After starting to build the Energy Transfer Model (ETM), we have the beginnings of an idea about change in energy. In order to start playing around with energy transfer, we need a common vocabulary.
The front of the packet gives students a place to write down the information. It is handy to use as a reference while they are still getting used to thinking about energy (and also later in the year when they forget an equation).
We won’t need the equations just yet, so we’ll save that column for later. Some symbols, names, and the beginning of an understanding of what the type of energy is would be helpful to start.
Today is going to be one of those days where I do most of the talking (ugh). I try to make it as brief as possible so that they can get back to learning. To that end, we’re only going to capture four types (enough to get going with most problems we will consider in this class anyway) right now. There are plenty of spaces to record other types of energies as we recognize them.
I missed this in the last post, but usually at the end of the spring/cart experiment (when we first say the word “energy”) I ask them to get out all of the English Language uses of the word energy that they can muster. As in: Ugh, I have no energy today. Turn off the lights and save energy. Energy drinks. 5 hour energy. She’s full of energy. Hey, yeah, you have a rad energy, man. Etc, etc. So we note the Physics is a Foreign Language That Sounds Just Like English problem (that we’ve been talking about since our 2nd unit—balanced forces). We’ll have to be really careful to define energy in physics and use that definition in class if we want our model to be good at making predictions.
Beauty—er, Energy—is Pain?
Here we go.
I choose the kid who I think will give me the best reactions (you’ll see what I mean in a minute or two). I walk over to them and quietly tell them to overreact. Then I give them a very dramatic (and very light) “punch” to the shoulder. They will usually cry out, “Ow!” or fall out of their chair (I did just tell them to overreact).
Moving back over to the front, I ask everyone what would happen if I put more energy into the punch. [“He would scream!” or basically, “It would hurt more.”]
So pain would be a crude way to get a feel for how much energy is stored (and maybe going to be transferred) as we start thinking about the types of energy. [Some laughter from the kids. Maybe nervous laughter from the chosen kid.]
For each type of energy, I try to make the name (and then symbol) the last piece of the discussion. First we need to identify the “new” way of storing energy, what it depends on, when it is present. Once we have that sense, then we can name it.
So, first up: Austin—let’s give the kid a name for easy description here. Anyway, so, Austin, would it be okay if I threw this ping pong ball at you with a speed of 1 m/s? You aren’t allowed to catch it. It has to bounce off of you. But 1 m/s. Slower than you walk. Are you okay with it?
[Sure, of course he is.] How about 5 m/s, a little faster than you jog? 10 m/s? [He’s pretty much okay with all of that, though he starts getting a little more nervous about it at 10 m/s. Though, note: I do not ever throw anything at Austin.]
Okay, well. How about this baseball? Are you okay if I throw it at you at 1 m/s? Remember, it has to bounce off of you. [Yep, that’s not that big of a deal.] 5 m/s? [Now we’re getting nervous.] 10 m/s? [Well, no thank you.]
Fine. Then, [pulling the final ball up from its hidden place on the chair behind the table and eliciting a great reaction from everyone] how about this bowling ball? 1 m/s? 5 m/s? 10 m/s?
So it seems like there is a type of energy that is being stored in the motion of these objects. Austin is afraid of the pain that would be caused by them. What does it depend on? [They will pretty readily come up with mass and velocity (or speed).] Is it speed or velocity? [They usually quickly say that it is velocity, probably because that sounds like the more physics-y word (and not for any other reason).] Well, Austin, do you care if I throw the bowling ball at you from over here or over there? It is going to be going the same speed and bounce off of you either way. Okay, so the direction doesn’t matter. [Ah, speed then, not velocity.]
When is this type of energy present? [Something is moving.] Are you ready for a name? [Often there are some who gathered facts in middle school and are ready to give name it Kinetic Energy without you saying anything at all. But if not, they are fine with that name anyway.] We’ll use a big, capital K for the symbol. [Of course, different classes/books/teachers/etc might use slightly different symbols. These are just what I use with my students, and I think they are pretty typical ones.]
Gravitational Interaction Energy
[Pulling an extra chair over behind Austin.] Let me know when you start feeling worried. [Honestly, just saying that is pretty troubling, but he’s willing to go with it.] Now? [Holding the ping pong ball just barely over Austin’s head.] Now? [Holding it about a foot above his head.] How about now? [Now I’m standing on the chair and holding it as far above his head as possible.] None of those really worried you? Alright.
How about this? [Repeat entire sequence with baseball. Austin worries a little bit about the 1 foot distance, but really worries when I’m standing on the chair.]
Okay, then—[I can’t even get through the sentence before the cries come from his classmates of, “Don’t do it!” But of course I do it anyway.]—how about now? [Holding the bowling ball just above his head. And yes, he’s already worried!] Now? And—[as I start climbing up on the chair]—it’s okay; I’ve only dropped this once. So how about now? [Yes! Very worried!]
So I think we’ve found another way of storing energy. What does it seem to depend on? [Weight (or mass). Height.]
