Last week on Tuesday, Energy 2 came across the term "mechanical energy." I already flagged this in a comment to Virginia's post about terminology here. Since this term came up again in Energy 2, this morning, I thought I should finally blog about it.
The conversation last Tuesday happened pretty much right after lunch break, the episode is located in (E2 110809 1242 T3) at 12:57pm. Unfortunately, it's rather long (7:11 minutes), so I'm just not going to post it here, also because I conflated the two audio tracks from the two cameras into one .mov file, for easier analysis and transcription (which would not work well with the blogger upload, I believe). I'm uploadin the episode to the server (file name "E2 110809 1242 T3 - Mechanical Energy - 2AudioTracks.mov"), then you can select one of the two audio tracks if you play the file back in VLC (Audio => Audio Track => Track 1 is the original audio, recorded with the mic attached to Jim, Track 2 is the other audio track, recorded with the mic attached to Derrick).
Nina asks what "mechanical energy" is at the beginning of the clip, and Tim answers that it's "motion energy" (kinetic energy) and gravitational potential energy together. Derrick adds, "It's the sum of the two," to which Tim responds, "It's . . . total," and Jim reinforces with "Yeah."
This answer wasn't totally satisfying to me, so I looked it up online. Wikipedia tells me that, "In physics, mechanical energy describes the sum of potential energy and kinetic energy present in the components of a mechanical system. Mechanical energy is the energy associated with the motion or position of an object."
OK, but that's "just" wikipedia. Daniel Schroeder (yes, THE Schroeder, think of Intro to Thermal Physics!) tells his students in his text to the class "Energy, Entropy, and Everything" (Honors 1500, Perspectives in the Physical Sciences), that "Mechanical energy is [...] any form of energy that’s directly associated with motion or with a force." Whooha, that sounds interesting, especially because we've been talking SO much about forces and the connection to energy, lately! In the introduction to the chapter about Mechanical Energy, he goes on to explain that, "Kinetic energy is one form of mechanical energy. In this course we’ll also deal with two other types of mechanical energy: gravitational energy,associated with the force of gravity, and elastic energy, associated with the force exerted by a spring or some other object that is stretched or compressed."
This is an interesting description, it's not just about motion or position, but Schroeder actually connects mechanical energy specifically with forces! Interesting!
And then, I found this one: physicsclassroom.com tells me that, "Mechanical energy is the energy that is possessed by an object due to its motion or due to its position. Mechanical energy can be either kinetic energy (energy of motion) or potential energy (stored energy of position)." How about that!? "Energy that is possessed?" Interesting use of metaphor ;-) They go on to say that "Objects have mechanical energy if they are in motion and/or if they are at some position relative to a zero potential energy position (for example, a brick held at a vertical position above the ground or zero height position)."
OK, so all these "authorities" tell me that Mechanical Energy is related to motion or position. But then, Leslie told me (this morning over breakfast), that the PET curriculum (I think; correct me if I'm wrong, Leslie!) uses the word "Mechanical Energy" to describe the process of transferring energy! She mentioned this also in class, this morning. In some weird way, this makes sense to me, using Schroeder's definition that Mechanical Energy is energy that is related to forces, and thinking about work as the mechanical process of transferring energy, which is also related to forces.
In the beginning, it seemed so simple, but then- it's not... Is that why the teachers discussed this issue for over seven minutes in the episode above? They could've just called it good after Tim's and Derrick's definition, but they went on talking about it. Jim pointed out that he had seen Mechanical Energy being used in different ways and that it's a rather complicated term.
I don't know if I have a point that I want to get across here, other then pointing out again that the use of terminology seems to be problematic if there is no consensus of what the term is supposed to mean exactly. And this goes toward the issue of unregimented vs. regimented discourse, I think.
This episode also seems to be related to Stamatis' "provocative claim" about what they're doing being a "common negotiation of meaning" (Amy's field notes for this morning, 9:51).
I apologize for the rawness of this post, I'm thinking while writing. Thoughts? Ideas? Suggestions?
In a flash of brilliance, I just figured out what mechanical energy is, at least to my own satisfaction. It is the energy that goes with visible, macroscopic motions or configurations. Visible, big. Not small, like thermal energy is kinetic energy of molecules. Not invisible, like the configurations of charges. But to your eye, how high is it, how stretched is it, how fast is it, etc. So, it is the set of easiest-to-see energies. (Obviously, ironically, excluding light.)
ReplyDeleteAnd by the way, just popping off about PET: I will go ahead and provocatively suggest that their choice to use mechanical energy as a term for an energy transfer was really not ultimately... good. I understand the spirit behind it to be some connection to work, but to make it sound so much like a form of energy that is not a form of energy is not pedagogically wise, in my opinion. If it IS ontologically different it should sound ontologically different. "Working" in modelling is a good example of this kind of alignment effort.
@Hunter: Hope you, Eleanor, Hazel, Alvin, and Dexter are well settled in Texas!
ReplyDeleteI'd like to understand better this "invisible" notion, especially with your example of configuration of charges. I include electrical potential energy in "my" "set" of potential energies (along with gravitational potential and spring/elastic potential - for which I have a model relating it to electrical potential energy - and more generally those energies associated with shape or configuration - or if I'm talking about forces, those energies associated with conservative forces). And "my" notion of mechanical energy is the sum of kinetic (though I will preliminarily agree to macroscopic kinetic) energy + potential energy of all the objects in my system.
I should take a step back, though. Why are we interested in mechanical energy?
we are interested in it only because people will not stop saying it.
ReplyDeleteby invisible, i meant literally, that charges are invisible, and also electric potential energy is usually excluded, i would guess, from what those who won't stop saying mechanical energy usually mean by mechanical energy. i think "they" mean gravitational, elastic, kinetic. this is informal observation though. it would be interesting to ask some physics faculty pointedly what they think it is. i see why you include it, i previously also included it, when i cared to define *for myself* what i thought mechanical energy is, and for the same reasons as you, as far as i can tell. now, though, i don't care to define ME for myself, except in so far as i need to define for myself what i think other people mean when they say it.
We're doing great in TX. Love it!
@Hunter's "we are interested in it only because people will not stop saying it"...
ReplyDeleteHa! And this ties in with one of Benedikt's points about problematic uses of terminology, which is of course a point about language itself.
Are there times when it is productive to gather up some forms of energy, lump them together, and then re-name that lump? I do have a notion of Mechanical Energy, but that's really only because, as Hunter points out above, I had to in order to engage with other people or textbooks who were using the term.
This might be a case where we should be prescriptive rather than descriptive, especially because who cares (or less pejoratively how is it productive) if we lump certain forms of energy together into mechanical energy. For the linguistic notions of prescriptive vs. descriptive, the Wikipedia entry is pretty good: http://en.wikipedia.org/wiki/Linguistic_prescription
I've tried to dump Mechanical Energy from my lexicon (though I'm not sure how successfully) since every time I use it, I mean the sum of those kinetic energies + those potential energies I happen to be paying attention to in my system. I hadn't really thought until Hunter pointed it out that mechanical energy might not include electrical potential energy - I had just assumed (possibly incorrectly) that since mechanical energy was introduced in the first semester of physics, we didn't talk about electrical potential energy then since we don't consider it in the typical sequence until after mechanics. It is incoherent for me at this point to exclude electrical potential energy from the set.
I don't know why I'm still engaged with this though, because if I'm just using the term because other people are using it, we can negotiate that meaning as needed. It's probably just an example of "someone on the internet is wrong!" syndrome.