tag:blogger.com,1999:blog-5088860151651047897.comments2017-08-08T19:49:18.516-04:00Jacobs PhysicsGreg Jacobshttp://www.blogger.com/profile/03854009948036330746noreply@blogger.comBlogger795125tag:blogger.com,1999:blog-5088860151651047897.post-61201759651434427462017-07-31T05:09:59.011-04:002017-07-31T05:09:59.011-04:00Yeah, everyone thinks Hewitt is great, and he is.....Yeah, everyone thinks Hewitt is great, and he is... for people who have had physics before, or for people who are especially curious. For a typical student - especially a young student - Hewitt is just irrelevant digression after irrelevant digression. <br /><br />Email directly and I can share my full conceptual course with you. Really, 43 days now counts a a full year's credit? I mean, my bugaboo is shortening baseball games, but if they played three innings over 45 minutes, I'd probably ask for my money back. :-). Not your fault, of course. Probably you can use my materials, but cut out 1/3 or more of the topics.Greg Jacobshttps://www.blogger.com/profile/03854009948036330746noreply@blogger.comtag:blogger.com,1999:blog-5088860151651047897.post-67883143266004127232017-07-31T04:26:36.324-04:002017-07-31T04:26:36.324-04:00Hi, I'm teaching conceptual physics this semes...Hi, I'm teaching conceptual physics this semester. I meet with Ss for 80 minutes each day. There are 43 days on the schedule and it will count as a full credit class. I tried using Paul Hewitt CP text last year but it seemed too wordy and I slogged through. Do you have a syllabus available or suggestions on the specific topics to cover? I would appreciate any help.<br />Thank you Show Me Your Work.....pleasehttps://www.blogger.com/profile/15201820623998824387noreply@blogger.comtag:blogger.com,1999:blog-5088860151651047897.post-51245138771653677422017-07-24T19:34:31.972-04:002017-07-24T19:34:31.972-04:00Nadia, I think you already convinced them! "...Nadia, I think you already convinced them! "Once they're used to the process and we're in review mode..." If you've worked through evidence-reasoning-conclusion all year on AP-style problems, then I don't think they'll have any issues adapting to the format of the actual AP exam. You'll have ingrained the correct reasoning process so that format doesn't matter anymore.Greg Jacobshttps://www.blogger.com/profile/03854009948036330746noreply@blogger.comtag:blogger.com,1999:blog-5088860151651047897.post-89444267078002250662017-07-24T18:43:42.246-04:002017-07-24T18:43:42.246-04:00It's crazy how such a small change makes a big...It's crazy how such a small change makes a big difference. I think once they're used to the process and we're in review mode, we can work on AP problems that aren't formatted this way since that's how they'll be on the AP exam. We'll have to practice ignoring the answer choices until we've finished working through the justification. Any idea how to convince students to do that?Nadiahttps://www.blogger.com/profile/17863223415849222693noreply@blogger.comtag:blogger.com,1999:blog-5088860151651047897.post-16229484623163681712017-07-23T05:30:59.698-04:002017-07-23T05:30:59.698-04:00I just saw this, but I wanted to share:
Like the A...I just saw this, but I wanted to share:<br />Like the AP reading, I grade one test question for ALL students. Then I start at the top of the stack and grade the next question. It helps with consistency because I'm grading the same question repeatedly and makes the process pretty anonymous (except for easily recognizable handwriting). I'm not influenced by scores on other questions and I move through the grading faster.<br /><br />I write the total points on each question with a fairly large, easily-found number. I have a spreadsheet with the students' name on the top and question number on the side. Adding up their scores is a simple matter of flipping through each test and entering numbers with the number pad, using a sum function at the end. (If you were going to do it in your head or type it into a calculator, anyways why not make it a more permanent record?) I don't alphabetize stacks of tests: I type the student's name into the spreadsheet as I pick up a test from the pile. Excel can alphabetize the spreadsheet for gradebook purposes. I don't even write the total score on the test, the students can add the points themselves and compare them with the total on the gradebook. (This works best when the test already says how many points each question is worth. If I don't have that information, I'll write it on the board when they get the test back.)