Milo's Solar System: quantitative replacement for the PHET "my solar system" orbit simulator

 

As 2020 dies, so does flash animation. I only care about two sites that will no longer be functional after flash is deprecated: homestarrunner.com, and PHET's "my solar system" orbit simulator (screenshot shown).  

Here's a link to the 2021 replacement: Milo's Solar System.  Read on for details.

See, I can't do live experimentation with gravitation. I can *predict mathematically* how to retain a circular orbit when the mass of the central planet is doubled:  Set the formula for gravitational force equal to the centripetal force.  Solve for speed v - you get root (GM/d).  So by Bertha's Rule of Ones*, doubling the mass means the factor of change for the speed is root(1*2/1) = root 2.

*Also known as the Factor of Change method

My students expect that the next step after any mathematical solution is to provide experimental evidence.  How to do that?  I can't very easily double the mass of the Earth, then speed up the moon by 40% to see if it still orbits in a circle.  And the gravitational force is too weak to set up a small-scale solar system in the lab.

For years, I've used the free PHET simulation linked above to show, if not real experimental evidence, at least something that has the look and feel of an experiment.  You can see me performing several calculations and experiments as quantitative demonstrations in this College Board video.  

Now that flash is gone, we need a replacement for the phet simulation.  We have one.

For his senior community service project, Milo Jacobs has essentially replicated "my solar system."  I'll call it "Milo's Solar System" for now.  Open it and press play - you'll see the yellow planet in a circular orbit.

To solve the "double the central planet's mass" problem I posed above, click on the big blue central planet.  You'll see its mass as "100,000" arbitrary units.  Change that to 200,000 and hit play - the yellow planet glides off the screen, obviously not in a circular orbit.  But, we know that the speed has to be root-2-times bigger than before to keep a circular orbit!  So, click on "undo motion", and change the vy value from 100 to 141.  Press play again... and you see a circular orbit again!  Yay!

I've written out twelve different versions of this question in this file.  I use it as a "come and show me" exercise - I give one randomly to each student.  They come to my desk to show me their solution.  I check that they've used Newton's law of gravitation and circular motion methods; then whatever their answer, I have the student use the simulation to check whether they in fact get a circular orbit with their result.  The student brings me a screenshot of a circular orbit with their new values of mass and/or speed and/or planet position.  If their solution was incorrect, they find out for themselves!  They either redo their prediction, or they come back to ask for help.  (Or most likely, they ask for help from a classmate.)

Milo's Solar System works best with chrome as your browser.  It works well for me on either my windows PC or my iPhone in chrome.  

Please explore - just as on the original PHET, it's possible to drag the velocity arrow, to add additional planets, and generally to explore trajectories.  The main benefit of Milo's simulation over the others I've seen online is that it's easy and fast to make quick quantitative changes to the initial speed, radius, and mass values to check whether an orbit is circular.  

(If you have operation requests or if you find bugs, please post a comment or email me - I can ask the author for changes.  We each tested the questions in the "come and show me" exercise, and they all seemed to work fine!)

Mail time: How should I adjust mid-year now that AP Physics 1 covers fewer topics?

Matt Freeze writes, in the comments section of the post about how AP Physics 1 will cover mechanics-only from now on:

I read your material here and other places often, though I rarely post. I'm the stereotypical "Long time listener, first time caller," guy. Anyways, before the news, I had gotten through the basics of motion, kinematics, force (including centripetal fore), and energy. So as I look at things, all I have left is momentum and collisions, and rotational dynamics. Any commentary on whether I should finish those units as scheduled and then have extra extra time to study for the test, or should I slow those units down significantly, or should I delay starting them, and for the first 3 weeks or so discuss the previous units? I suppose this is a good problem to have....

Thanks for the question, Matt!  I'm sorta in the same place - only difference is I've done momentum but not energy.  

