# Just Enough Physics Video Update

Perhaps you haven’t noticed, but I’ve been trying to make a bunch of physics videos. Oh, sure—I already have a ton of stuff on youtube, but it’s not linear. For my old stuff, I will solve a problem on work-energy and then do electric field stuff and then go back to kinematics. It was just whatever topic came up in class or online or whatever.

I figured I should start over and do a whole physics course—well, not EVERYTHING. No, I would do just enough to get you through the course. I hope you get the title now. Also, this is the title of that self published ebook I wrote a long time ago—since it really is the video version (but updated).

So, where am I now? I’ve got 4 chapters completed. I think it’s a pretty good start. Here are the chapters (as playlists) along with descriptions.

Chapter 1: Kinematics

Notice that I started off by making a title screen and all that cool stuff. I will end up dropping this so that I can make videos faster. Also, in my previous videos I was in front of a whiteboard. In this case, I’m writing on paper. Still not sure which way is better.

In this chapter:

• Introduction.
• Constant velocity in 1D.
• Example of constant velocity.
• Introduction to numerical calculations (1D constant velocity).
• Constant acceleration in 1D.
• Numerical calculations with constant acceleration (in 1D).
• Solving the “cop chasing a speeder” problem.

Chapter 2: Forces and Motion

It’s tough to start in physics with forces. There are so many things to cover. This is a shorter chapter that looks at the fundamental ideas of force and motion.

• Introduction to forces and motion. I really like this first video. It’s a conceptual look at the forces, the momentum principle and “Newton’s 2nd Law”. Guest appearances by Galileo, Aristotle, Newton.
• Forces in 1D – falling objects.
• Modeling the motion of a mass on a spring (and finding the model of a spring force). This one is long (but pretty nice).

Chapter 3: Vectors—2D and 3D Stuff.

The goal here is to expand kinematics into using vectors—but then you need to know about vectors.

• Intro to vectors.
• Kinematic equations with vectors.
• Example of constant velocity and position update formula in 3D.
• Intro to projectile motion.
• Finding the range for projectile motion.
• Numerical calculations for projectile motion.
• Acceleration of a block on an inclined frictionless plane. This is an example of forces in 3D.
• The physics of flying R2-D2. Using forces and air resistance.

Chapter 4: Calculated Forces

There are really two kinds of forces. There are forces that have an equation to determine the vector value (these are calculated forces). Then there are forces that don’t have an explicit equation (forces of constraint). This chapter just focuses on calculated forces.

• Universal gravity.
• Example of gravity—calculating the net force on the Apollo 13 spacecraft.
• Introduction to visual objects in VPython. This is a setup for the next video.
• Modeling the motion of an object near the Earth.
• Modeling the Earth-moon system.
• Mass on a spring (again) – but this time with visuals AND 3D motion.

# Top 10 Blog Posts from 2019

It’s always difficult to pick the BEST of stuff. This is especially true when it’s all your own stuff.

So, let’s just say these are 10 nice posts from 2019.

How Does the Mandalorian See Through Walls?

You know I love to write about stuff that gets me excited—and I’m super pumped up about The Mandalorian (just finished season 1). In one of the episodes, Mando sees through a wall with his sniper rifle. How would that work?

No, it probably wouldn’t be with infrared.

Modeling the Water from a Spinning Sprinkler

You don’t really understand something unless you can model it. In this post, I use python to model the motion of water shooting from an inward pointing and spinning sprinkler (based on the Steve Mould and Destin video).

This gif pretty much sums it up.

Orbital Physics and the Death Star II at Endor

This is my favorite thing to do (which I also did in the Mandalorian post above)—take some scene from a movie and and then use that as an excuse to talk about physics. In this case, it’s all about geostationary orbits from Star Wars: Return of the Jedi.

Bonus: more python code in this post. Double bonus, I use data from ROTJ to estimate the length of a day on the planet moon of Endor.

All Measurements Are Really Just Distance—or Voltage

I was in lab when I realized that pretty much all of our measurements were actually measuring distance. Well, originally that was true. Now we can make measurements by measuring a voltage.

Here are some measurement devices—this wasn’t in the original post.