<br /><br />Like the "bubble room", my spreadsheet becomes a gold mine of data to sift through. With a few formulas and conditional color formatting, I can immediately see how each student did on the test and how the class as a whole performed on each question. I can better target my reteachig to the class and identify the students who need one-on-one help. This is probably the best part about the process. I acquire all this information without any real extra work on my part and it makes grading consistent and relatively quick. Nadiahttps://www.blogger.com/profile/17863223415849222693noreply@blogger.comtag:blogger.com,1999:blog-5088860151651047897.post-91534858410661984752017-07-20T15:04:16.500-04:002017-07-20T15:04:16.500-04:00I'm really impressed that you can get to April...I'm really impressed that you can get to April having learned all the fundamentals. I love the idea of letting the class play at the end and really apply what they've learned, but in my planning and from the AP pacing guides I can't seem to find enough time. Do you have your own pacing guide or schedule that you would be willing to share?Anonymousnoreply@blogger.comtag:blogger.com,1999:blog-5088860151651047897.post-48953874276114875142017-07-17T21:34:22.230-04:002017-07-17T21:34:22.230-04:00Please, do elaborate! Do you have any particular s...Please, do elaborate! Do you have any particular strategies you use during mechanics to prepare students for the field concept in electrostatics?<br /><br />-AmandaUnknownhttps://www.blogger.com/profile/04763182754503624840noreply@blogger.comtag:blogger.com,1999:blog-5088860151651047897.post-27524049709882012762017-07-14T10:08:14.928-04:002017-07-14T10:08:14.928-04:00Excellent, excellent advice. I'll elaborate l...Excellent, excellent advice. I'll elaborate later. I start my second semester class with the point charge E field rather than the Coulomb force because I want to emphasize the field concept from the start. Starting with the electric force suggests that the course is just like mechanics but with a new force. It isn't, and the jump to Gauss' Law is quite challenging if you start with forces. CCPhysicisthttp://doctorpion.blogspot.comnoreply@blogger.comtag:blogger.com,1999:blog-5088860151651047897.post-18024629541232009152017-07-12T07:50:57.424-04:002017-07-12T07:50:57.424-04:00do you still have the exam bank, problem solutions...do you still have the exam bank, problem solutions, reading quizzes, and guide to using?<br />Unknownhttps://www.blogger.com/profile/14979158879272465236noreply@blogger.comtag:blogger.com,1999:blog-5088860151651047897.post-28311689538493439262017-07-09T11:48:04.876-04:002017-07-09T11:48:04.876-04:00Thanks for the info!
My guess is that it works m...Thanks for the info! <br /><br />My guess is that it works much like the rotation detector we also have, sensing fractions of a revolution and turning the accumulated displacement into distance by knowing the radius of the wheel. I've always assumed that the rotation detector uses a toothed wheel encoder with what looks like 0.5 deg resolution, but there isn't room for that in the Smart cart. <br /><br />What I like is that the Smart cart also has a load cell. We only have a few of those, for demonstrations rather than lab or active learning experiments. CCPhysicisthttp://doctorpion.blogspot.comnoreply@blogger.comtag:blogger.com,1999:blog-5088860151651047897.post-59317996932195542062017-07-08T21:33:31.179-04:002017-07-08T21:33:31.179-04:00I may not know much, as some character once said ....I may not know much, as some character once said ... but I was taught that the best thing to do is assume everything is a point, a line, a sphere, or a cube. Why? Because it simplifies things enough so that we can think about ... and model them. If you follow this path, then all the questions ... what if it isn't uniform? what if it ... ? simply disappear. It's what I constantly tell my students, what I constantly model for them. I tell them SIMPLIFY. ASSUME symmetry. "Much less than" means "IGNORE it. Think "a mosquito hits a speeding freight train" and think "what effect does it REALLY have?" If we try and account for all the "lumps" we will never arrive at a reasonable approximation. I also teach them that we are approximating reality and that as we use increasingly more sophisticated math ... in later courses ... we get "closer" approximations. The idea here is that we need to keep the moving parts to a minimum.Jeffrey Stantonhttps://www.blogger.com/profile/17086589576385374773noreply@blogger.comtag:blogger.com,1999:blog-5088860151651047897.post-47208192007023883642017-07-07T21:51:14.723-04:002017-07-07T21:51:14.723-04:00CCPhysicist, I completely agree with you - I love ...CCPhysicist, I completely agree with you - I love the bumpy track problem. It's exactly what we are looking for in a quantitative-qualitative translation question: connect mathematics to a thought experiment about a situation that the student is unlikely to have seen specifically in her or his own laboratory experience. <br /><br />As for the PASCO carts... you're right that the data is hardly different from what.you'd get with a motion detector. My instinct is that your point about taking numerical derivatives is dead-on. However, I still haven't figured out exactly how these new toys work. <br /><br />If you already have motion detectors that are easy to use, then there's no reason to break the bank to buy smart carts. But if you need to replace a dead motion detector or labquest, a smart cart might be a good alternative.Greg Jacobshttps://www.blogger.com/profile/03854009948036330746noreply@blogger.comtag:blogger.com,1999:blog-5088860151651047897.post-69823103991741188492017-07-07T21:43:49.156-04:002017-07-07T21:43:49.156-04:00Oh, and kudos to Zach! That is the way to think l...Oh, and kudos to Zach! That is the way to think like a teacher who is also a