My plan for now is to move through the first contact with energy at a bit slower pace than normal - say, three weeks rather than two.  The new pace will mean spreading out the problem sets so there's generally only one short question per night, and probably doing an extra full-on linearization experiment with springs.  

I'll move quickly through harmonic motion, as always.  No need to belabor a simple concept.

Then, I'll move into rotation.  Usually I introduce the concepts of rotation very quickly - like in the week before spring break - and then let the students do a lot of independent learning after break in the context of reviewing linear mechanics.  This year, I'll spread the rotation discussion out over three weeks or more.  I'll assign more problems, we'll do some in-class lab exercises with rotation that I didn't previously do with the whole class.  

Throughout, we'll spend even more time than usual emphasizing fundamentals with daily quizzes and the 4-minute drill.  After spring break - like, starting about March 25 - I'll be in full-on "putting it all together" mode, except with even more independent experimental work than normal.  

My goal will be to bring as many students as possible up one score.  Those who earn 5s will still do so, and they won't be bored because of the plethora of experimental work.  Everyone else will benefit immensely from the extra time to learn and remember their fundamentals, and from more but less intense practice.

Once we've reached the latter part of the year, pretty much all of my class has learned the necessary skills for success in the course.  Usually, the big difference between the 5-level students and the 3-level students is their performance on recall quizzes.  If someone doesn't remember that the area under a force vs. time graph is impulse, or if they still can't articulate the difference between impulse and momentum, then that someone has no chance when asked advanced questions involving a force vs. time graph.  The more my students can learn their facts through use, through active study, through games, through corrections, the more likely they are to improve their exam performance.

Yes it's true and permanent: AP Physics 1 now covers mechanics only.

The announcement came Friday afternoon, first on the AP Physics 1 homepage and then via email to teachers with an AP course audit:

Colleges agree that Units 8–10 can be removed from AP Physics 1 since they are covered in AP Physics 2; accordingly, Units 8–10 will no longer be tested in AP Physics 1, effective this year.

Is this just for the 2021 pandemic year?  No, this is permanent.

Will AP Physics 2, C-mechanics, or C-E&M be shortened as well?  No.  All other AP courses in all disciplines are remaining as-is even in the 2021 pandemic year.  AP Physics 1 is an exception - it is the only course out of 30+ to be shortened. And this is not a pandemic-related decision (though I'd not be surprised to hear that the pandemic was a catalyst for making this decision sooner rather than later).  

Why are colleges okay with cutting topics in AP Physics 1?  In surveying colleges and universities about their first-year algebra-based physics courses, the College Board found substantial uniformity in the first-semester topic coverage: it's all mechanics all the time.  Yet, in the horse-trading that went into attempting to satisfy everyone when the P1/P2 sequence was established for 2015, waves and circuits were put into P1.  I suspect that this decision was made in large part to meet big-state public high school curriculum standards.  For example, including waves and circuits aligned P1 very nicely with most of the New York Regents curriculum.  

But because waves and circuits are hardly ever part of a college's first-year algebra-based course, the College Board found that universities are happy to give that first semester of credit for just mechanics.  

Well, when I took college physics, things were different.  I'd do things differently.  I don't doubt it. The College Board's job isn't to provide the physics course that you took in college, nor the one you personally want to teach.  

Their mission is a balancing act.  They are trying to increase equity and access to college-level physics.  They are trying to provide a course that meets the needs of a wide variety of university programs and high school programs.  And, because the College Board is the de facto national leader in physics curriculum design, they're trying to lead with a course based on the instructional best practices evidenced over the last several decades of physics education research. 

You might personally have balanced competing ideas and interests differently.  Fair enough.  Just know that the people making these big-picture decisions in physics are people I trust.  I'm aware that teachers are used to assuming incompetence and bad faith in decisions made by educational administrators, likely because so, so many educational administrators across the country have unfortunately earned that assumption.  The College board physics leadership has earned my assumptions of both good faith and competence.  They are experts doing what they think best fulfills their mission.