You Can’t Calculate the Work Done by Friction

This was a post I wrote after a discussion I had with Bruce Sherwood. He told me this story about how it’s easy to use the momentum principle with a sliding block (with friction), but you can’t use the work-energy principle.

We like to think friction is this simple thing—but it’s not. The above image is an illustration to show that the distance a friction force is applied is not the same as the distance the object moves.

Video Analysis of Captain America vs. Thanos

There is the perfect scene in Avengers: Endgame. It’s not only perfect because of what Captain America does—but it’s perfect for video analysis. So, in case you haven’t seen it, Cap takes Thor’s hammer and smacks Thanos hard.

Here is the frame corrected version after using Tracker Video Analysis.

No, momentum is not conserved. But that’s OK.

What are Maxwell’s Equations?

Yes, Maxwell’s Equations can be tough.

Here is my attempt to explain these equations in a simple way to describe the electric and magnetic fields.

Every Jedi Jump in Star Wars

OK, not every Star Wars movie. I didn’t have Episode IX to include at this time (I will have to wait for the digital version of the video). But the idea is to analyze ALL the jumps. Here they are.

There are too many jumps for me to do a complete video analysis. Instead, I just estimated the jump height and the jump time. From these two values, I can make a graph—if the vertical acceleration is constant then there should be a linear fit.

The best part is that most Jedi have a vertical acceleration LOWER than g (free fall acceleration on Earth). Yoda has a vertical acceleration HIGHER than g because he takes so many short jumps. I need to write a future post just looking at Yoda.

All the Hacks and Science from MacGyver Season 3

Maybe this is cheating since it’s really not just one post. This is a list of all my science explanations for MacGyver Season 3. Oh, just to be clear—I’m the Technical Consultant for the CBS show MacGyver (season 4 starts in February).

It’s a lot of work to help the writers come up with new science tricks for MacGyver, but it’s also super fun. I also really enjoy making these MacGyver at home videos.

I’m really looking forward to sharing more science for season 4.

Projectile Motion in Polar Coordinates

I’ve had this secondary blog for over a year now—and I really like it. It’s like the old days of blogging. I can write whatever the heck I want (example—the top five lightsaber fights in Star Wars). Also, I can go into super complicated physics stuff.

Here is an example from my upper-level classical mechanics course. Can you use polar coordinates for projectile motion? Yes you can—but it’s obviously not the best choice.

There’s python here too.

# Best Posts for 2018

In order to keep my blogging certification up to date, I am required to post some type of year end review.

OK, here it is. These are my “best” or “favorite” posts from 2018. Maybe these didn’t get the most traffic, but they are ones that I like the best. It’s all about me.

You might think this list is long, but I just counted. I had 106 blog posts for the year of 2018. So, these all “made the cut”. Also, I normally just list the posts but this time I will give a brief description.

Let’s do it.

There is indeed gravity in space. Common ideas about gravity.

Really, this post is all about a TV show – The 100. In one episode, a boy is floating around in a space ship during the re-entry process. This leads to a discussion about how gravity works and what happens during re-entry.

Flying planes with tiny collisions.

This really isn’t a blog post—at least not like a normal post. This is really just a holder for my WIRED video on how airplanes fly. This short explanation covers flying using the momentum principle instead of Bernoulli’s Principle.

Finding the gravitational constant with a mountain.

The gravitational constant is needed to find the gravitational force between two objects with mass. The problem with finding this value is that it’s very small and we (humans) didn’t initially know the mass of the Earth.

Here is a method to find the gravitational constant by estimating the mass of a mountain and detecting the change in gravitational field with a pendulum. It’s just so crazy it might work.

Oh, the real tricking part is find the direction of “up” and “down”.

Calculating the angle of reflection with a numerical calculation.

Everyone knows (or should know) how much I love numerical calculations with python. Here is a demo to show the angle of reflection is equal to the angle of incidence using Fermat’s Principle. This says that light takes the path of shortest distance.

So, python code just “shoots a ball” at different angles and then calculates the travel time. Check it out. https://trinket.io/glowscript/1e2a30cab5

Why is it so difficult to predict where a satellite will crash from orbit?

Here is another numerical model (python) to show that with slightly different initial conditions, the Taingong-1 spacecraft will crash at a different location. Code included.

Random walks in 1D, 2D, 3D, 4D—and why we live in 3D.