physicist. CCPhysicisthttp://doctorpion.blogspot.comnoreply@blogger.comtag:blogger.com,1999:blog-5088860151651047897.post-13159156111530931532017-07-07T21:39:48.414-04:002017-07-07T21:39:48.414-04:00I must not have been following you closely when la...I must not have been following you closely when last year's exam was released. I have to say that I rather like the bumpy track question because there shouldn't be any guessing at all! They tell you what happens, and the rest starts with basic physics based on just a few obvious forces. Sort of the same point you made about this year's exam. But that isn't why I posted a comment. I have a question about those Pasco Smart Carts. <br /><br />The data show the same kind of drop outs that we get with a Pasco motion detector and a cart with a reflector. Annoying, which might be a good enough reason to spend money (money we don't have right now) to get some. How does it happen in this case? Is it because they get distance from a wheel encoder, which fails when the cart bounces off the track momentarily in this experiment? If so, the data should be "perfect" otherwise, but I see odd defects for smooth sections of the track. Are those just the usual minor effect of increased uncertainties resulting from subtraction when the program calculates the velocity estimate, like we also get with a motion detector? Are they worth the money? CCPhysicisthttp://doctorpion.blogspot.comnoreply@blogger.comtag:blogger.com,1999:blog-5088860151651047897.post-73410642720884710212017-06-30T18:29:21.040-04:002017-06-30T18:29:21.040-04:00Not going to lie, I didn't much care for that ...Not going to lie, I didn't much care for that particular problem. When I've run this by my students who took that exam and then took my AP Physics 2 class the next year, the results basically read like an IQ test more than a physics test, because it's written as a thought experiment and not a real experiment. What I mean by that is, students fell squarely into two camps: those who guessed wrong and then tried to justify their guess with physics reasoning that was doomed from the start, and those who guessed correctly and whose answer probably looked reasonable pretty much no matter what reasoning they followed it up with. Furthermore, every student who got it wrong came away with the conclusion, "Oh well, I guessed wrong [about the graph shape]," rather than learning anything from it.<br /><br />I'd have liked this problem better if they had done what they did with the Lab Groups Finding Coefficients of Friction problem from this year's exam, and collected class data and asked the students to analyze it rather than predict the shape of a graph. That way, students could show their analytic skills in a more step-by-step manner that more accurately reflects their level of scientific knowledge.<br /><br />Lastly, a more general point: "Predict what will happen," or, "Predict what the data will look like," is always going to be a tricky format of question in a course where hypotheses can and will be wrong all the time, and that's supposed to be okay, as long as one knows how to test it and determine from data the reason why the hypothesis turned out to be incorrect. So I guess what I'm saying is that Bumps was a theorist's question in what I thought was supposed to be primarily an experimentalist's course and exam.Willnoreply@blogger.comtag:blogger.com,1999:blog-5088860151651047897.post-47128621992206416252017-06-30T18:03:57.372-04:002017-06-30T18:03:57.372-04:00My initial thought seeing the disk hit the rod was...My initial thought seeing the disk hit the rod was that the problem was going to ask about the "sweet spot" on a bat, which involves a different imperfection: that the rod isn't perfectly rigid. But since the sweet spot would still be closer to the far end of the rod than toward the center, it wouldn't change the answer at all (as far as I can see). It might however lead to some interesting data sets and class discussion if a class's lab method of testing this is to launch some clay at a baseball bat.Willnoreply@blogger.comtag:blogger.com,1999:blog-5088860151651047897.post-2677991217131105142017-06-19T13:22:25.089-04:002017-06-19T13:22:25.089-04:00(1) From a force point of view, the two forces (gr...(1) From a force point of view, the two forces (gravity and normal) are equal and opposite and cancel out. When forces cancel this means there is no acceleration - which matches our scenario. <br /><br />(2) From a Work point of view, the two forces (gravity and normal) are doing the same amount of work, but Normal is doing positive work, and gravity is doing negative work. When Works cancel, there is no change in kinetic energy - which matches our scenario. <br /><br />(3) From a Potential Energy point of view, as the elevator rises, it gains gravitational potential energy. Also, Normal force is doing positive work. We see that all of the positive work that the Normal force does is stored as gravitational potential energy; so, the elevator's KE hasn't changed, but it has gained potential energy - which matches our scenario. <br /><br />----------------------------------------------------------------<br /><br />The key point is how gravity is considered with respect to energy. It can EITHER be considered as a