Isn't this new P1 just Physics C-mechanics without calculus?   In terms of topics, essentially yes.  In terms of style, well, certainly not. The fundamentals of AP Physics 1 haven't changed.  P1 students will be required to explain physics using words, equations, experimental design, diagrams, graphs, etc.  The fundamental "best practice" emphasized in P1 is that students shouldn't just be able to solve mathematical problems, they should be able to explain the concepts underlying each problem, as well as the experimental evidence underlying each concept.  P1-level conceptual understanding is a prerequisite for true success in Physics C-mechanics.  

Physics education research has shown, again and again, that mere breadth of content, mere ability to solve equations and get answers, does not lead to long-term success and understanding.  Research has shown that it's less important which topics are covered than how they are covered.

Will cut scores for P1 be adjusted? I have no idea.  We'll find out somewhere around September!

How are you adjusting your overall program, Greg?  Well, this year, I'm gratefully accepting the gift of time that I've been offered.  In a pandemic year, I'm suddenly in good shape with my course pacing.

In future years, I'm going to be able to expand the pool of students who can do AP Physics 1 in their 9th grade year.  Even my most advanced 9th graders have been sufficiently challenged by P1.  Now, though, some of my students who were getting 3s will likely be able to get 4s, because they'll be able to concentrate on fewer topics.  And, some students who couldn't previously handle the pace of P1 will be able to take the course - I can ease more gently into AP-level work, because I have more time.  I'll similarly be able to expand the number of conceptual physics students who come back to take our P1 course their junior or senior year.  

And, since I can expand the number of 9th graders in P1, I might eventually have enough students to offer P2 as a scheduled full-year course.  Right now I'm offering it to just a few folks haphazardly as a sort-of independent study in the second half of our Science Thesis Seminar.  

Should I teach circuits and waves anyway?  I like teaching those topics.  I mean, I personally would not.  Especially this year, when nothing at all is as efficient as anyone is used to.

Though circuits and waves are fun, students tend not to appreciate discussion of topics that aren't on the exam, except if there's a real excuse for an enrichment-only time.  For example, if you had all your seniors gone for a week and just the juniors left, it would be fun to do circuits with just the juniors.  Or, if your state mandated exam (like the Regents exam) includes circuits and waves, then by all means, you'll have plenty of time next year!  In general, though, you'll gain a lot more political capital by minimizing your topic coverage but increasing your demands for depth of understanding.  

On the reduced P1 exam, what are the new topic percentages?  This isn't clear to me yet.  If I had to bet, I'd pro-rate the unit percentages in the currently published course description based on eliminating units 8-10.  It doesn't matter that much, though - because going forward you'll have plenty of time to help your students understand everything in units 1-7 very well.  

I'll add to this post as I hear more.  Feel free to post questions in the comments, or to me via email.  Know that I will simply delete passive-aggressive comments - I've been seeing too many of these.  If you have a good-faith question, though, I'm happy to help where I can!

Zoom parent conference day - I loved it.

I'm well practiced at performing for the open-house parent showcase, and at parent-teacher conferences.  By the time these roll around in mid-October, my students have usually adapted to approaching problems that aren't pure recall, they've begun to recognize that physics class is, comparatively, a fun part of their day; and they've communicated this positive energy to their parents.  My conferences are usually relaxed and not-hostile.  

Yet, I do not like meeting with one set of parents after the other.  I feel like I'm performing, all day, even in between conferences.  What do I say to the family that has arrived 20 minutes early and is staring daggers at me while I wait for the (late-arriving) family on my schedule?  Has my tie come out of place, has my hair mussed, in the five hours since we started?  I hope not, I'll be judged.  Oh, gawd, the one student out of fifty whose parents are huffy is waiting outside and talking to two other sets of parents, probably spreading poison.  I really need to let the dogs out and to make another cup of coffee, but the 15-minute break isn't enough for all that, especially when half of break is taken up with the couple who arrived late for their meeting and wouldn't leave until they had their full allotment of time.