First, there is a random walk. Second there is a random self-avoiding walk (SAW). A self avoiding walk doesn’t cross its own path. So, a SAW is like a long protein—which is important for life.

3D works the best for proteins (and not 4D). In 4D, there is no big difference in length vs. step number for SAW and normal walks. In 3D, it’s more likely a walk will cross itself—which is important for protein folding.

Yes, there is lots of python code here. Note: random walks in 4D are tricky since you can’t just use the position of the walk as a built in 3D vector class.

Video analysis of a race with The Freeze.

The Freeze is this guy that races mere mortals on a baseball field. He’s fast. Very fast. He gives the victim a head start and still wins. So, here is my analysis along with some physics homework.

Celestial navigation with a protractor and a watch.

This is one of my blog posts that goes along with an episode of MacGyver (since I’m the technical consultant for the show). The idea was that MacGyver would use some stuff from a car to find out how to get to a base from his location in the desert.

The cool part is that navigation is really just using a compass to measure angles and clock to measure time.

Note: for this particular episode, I did a bunch of calculations to get the exact angle and time measurements they would use in the show. I don’t think they made it in the episode, but I did it. Also, I had to cheat since everything happened at night and MacGyver couldn’t find the time of local noon.

Modeling the trajectory of turbo lasers in The Last Jedi.

Yes, a Star Wars post. SPOILER ALERT – there is a space battle in Star Wars The Last Jedi. They show this First Order ship firing on the Resistance. In order to make it look like a WWII sea battle, the turbo lasers have an arc to them—that looks cool, but it wouldn’t happen.

So, what do I do? Other than enjoy the movie (which I do), I first do a video analysis to determine the vertical acceleration. Then I make a python model to recreate the arc. Fun.

Oh, here is the code. Also, I think I did this same type of thing with TIE bombers from Empire Strikes Back. Maybe that should have been in this list too.

Some science communication mistakes are worse than others. I’ll just leave it at that.

Physics model of a running human.

How do you model the motion of a running human? How do you take into account the idea that they can’t keep speed up forever? Here is my basic model.

• Humans are like a ball that impacts with the ground.
• When a human hits, they can only exert some maximum force to change the momentum.
• The vertical component of this force pushes them back up to keep them in the air for some amount of time.
• There is a minimum time the human must be in the air to switch back and front legs.
• The faster the human runs, the lower the ground contact time.
• Eventually a human reaches a speed such that the contact force is only up and not increasing the speed anymore.

I really like this model. It makes me happy. Code included.

Does the Sun orbit the Earth or does the Earth orbit the Sun?

OK, we all know the Earth orbits the Sun. But how can you tell? Here’s the answer. Oh, and I also include a python model of retrograde motion.

Boiling Water at Room Temperature

What is boiling? Great demo. You should try this yourself.

What’s the difference between mass and weight?

This is a surprising confusion for students. Here is an explanation of mass vs. weight. Also, here is a great experiment to calibrate an inertial balance (that doesn’t need gravity).

Deconstructing a special effect from Star Wars

In Star Wars A New Hope, there is a scene that shows the escape pod leaving the blockade runner (near the beginning of the movie). It turns out that this shot was created by dropping a model and viewing it from above.

Here is my video analysis (with angular size) to show that the model is indeed accelerating as it moves away.

Bonus: one of the guys that made this special effect sent me an email after I posted this. Winning.

Build a radio transmitter – mostly from scratch.

I started off writing a book review (How to Invent Everything: A Survival Guide for the Stranded Time Traveler – Ryan North). In the book, he suggests that if you were starting from scratch it would be easier to build a radio transmitter than it would be to build a clock. Of course (from a previous post), a clock is important for navigation. If you had a radio transmitter, you could just broadcast the time.

OK, but how difficult is it to build a transmitter? Not too hard. I did it. Here is my spark gap transmitter.

The End.

Why are you still here? Oh, you are waiting for just one more post? Or maybe you think this was too many? No, it’s 16 out of 106. That’s just 15 percent of my posts.

# Thanksgiving Physics

I am honestly not quite sure how many blog posts I have about Thanksgiving.  It’s probably about 1 per year for 8 years.  I’m going to guess it’s 8.  Here goes my internet search.

This is what I found.