# Update on Python Physics Curriculum

So here is the deal.  I had this idea.  The plan was to include numerical calculations into the intro physics curriculum by writing a sort of online textbook.  Or maybe just redo my Just Enough Physics ebook to include more numerical calculations.  Anyway, this is what I came up with. It’s written with trinket.io – an online implementation of python that pretty much rocks.

Here is my curriculum (it’s incomplete – but totally free).

Introductory Physics with Python

Here are some of my own thoughts on this curriculum (including using trinket.io):

• It’s free and online.  That’s mostly good – but I don’t know if online is the best format for physics.
• There is one thing about trinket.io that makes this rock.  There is python RIGHT IN THE PAGE.  Readers can view and run code – no logging in, no saving, nothing.  Just edit and run.  No barriers.
• It has the same idea as Just Enough Physics in that it goes over the basic stuff – but doesn’t overload the student with tons of different ideas (no fluid dynamics, waves, buoyancy, sound…).  It’s not that those are bad topics, it’s just too much.  Too much.
• Homework.  Students want homework questions.  I sort of added those in – but students seem to want traditional homework questions.

Now for the part that needs work.  Well, all of it needs work – it’s not complete.  But I made an error – I figured I would finish this curriculum as I was using it to teach the summer session of physics, but the pressure was too much.  In the end, I think I made it too much like the traditional format of a textbook (with the traditional order of topics).  Really, I started along the best path – but went off the rails when I wanted to do a problem that involved new physics.  So, I just added that new stuff in there.

I need to rethink just what I want to cover – and here is my new plan.

• Kinematics in 1-D and 2-D. I like starting with kinematics because students can model motion and this works great with numerical calculations.  The one problem is that you have to use acceleration instead of change in momentum – and this messes up with my momentum principle.  Actually, maybe I will just do 1-D motion so that I don’t need vectors.
• Forces. I don’t really want to focus on forces and equilibrium, but the students need this to do more stuff.  In this, I need to do the following.
• Vectors.  Boom – need vectors.
• Special forces: gravity, real gravity, maybe Coulomb force.
• What about friction, and forces of constraint (like the normal force)?  Here you can see how it gets out of hand.  Friction is super crazy if you think about it – so are normal forces.
• What if I just did simple forces – like pushing with your hand or rockets?
• Momentum Principle.  Here I need to make a connection between forces and motion.  Since I used acceleration before, I need to make a connection between the momentum principle and $\vec{F}_\text{net} = m\vec{a}$.  Honestly, I hate calling this Newton’s Second Law – it seems wrong.
• But what about circular acceleration?  How do you deal with that?  I don’t know.  Maybe just avoid it for now.
• Work Energy Principle. I think this is mostly ok – except I need to introduce the spring force and spring potential energy.
• Angular Momentum Principle.  My initial idea was to cover “Three Big Ideas” – momentum principle, work-energy, angular momentum principle.  However, there is SO MUCH baggage associated with angular momentum principle.  Much of this stuff is just beyond intro-level students.

I think I have a new plan.

• Forces – but simple stuff.  No friction.  No normal forces.  All the examples will be in space or something.
• Momentum Principle and acceleration. Again, normal stuff.  No forces of constraint.  Mostly space stuff because that will be fun.  Projectile motion stuff too.
• Work-Energy Principle.  Springs, gravity, dropping objects.  Orbits.
• Special cases.  Instead of Angular Momentum, I’m going to go over forces of constraint, friction, normal forces, circular acceleration.

The end.  Oh, I need to make sure there are plenty of exercises for students.  Rewrites coming.

I need to redo all my physics labs.  They are terrible.  I want to make them even MORE about model building.

With that in mind, I saw this:

One sentence labs.  Leave the procedure up to the students.  I think I will need some type of turn in sheet for these labs though.  What about informal lab reports?

# The worst high school physics question EVER

Here is a multiple choice question from an online high school physics question.  It’s bad, but it’s probably not actually the worst ever.

It goes something like this:

You have three objects that start at the same temperature.  Which one cools off the fastest?

1. A dry bean
2. Toast
3. Water

I’ll be honest, I answered this question incorrectly – well, I should say that my answer didn’t agree with the key.  Let’s go over the options.

Water

I’m starting with water because this is the answer I chose.  Why would water cool off the fastest?  My assumption was that the water would evaporate and cool off the liquid more than the other two objects.