In-person parents' weekend reminds me a bit of high school homecoming.  People are dressed so carefully, performatively playing a role rather than being their authentic selves.  When junior Tricia gives a big hug to returning alumnus Scotty, is it really cause "it's so good to see you, I miss you so much?"  Or is she saying that to get in the sack with Scotty's brother?  And Scotty's return compliment about Tricia's dress - he knows darned well that she paid $250 for what looks like aluminum foil wrapped around a cylinder.  (Right?)  It's all a game teenagers play, an agreed-upon reality that no one is willing to bust - everyone is using the same tone of voice they use to compliment Aunt Gloria's green bean casserole.  "Oh, I'm so glad that Mr. Jacobs is Will's physics teacher," they say loudly to each other.  I suppose that might be true, just as there might be someone in the universe who likes green bean casserole or Tricia's godawful dress.

Online parent conference day was, um, entirely unlike that.  Phew.

Of course we did conferences online out of necessity, not desire.  Due to restrictions on campus visitors, most parents had dropped their kid off at our boarding school on August 31, and wouldn't see their kid again until November 18.  That's hard for parents.  Especially for parents of 14 year olds who have never been away from home for so long.  As much as parents' weekend irritates me, I know how important it is to the parents, to our school's overall culture.  I fix my best customer service smile and have empathy for the parents who are making a significant sacrifice to give their sons [we're a boys school] an amazing four year educational experience in a caring community.

This year, online parents' weekend held a different dynamic.  

It seems like such a small thing, but it felt important: I was free to take my laptop anywhere for these conferences.  The fact that I wasn't tethered to my office, that the next group of parents sat in a *virtual* waiting room, meant that I felt more relaxed all day.  In fact, I had far fewer and shorter breaks than in a regular parents' weekend.  (This is in large part because parents could schedule conferences without traveling to campus - so even families from airplane distance away showed up.)  But even a 15 minute break allowed me to get a snack, use the bathroom, take care of dogs, even change my location.

Everyone was on time; everyone was respectful of the fact that the next parent was waiting, and so ended the conference promptly.  Because they couldn't see whether or not someone was actually waiting, they assumed someone was.  And parents who scheduled each of their kid's six teachers back-to-back didn't have to walk from one building to the next, taking their own unscheduled breaks, sniping at each other for turning the wrong direction and losing five precious minutes with a teacher.  They just sat in their living room and clicked on one link after the other.  

The conference content was far more positive, too.  The parents didn't see each other all over campus; they didn't have any opportunities for the performative "well, I definitely need to talk to Mr. Lipshutz, because Will says he lost credit on his homework even though the answer was right."  Rather, the parents were so happy for any lifeline into their son's world, a world that suddenly didn't include parents at all.  I always refuse to discuss grades at parent conferences.  For once, my refusal didn't meet with pursed lips; in fact, few parents even tried to mention grades.  They heard me start telling stories about their child.  They noticed that I noticed little things about his personality, about his character.  They felt better knowing that I noticed.  They suddenly forgot about, didn't care about, whether Will had an A- or a B+.  Here was someone who could give them insight to their son's life far from home.  The parents were... grateful.

As am I.  Somehow I doubt that zoom parent conferences will become the norm, but nevertheless (like for zoom faculty meetings) I truly hope so.  My classes themselves require in-person team building over the course of many months.  Online classes were as thin gruel to the real thing, the difference between Beyonce live in concert vs. a 12 year old Beyonce impersonator recorded acapella on a Fischer-Price My First Cassette Deck.  But the talking-about-classes that goes on in a parent conference or faculty meeting?  That's better on zoom.  I've felt so much more positive about the parents and my colleagues this year.

"My Physics Teacher Hates Me"

Yep, I heard this in October, reported from my student to his parents to his advisor and back to me.  In my 25th year of teaching, with a pretty danged consistent track record of positive feedback from every student phenotype.  Have those 25 years earned me any good will at all?