Weight or mass? [Similarly to the speed/velocity question, some will decide mass just because it is the “physics word”(?!).] Okay, so would Austin be just as worried if we did the same exercise on the moon? [Well, no, not as worried.] How about if we did this far out in space, far away from all other things? [No, he wouldn’t be worried at all. The ball wouldn’t fall. This is often the first point at which they start to want to yell out their middle school memorized facts, and I try to hush the “potential” word usage as much as possible. We’re not even ready for a name yet, anyway, much less a confusing one like that!] So, weight, then, right?
It depends on weight—and the other one you mentioned before was height. Would Austin be less worried if we did the same thing downstairs in the Bio Lab (directly below our classroom)? [Yes, that wouldn’t matter.] So it’s not exactly height, right? [We get around to an idea that it is more like how high the ball is above Austin, not height above the floor, not height above the surface of the Earth, etc. They come to some sort of idea about how it depends on ∆y, though of course there was no ∆y in our exercise (just an imagined ∆y). But we also know that it doesn’t really depend on Austin (because if it had been Brandon or Noah sitting in that seat, it seems like the same amount of energy would be stored in the raised bowling ball).]
So it depends on weight and also a relative height. And since it depends on weight, it would only exist if the object is near a planet. So it depends on the relative distance between an object and a planet. And it depends on there being an interaction between the object and the planet (the gravitational force).
There is a sort of class of energy types that depend on interactions and relative positions. We call them “Interaction Energies.” So this is one flavor of Interaction Energy. [To consider: was Kinetic Energy an Interaction Energy?] For interaction energies, we use a big capital U, then give it a subscript for the type of interaction energy. So for this (Gravitational Interaction Energy) we get Ug.
Spring Interaction Energy
There’s another way of storing energy that we’ve really already started thinking about. When we did the experiment with the springs giving their effect to the carts, there was some sort of transfer of energy, right? How did the carts store the energy? [As kinetic energy.] Where did it come from? [It came from the spring.] So what kind of energy was stored in the spring?
A new type, right? Okay, now is this type more like Ug (depends on an interaction and the relative position of multiple objects) or more like K (can be stored in a single object, does not depend on position)? [After a little talking, we decide it depends on how much the spring is stretched (or compressed), and that there has to be another object for there to be a spring force (interaction).]
So, spring interaction energy, then. And symbol? [Us.]
Change in Thermal Energy
Okay, check this out: I push the marker box and it starts sliding across the table. It has a bunch of kinetic energy and then—where did it go? [Immediate answer: friction.]
That’s a force, but before (in the cart/springs experiment), we saw that energy was transferred from one object to another [Us to K]. Is this different? Or is it being stored somewhere? What is storing the energy? [The table is storing it. Friction interaction energy? There’s definitely an interaction, but does it depend on the relative position of the two objects? Not in the same way as the object/earth or object/spring from before.]
Let’s look at what happens with the matter models when you slide them against each other. Look at the atoms and see what happens to them.
The get jostled, right? They shake. Wait, the atoms in the table shake or the atoms in the box shake. [Try it again with the two matter models. The atoms in both shake!] Okay, so at first there was Kinetic Energy stored in the marker box. Then the box atoms and the table atoms jostled each other. So the box slows down, but the atoms are moving in faster random motion. So it is kind of still kinetic energy, but it is teeny tiny kinetic energies stored in the atoms moving randomly, not the whole box moving in one direction.
But do you ever see it happen in reverse? That is, everything gets colder and the marker box suddenly starts sliding across the table? That would freak you out, right? So there’s another thing that is different about this flavor of energy. Once energy is stored as the tiny random atom motion, it mostly seems to stay that way.
And one more thing: does this only happen when there is friction? Is there any other way that the atoms can get jostled and have faster random motion? [Try hitting the matter models against each other. So an impact (normal force) can also jostle the atoms.]
Now, aren’t the table atoms and box atoms already moving even before I slide the box across the table? How much teeny tiny kinetic energy is there beforehand? That’s hard to figure out. But it is pretty easy to see how much it changes (because it is easier to see how much macroscopic kinetic energy the box has when it is first sliding). So we usually just keep track of how much this type of energy changes.
We use the word “heat” to mean something specific (and different from this) in physics. We call this Thermal Energy (and so we will keep track of Change in Thermal Energy, or ∆Etherm).
Interaction, not Potential
It will come up eventually (thank you, middle school), but I try to keep it from happening on the first day (for the sake of those not already burdened by the English Language problem of “potential”). We’ll talk about why “interaction” does a better job of helping them think about when the energy is present (when there is that type of interaction; also, more than one object has to be involved (can’t have an interaction between one object)) than “potential”. And I give them the money quote from a student my first year:
“Everything has potential energy. Someone could potentially come up and push it and then it would be moving.”
Now, packing 4 flavors of energy (20 to 40 minutes worth, just depending on how the students in that particular class react to everything), we’re ready to start drawing some pie charts (next post in this series
, and coming soon update: pie chart post complete!).