Of course the evaporative cool depends on several things:

• The water temperature
• The air temperature and humdity
• The volume of water
• The surface area of water.

If I take some water and pour it into a very shallow pan with a large surface area, this stuff is going to cool off quick.  Note: here is an older post about evaporative cooling.

Toast

This was my second answer.  What is special about toast and why would they choose it?  In my mind, toast is special because it has lots of holes.  Lots of holes means that it has a large surface area to volume ratio.

Since things radiate thermal energy through the surface area, things with high surface area to volume ratios cool off faster.  This is why small objects cool off faster than large objects.  This is also why the moon’s core is cooler than the Earth’s core (the moon is smaller).

Oh, this is also how a heat sink works.  Large surface area to volume ratio.

Dry Bean

A dry bean could cool off the fastest because it is small (high surface area to volume ratio) and it is low density.  I assume if it has a low density it has a low specific heat capacity.  This means that with a low specific heat capacity, the dry bean has a small amount of thermal energy even though it has the same temperature as the water and the toast.

This is essentially the same reason that you can put pizza on aluminum foil in the oven.  Once it is hot, you can touch the aluminum foil, but not the pizza.  Although they are at the same temperature, the aluminum foil has less thermal energy to burn you (because of the low mass).

This was the correct answer (according to the people that wrote this dumb question).

Writing questions isn’t so simple

I think what the author really wanted to ask was “which has the lowest thermal energy?”  But even then, you have to take mass and specific heat capacity into consideration.

It’s really just a super bad question.  Super bad.  Oh, but it’s probably not the worst one.  I saw some others that were just as bad if not worse, but I have blocked them from my memory.

# Reflections on Student Video Assessments

After the summer session of physics (algebra-based), I have the following comments.

• It seems like every other video has a problem with vector notation.  Students often set a vector equal to a scalar.  Frustrating.
• Students seem to confuse two standards: The Momentum Principle and Collisions.  I have students submit videos for the momentum principle that are just a collision.  The key point is that the momentum principle deals with force, time and change in momentum.  I guess this is my fault. I offered suggested homework problems from a textbook and it covered momentum and collisions in the same chapter.  I guess they thought they were the same thing.
• Students are not very skilled at picking problems to solve.  They like the lowest level of something like “mass is 2 and velocity is 3, what is the momentum?”. I tried to help them, but it didn’t seem to work.  I showed a bunch of questions in class and had them “rate” them then discuss what makes a good problem. (I think I wrote about that here on my blog).
• I’m still not happy with the “student review”.  I want students to watch other student videos – but I don’t know how to implement that.
• Students like to procrastinate.  I’m getting a bunch of redos on the last day of submissions.  That sucks to grade.
• I hate vertical videos – but I hate videos that are recorded sideways even more.  I stopped accepting the sideways videos since they can fix it and send it back to me.
• I try to give meaningful feedback in my responses – but sometimes I just give a grade (score out of 5 points).
• I’m trying to give higher scores.  If they do well on the in-class assignment and submit multiple videos that aren’t wrong, I typically will at least give a 4/5.

# Working notes for my bouncing ball running model

I’ll be honest.  I had some problems getting my bouncing ball running model working.  Oh, here is the model.

https://www.wired.com/story/how-is-a-runner-like-a-bouncing-ball/

Basically, this models the speed of a running human by assuming they are bouncing ball.  When the human impacts the ground, there is some maximum impact force and an impact time.  The impact time decreases with horizontal velocity such that eventually, all the force is used in the vertical direction to keep the human off the ground long enough to switch feet in the air.  The end.

As I was making this model, I took some notes because I couldn’t get it to work.  Here are my notes.  Hopefully you can use this to see how to troubleshoot a program.

Running model notes

I think I mostly have it working:

http://www.glowscript.org/#/user/rhettallain/folder/blog_posts/program/runningbouncemodel/edit

Here is basically how it works.  Two big ideas:

• Humans can push off the ground with some maximum force.  This force does two things – gets them off the ground and in the air so legs can move and pushes them forward
• The contact time with the ground is small and gets smaller as horizontal speed increases
• This means as the human speeds up, the ground force eventually gets to where it can only push up and not forward