conservative force (i.e., the work it does is CONSERVED as potential energy as in explanation 3 above), or a non-conservative force (i.e., the work it does is NOT conserved as potential energy as in explanation 2 above). In explanation 2, we don't consider PE related to gravity, and in explanation 3, we don't consider the work related to gravity. <br /><br />I hope that helps!Michaelhttps://www.blogger.com/profile/06967354327638923275noreply@blogger.comtag:blogger.com,1999:blog-5088860151651047897.post-44565282893318166652017-06-16T18:58:53.545-04:002017-06-16T18:58:53.545-04:00Sorry I wasn't clear, but my context was learn...Sorry I wasn't clear, but my context was learning (or perhaps using without error) rather than the grading rubric or getting them to annotate their work. But I also think that equations and description sometimes come into conflict with one another, and that might lead to resistance by students. Let me try again. <br /><br />I take them directly from Delta E = W to Ef = Ei + W and then use the latter when doing examples of how to solve problems for only one reason: it mirrors Vf = Vi + a*t and is thus more consistently memorable. Teaching philosopy, or pedagogy if you prefer. Other orderings, whether Vi + a*t = Vf or Vf-Vi = a*t will convey the same physics, but those do not convey the idea that Vf is actually a function V(t), and functions are always defined on the left side of an equation. [I've never seen a textbook present Vi + a*t = Vf as The Equation.] I want them to think of momentum and energy the same way, as an alternative way of understanding the dynamical evolution of a system, and use that standard ordering to get work in the right place every time. <br /><br />Order is unimportant in Pf = Pi or Lf = Li problems, and that is about all you get at your level and mine. External impulse problems are rare in intro classes. I don't care how they write it, but I approach momentum consistently just for consistency. Pf = Pi + Favg*t, if you like, even if that is rarely used on a test. <br /><br />What complicates all of this is the descriptive process tends to be in temporal order. That is what leads to your statement (and mine) about what happens in that collision. We draw or describe motion diagrams from initial to final, and we should translate that into mathematics in the same sequence, Ei + Work = Ef, which is the exact opposite of how the equations are presented in textbooks! One of the two has to give if we are to push description first, before math, because students take The Book as gospel. CCPhysicisthttp://doctorpion.blogspot.comnoreply@blogger.comtag:blogger.com,1999:blog-5088860151651047897.post-80521474871477697772017-06-15T08:37:50.111-04:002017-06-15T08:37:50.111-04:00Hi Greg! I would also love to get some information...Hi Greg! I would also love to get some information. I'll be a first year teacher this year and will be teaching some conceptual physics. I've just sent you an email as well I'd love to make it to one of your summer institutes, but I don't think that will be an option this summer. Brittany Neashttps://www.blogger.com/profile/07654225011343013750noreply@blogger.comtag:blogger.com,1999:blog-5088860151651047897.post-75184981694846796572017-06-14T14:53:55.093-04:002017-06-14T14:53:55.093-04:00Edited for reasons of previous stupidity!
Hi Micha...Edited for reasons of previous stupidity!<br />Hi Michael. Thank you for your explanation but I'm still struggling; I must be missing something but please indulge me! Surely in your first system, gravity and normal forces are "conservative" and cancel, not as you describe. In your second, gravity is conservative and the elevator force (generated by the motor) is non-conservative. By this I understand that the elevator motor must perform more work than the increase in energy of the system because some is lost as heat, noise, vibration etc. I don’t understand what you mean by “gravity’s energy”. At the end of the ascent the increase in energy, mgh Joules, is the same work performed, F*d, also Joules (less losses in the second case) but I can’t see how F is derived. It still seems like no net work has been done at the top of the elevator journey, but it apparently has. I can't quite picture why just because one force is conservative and the other is not they can't be calculated in the ordinary way for vectors, and therefore sometimes cancelled. In this case I would find it hard to draw a vector diagram showing a force which has moved. Would you be able to do that?Hamish griffithshttps://www.blogger.com/profile/09336250080012848887noreply@blogger.comtag:blogger.com,1999:blog-5088860151651047897.post-47828681878448076082017-06-14T11:06:03.962-04:002017-06-14T11:06:03.962-04:00This comment has been removed by the author.Hamish griffithshttps://www.blogger.com/profile/09336250080012848887noreply@blogger.comtag:blogger.com,1999:blog-5088860151651047897.post-35640534342442117872017-06-13T22:02:21.880-04:002017-06-13T22:02:21.880-04:00Hamish, great question! I believe the issue lies w...Hamish, great question! I believe the issue lies with "double-counting" the force of gravity. The general formula for the conservation of energy is:<br /><br />PE+KE+Wnc=PE+KE<br />In words that would be: potential energy initial + kinetic energy initial + work done by non-conservative forces equals potential energy final + kinetic energy final<br /><br />So, where do you consider gravity's energy? As Work =F*d=mg*h? Or as PE=mgh?