cause confusion when students encounter the term "potential energy" elsewhere, since it is so commonly used. I agree that the term may get in the way of understanding the concept, but banning the term might also get in the way of understanding other scientists.

    What about "electric potential" (V)? Same problems? If so, any ideas on a better name for it?

  2. While I like the idea of omitting the word "potential" from the different types of potential energies, I too am concerned about students encountering it elsewhere, including on AP exams. Are you suggesting to drop this word only for conceptual physics courses geared toward younger (9th and 10th grade) students, or are you advocating its banishment altogether, at all levels of high school physics?

  3. Yet another vote for dropping the word potential in potential energy. With AP/honors students I still tend to use it as the generic term for all energies that are not kinetic or dissipative but the individual energies get named after their associated force. I started doing that because I wanted to keep the word potential for use in electricity.

  4. Great comments. In order:

    Matt: I see your point about understanding other scientists -- after all Feynman hat that very issue with the mathematical symbology he invented. But scientists understand "spring energy" just fine. And your students will figure out the textbook just fine. (I dispute that more than a select few first-year physics students can get much out of a textbook as opposed to in-class work, anyway. Those select few won't have an issue.

    Drew: Jeff, who initiated the conversation with me, is an AP exam leader. I'm going to remove the word "potential energy" from all my courses. I'm not *advocating* anything; my approach is just that, my approach. I'm offering up a suggestion, but you will not find me criticizing those who choose a different route.

    David: Wow, I hadn't even considered the "potential" issue. Of course we've all had difficulty with the terminology difference between "electric potential" and "electrical potential energy." Well, if it's just "electric potential" and "electric energy," then there's no confusion. Woo-hoo!

  5. To annotate Greg's final scenario:
    So consider a ball dropped from 2 m high onto a vertical spring. What's the energy conversion? I've trained my students to write "gravitational potential energy is converted to spring potential energy." But having read through stacks of papers, I see that Jeff is right. How often have I read "potential energy is converted to potential energy?" Or even with a correct statement of energy conversion, how many times have I seen the resulting equation written as "mgh = mgh"?

    I would say, "The energy is initially stored in the ball's height (elevated position) in earth's gravitational field. The energy then progresses to being stored more and more in the motion of the ball as it falls down from higher heights gaining speed. The energy is then stored more and more in the compressed spring, BUT that compression AND MOTION are mpt storing MORE of the energy until the net force on the ball becomes zero (spring force balances gravitational field force). Eventually all of the energy changed from being at a higher height is now in the compressed spring.

    I'd DO this conversation around a series of pie charts that all have the same area and three different pie pieces of varying sizes. Create a pie chart at H, 3/4 H, 1/2 H, 1/4 H, and 0 where H is the height of the unstretched spring with the object attached and not moving relative to the lowest position where the spring is maximally stretched and the object is not moving. IF the students know the relationships for the various forms of energy storage (not type of energy), then they can use the pie charts to build quantitative bar charts and/or a single graph of Eg, Ek, and Eel vs h from 0 to H.

    Understanding that graph IS a high level exercise. It can bring into sharp focus that the position where the FORCES balance is NOT the position where the energy stored in g-field equals energy stored in spring. But, instead, the position where the forces balance is the position where the Ek is a maximum.

    Pedantic to a fault? Yes, but this is talk among teacher not with students.

  6. If you eliminate the idea of "potential" energy, do you have a clever way of writing a large conservation of energy formula? Maybe just E0 + Wnc = Ef? That's definitely shorter than U0 + K0 + Wnc = Uf + Kf, but I'm not sure it'd be easier for students to use. Maybe I'm wrong though, and with practice they'd figure it out.

  7. I like your formulation... At the general level, and when I teach AP Physics 1 eventually, I'll go with the verbal approach: "Spring energy is converted to kinetic energy and work done by friction." Then they can use that verbal description of energy conversion to write an equation to solve.