Here is what it looks like so far

Here is a graph of speed vs. time

• This model reaches a max speed of about 3.5 m/s in just a couple of strides – that doesn’t seem right
• I think my Fv calc is wrong – it gives back the same speed not the needed vertical speed to get the stride time
• Need to recalcualte Fv based on pfinal
• If you want to be in the air for ts seconds, then your initial vertical velocity must be -g=dv/dt.  dv=g*dt dv = 2vstart. start=(½)gdt
• Now to calculate the force. I know tc (contact time) so F = dp/dt = m*(vy2-vy1)/tc – this is the total force = Fv-mg so Fv = that stuff +mg

Something isn’t right.  Here is a plot of position vs. time

It’s getting higher and higher (and going lower – weird)

• I’m getting stride (in air) times of 0.09 to 0.13 – that’s wrong

Ok – I think I know the problem.  I need to set the force push time loop and forget about while human.pos.y<R – I think that’s my problem

1. Once human hits the ground – calculate Fv, Fx, and tc set tcount = 0
2. While tcount < tc – set human.pos.y = ground. And set the forces
3. When tcount = tc, turn off the forces and stop holding the person

It appears there is something wrong with my Fx.

• Fx is some value for the first push – but after that it goes to zero and the Fv is maxed out.
• Werid
• There is a problem with both Fv and Fx

The problem is the time of impact – it gets too small such that the required force is HUGE

• How about a min time – and it can’t go lower?

Fmax = m*2v/t

t=m*2*v/Fmax

I think the problem is that during the contact time, the horizontal force is too much so that the human ends up going faster than the theoretical speed.

I can use the time and force and velocity to estimate the average velocity and then recalculate the time

This is the paper

https://www.physiology.org/doi/pdf/10.1152/japplphysiol.00947.2009

It has this plot.

This shows a decrease in contact time with speed

Here is what I get for a fit

This gives a contact time function of

Although this “blows up” at v= 0.  Maybe I should say tc = 0.3612 for v < 2 and this expression for v>=2

End of notes – it finally worked.

# What is a good problem?

Part of the reassessment process has students pick problems to solve that they think are good demonstrations of their understanding of the material (or the standard).

For me (as the evaluator), I can learn quite a bit about what a student thinks just based on the problem they pick to solve.  However, it seems that students really don’t want to pick problems.  They would prefer to have me just tell them what problems to solve.

OK, let’s do this.  Let’s look at some problems and see which ones are good and which ones are not so good.  In this case, it will be for the Position-Velocity-Acceleration standard.  For this standard, students should show that they understand and can use the definitions of position, velocity, and acceleration in 1 dimension.  So here are some questions.  You get to pick which one is the best.  Actually, why don’t you score them from 0-10 (11 being the best).

Problem A.

A plane has a mass of 1120 kg and is landing on a runway.  The landing speed of the plane is 50 m/s and the runway is 2140 meters long.  What is the acceleration of the plane?

Problem B.

Your car is the fastest all around.  No one can beat you.  It has an acceleration of 8.2 m/s2.  Suppose you start from a rest (because, don’t all drag racers do this).  How long would it take your awesome car to get to a speed of 55 m/s?  What is this speed in mph?  What is the average speed during this time?  How far did you go?

Problem C.

A police car starts from rest and can accelerate at 5.5 m/s2.  The police car starts accelerating as soon as a speeding car passes by with a speed of 25 m/s.  Assuming the police car has a constant acceleration and the other car has a constant speed, where does the police car catch up to the other car?

Problem D.

Can you have a hang time of over 2 seconds when jumping?

Problem E.

A rocket is in space traveling with a speed of 328 m/s.  It fires its rockets to create an acceleration of -10.7 m/s2 (slowing down).  What is the speed after 5.8 seconds?

Problem F.

no words

# Letter to High School Students: What to Major in

Dear High School students,

How are you? I am fine. I am very glad that I am no longer in high school. Maybe you enjoy high school, but for me, it was not so good. Don’t get me wrong, I went to an excellent high school (Waubonsie Valley HS). There was something in high school that didn’t feel right. Maybe it was being in classes for too long and the lack of time to work on my own projects. Maybe it was lack of freedom in choosing my own classes (there was some freedom to chose). Or perhaps I was just not mature enough to enjoy it. Needless to say, I am past that now.

I think now, how could I help high school students? In particular, how could I help them choose a major in college (if they choose to go to college). So, this will be the topic of this letter today. Please don’t hope that I will write more useful high-school letters such as “how to find a prom date”. That is definitely one area I failed at.