Well, yes, kind of.  The advisor is new to the school, but knows me by reputation.  The advisor related the story sorta tongue-in-cheek - "ha ha, isn't that silly, my advisee told his mom his physics teacher hates him."  We could both laugh, especially as I had just sent the advisor a very complimentary email about the student's recent work.  I said, "Ha ha, of course I don't hate your advisee, but I do dislike that he spent class time this week clowning and spraying people with white board cleaner.  He's doing better now that he's joined in the positive class culture."  

I have this clowny type of student every year in conceptual physics.  The AP class analogue is the student who gets angry when told they're wrong, gets angry that I won't do their work for them but rather make them figure out how to deal with new situations.  These folks tell all who will listen that I'm mean, I hate them, I'm unfair, I don't fit their learning style, whatever will get them a sympathetic ear.  Every year.

Is that okay?

Well, on one hand, no.  It's emotionally draining.  It's frustrating to have to explain my methods to a distrustful colleague or parent, especially when they try to mansplain physics education best practices to me.  They make ridiculous charges, charges that can't be defended. (And shouldn't be - I can't prove a negative, I can't prove I *don't* hate someone, and if I tried to defend myself I'd merely further establish the meme that aha, see, you *do* in fact hate this boy, gotcha!)

On the other hand, we're dealing with 14-18 year olds.  It's unacceptable, but nevertheless understandable, that when they encounter their first true difficulty with academics they fight dirty.  It's unacceptable but understandable that they conflate difficulty of the subject matter with obstinance on the part of the teacher.  A 15 year old who was losing his playoff soccer game this fall blatantly pushed another boy in the back, then wouldn't shut up about how awful a referee I was when I called the foul.  Unacceptable, but understandable.  I shouldn't be refereeing if I can't deal with this level of unjustified criticism.  I shouldn't be teaching physics if I take personal offense that a 14 year old told his mom that I hate him.

On the third hand... I wish I had three hands... we teachers all need to stand firm against this insidious emotional manipulation which can indeed damage careers.  All it takes is one administrator without an understanding of my positive work with students over years, one administrator who tries to advance their career by currying favor with influential parents, one administrator to whom "our mission is to serve our students" translates into "it is the teacher's fault if a student is momentarily upset."  Reputation or not, we're in trouble.  And lord help the new teacher who can't fall back on decades of good work.

What can we do?  We - that is, experienced teachers who have been successful for a long time - can and must stand up for colleagues.  Like the advisor of the student-who-hates me did.  Like I do when necessary. 

When a student or parent says anything, even in jest, about whether my colleague dislikes a student, I am firm: "That's not a fair statement.  Like all of us on the faculty, Mr. Lipshutz is directed to know, challenge, and care for his students.  He has dedicated his life to that purpose.  I understand that you are frustrated with Mr. Lipshutz, but please refrain from attacks on his character or motivation.  Let's instead focus on how to help your son understand how to be successful in Mr. Lipshutz's class."  I will and have said such a thing even about colleagues whom I don't particularly like.*

* Of course, this calculus would change if I had a colleague who unprofessionally, repeatedly, and egregiously violated our mission to serve our students. Then (a) I would have already expressed my concerns to the headmaster, and (b) I would refuse to discuss the matter other than to suggest that this parent should share their concerns with the headmaster, too.  I'm not advocating a Blue Wall of Silence.

I've put my foot down similarly about umpiring partners to baseball coaches; I've had this conversation with colleagues outside my department about people inside my department.  I'm on a team.  I can and will constructively criticize others on my team, as they will to me.  It's important that my teammates and I listen to each other, that we ruthlessly self-evaluate, that we change things when we aren't perfect.  Of course.  

Yet.  I'm not talking today about dealing with rational, evidence-based criticism of colleagues.  "Mr. Lipshutz hates me" and the like is as far from "rational" and "evidence based" as Oz is from Kansas.  Bad-faith insinuations need to be squashed hard.  Like the Wicked Witch of the East.