<br /><br />If the earth IS NOT part of the system, then there are two non-conservative forces: gravity and normal. Since they point in opposite directions, they cancel (or since one adds energy and the other removes energy, they cancel). Since the earth is not part of the system, then no PE can be stored. The equation boils down to 0=0.<br /><br />If the earth IS part of the system, then there is one conservative force (gravity from the earth) and one non-conservative force (normal force from elevator). The normal force does positive work as it lifts the person up, and on the other side of the equation, positive PE is gained since the person has been lifted off the ground. The equation boils down to N*d=mgh (with N=mg due to equilibrium and the distance moved is h, so mg*h=mgh). <br /><br />I hope that makes sense!<br /><br />Thanks,<br /><br />-Michael<br /><br />Michaelhttps://www.blogger.com/profile/06967354327638923275noreply@blogger.comtag:blogger.com,1999:blog-5088860151651047897.post-12062840783937832862017-06-13T10:29:24.807-04:002017-06-13T10:29:24.807-04:00I hope you don't mind me joining this rather l...I hope you don't mind me joining this rather late. I used to fly aeroplanes and intellectual difficulty arises there also. My problem and, it seems that of others, is this; in the "constant velocity" ascending elevator the normal force exactly equals the "contact force" which is exactly opposite. There is no "net force" because the vector quantities exactly cancel. Even if you allow that the elevator does "positive" work and gravity does "negative" work the two also appear to exactly cancel. How is any net work accomplished? (It obviously is because the system ends up with more potential energy but I can't see that a net force has been moved through a distance, which was always my understanding of the definition of work. I have ignored the acceleration/deceleration which also cancel in a friction-less system). Interestingly (I think!) in the aeroplane example there is an analogous situation in flight at constant velocity; there thrust and drag are equal and opposite and similarly cancel but despite movement of the system there is no change in energy.<br /><br />Hamish Griffiths Hamish griffithshttps://www.blogger.com/profile/09336250080012848887noreply@blogger.comtag:blogger.com,1999:blog-5088860151651047897.post-4916827473704524772017-06-13T09:03:37.034-04:002017-06-13T09:03:37.034-04:00Great point, CC... I certainly teach the mathemati...Great point, CC... I certainly teach the mathematics of energy conservation by starting with the change in the system energy, setting that equal to the sum of changes in each particular form of energy. Your approach is a good one.<br /><br />So how does that square with my discussion above? Perfectly. A typical student would start the derivation with L = L', and go from there. Your student would start with 0 = L'-L. Works for me! <br /><br />The AP exam is graded by humans, not robots, for a reason. We look to see if the student clearly communicated the starting point of the derivation -- that is, didn't just plug variables into equations, didn't just write a final answer, but said clearly what fundamental principle was used as a springboard.<br /><br />Thus, even the words "conservation of angular momentum" would be sufficient. So would "the rod-disk system's angular momentum can't change." <br /><br />And, note the point I made about annotation. I did see a number of papers with odd groupings of terms... often these were clearly labeled with the words "before collision" and "after collision". My overwhelming preference is for a student to communicate in both words and mathematics, in order to make discussions of mathematical order unimportant. :-)<br /><br />GCJ<br /><br />Greg Jacobshttps://www.blogger.com/profile/12121422726610824760noreply@blogger.comtag:blogger.com,1999:blog-5088860151651047897.post-53310501639801088392017-06-13T08:50:57.487-04:002017-06-13T08:50:57.487-04:00I have a question about teaching philosophy. You ...I have a question about teaching philosophy. You wrote "with before the collision left of the = sign". All of dynamics is structured as change (final minus initial) equals whatever drives the dynamical change. That naturally leads to final velocity equals initial velocity plus a*t, or final energy equals initial energy plus net work. I encourage my students to stick with that ordering even when the change is zero. Do you have a reason for writing inital plus physics equals final? I have toyed with, and found success with, that reordering for collision and energy problems but would not dare try it with the kinematics equations. As I see it, the conflict is between consistency (which tends to eliminate fundamental sign errors) and linking the equation to a pictograph or textual description of what is happening. The latter tends to run from past to present to future. <br /><br />Your thoughts? CCPhysicisthttp://doctorpion.blogspot.comnoreply@blogger.com