**Major in Physics**

I know. You saw that coming. Why should you major in physics? Here are some of my points:

**It is difficult**

Wait. BECAUSE it’s difficult? Shouldn’t you do something because it is easy – not difficult? Well, it depends on why you are doing it. If you are doing something to improve yourself, difficult is good. Imagine you were going to exercise. Should you walk around the block or run 3 miles? Well, if you find that walking around the block is very easy, it probably won’t do much for you. If you find underwater basket weaving to be easy, maybe it is not really helping you grow. So, physics IS difficult, but that is a good thing.

**What else are you going to do?**

I hate to be a negative person (I am not really) but look at where we are today. There are financial problems, energy problems, problems with that rick-roll stuff. What are we going to do? You could help. Maybe it will be you that contributes to the energy problem (well, we all contribute to the problem, maybe you could contribute to the solution). Physics gives you are start in the fundamentals of nature. We NEED people to understand the basics so that we can defeat the energy problem.

What are your other options? Underwater basket weaving? Maybe there are lots of people who can do underwater basket weaving. This means there are lots of people capable of UBW (underwater basket weaving). Job competition for UBW is high and maybe companies will just start sending their UBW tasks to India where it costs less. Plus, does anyone really NEED UBW? Maybe it is the first thing to go when the economy is taking a turn for the worse.

A common idea that comes up is that students think they can not major in physics because they did not take physics in high school. I don’t think this is a disadvantage in any way. In introductory physics, the common problem is that students must “*unlearn what they have learned*” such as the idea that *constant force causes constant motion* (just to be clear -that idea is wrong). What about math? Yes, you need to be proficient in math. Most students entering school (at least here) are not quite where they need to be in terms of math understanding. However, that doesn’t mean they can’t catch up. Ideally, students should be ready to take Calculus I when entering college. You have time, work on your algebra and trig skills so that you can do well on the Math part of the ACT and place into Calc I.

**Other**

This is longer than I usually write without including an equation, so maybe this is a good place to stop. If you want to major in physics, there are lots of things to consider. Here are some resources:

# People need to play with magnets

Question from class: *What do magnets interact with?*

Basically, everyone said “metals”. I am quite surprised. No one specifically indicated that magnets only interact with iron and steel (of the materials they would likely see). I understand that steel is a very common material they are likely to encounter, but what about aluminum? I think this points to the idea that very few of my students have actually played with magnets. This is a shame. Everyone loves magnets.

So, I propose you go out and give someone you love some magnets today.

# A few tips for new faculty

So, we have some new faculty. New to the university, and new to teaching. What advise can I offer? Here are few things to consider:

• Never show fear. Students can sense fear. They see it as a sign of weakness. They may attack. If you are afraid, act like you are not.
• When in doubt, imitate. Don’t try to reinvent anything. Don’t try to find your own style, that will come with time. The goal is to become familiar with teaching and to become familiar with the content. Once you do that, you will have a better idea about how YOU want to run things. So, in the mean time, find an experienced faculty member and ask for help. He or she will probably share any materials they have with you. Maybe you could sit in on their class and just do almost the same thing as him or her. This may seem like a cheat, but the semester is starting now. You need to do something.
• If you are unsure of level, aim high. Is this test too hard or too easy? If you are not sure, err on the side of difficult. It is much better to start the semester off too hard than too easy. You can always curve to bring the grades up at the end of the semester, but if you do it the other way, there will be blood.
• Fairness is important. Don’t change test dates once you have set them. Don’t change the “rules” you create in the syllabus. If a change needs to be made (like for a hurricane or something) make sure the change has a positive effect on student grades. Grades may not seem like a big deal to you, but they are HUGELY important to students.

# Mr. Miyagi and learning

So here I was in thermal physics class. The students were talking about the assigned homework and then asked: “can’t we get some homework credit for this? Why are we even doing this?” Immediately in my head popped “wax on, wax off”. This was the same situation Mr. Miyagi (from [The Karate Kid](http://en.wikipedia.org/wiki/The_Karate_Kid)) was in with Daniel-san. Homework should not be done just for the points. Homework should help the students become more proficient at blocking blows from the test.

I really like the movie karate kid. Mr. Miyagi brings up some good points. How does Daniel-san learn about karate? Is it by sitting and listening to Miyagi? No, he learns by doing some stuff. At first Daniel-san does not see the point of the exercises, but in the end, he wins.