# MacGyver Season 3 Episode 13 Science Notes: Wilderness + Training + Survival

There is no funny intro for this post. Oh wait, this is an intro.

Rock and steel to make a spark.

OK, this isn’t actually a MacGyver hack in this episode but I’ve talked about it before. Here is a video.

Other fire stuff

Are there certain plants that ignite more readily than others? Yes. Here is a nice article from Field and Stream that goes over the basics.

Again, not a hack—but cotton is terrible when wet (so is denim jeans). When wet stuff is next to your body, the water evaporates. In this process the liquid water turns into a gas water (water vapor). The phase transition requires energy. Guess where the energy comes from? Yup, it comes from the human. This makes the human colder. Here is my more detailed explanation.

What about wool and other materials? The key to these better fabrics is that they “wick”—they pull the water away from the body. Here is a nice post on that.

Zipper for ice traction

MacGyver takes the tent zippers and uses them on people’s shoes for added traction on ice. It’s sort of like mini-spikes on your shoes. Classic.

How to make drinkable water

MacGyver uses a tree branch to act as a water filter. This seems to be real (from MIT) so you know it’s got to be good.

Here is a nice video showing how this would work.

Making rope (string)

I’ll admit it—I’ve never really understood how this works. If you take some vine or some other material, it has a certain maximum strength in its tension before it breaks. If you take two of these things together, it doubles the strength. If you take these two things and then twist them—the strength is more than double. What? But it does indeed work (I need to do an experiment sometime to really understand this).

Here is an older video in which I attempted to make rope from a TV guide.

Size and weight of 18 million dollars

This is a classic MacGyver estimation problem. How do you find the size and weight of a bunch of money? Why does it even matter? Well, one big thing is to find the density of the money. If the density is less than the density of water, then the crate of money would float and then be swept away in a flash flood.

Yes, you don’t need the weight and size—but just the density. However, if you want to estimate how long the crate was floating you DO need the size. Bigger crates will “hit the bottom” before smaller crates.

If you want to look at more stuff about the density of money, here is an older post in which I find out how far 1 trillion dollars would stack. Would it make it to the moon?

Surveying tools

How do you find the slope of the ground? This is where you need surveying tools. Here’s how it works. Get a scope (from a rifle) and make sure it’s aimed level. There are plenty of ways to level a scope—those little bubble levels work great. Next get a survey stick. Make sure this is also vertical and then measure where the scope points at the stick.

If you know the change in elevation and the distance between the stick and the scope you have the slope.

Float distance calculation

Yeah, this is pretty tough—but that should never stop anyone from trying. How do you estimate the distance a crate will float in a flood? Here are some things to consider (some of these would be tough to estimate).

• How fast was the water flowing?
• How deep did the water get?
• How long did the flood last?

Really, if you know those things you can calculate the speed and time of the floating crate. This would then give you the distance. From that you can find the location on a map.

Drag sled

To move a crate (or an injured MacGyver), it shouldn’t be too hard to make a drag thing—called a travios.

Grab hot coals

Don’t try this at home, but it is indeed possible to grab hot coals. Essentially, you can grab hot stuff if you are really quick. There isn’t enough time to transfer energy to cause a major burn.

It’s just like walking on hot coals. Here is a nice physics post on that.

# MacGyver Season 3 episode 12 Science notes: Fence + Suitcase + Americium-241

It’s not a super Mac-hack, but it works. The only thing MacGyver does is to put a toothpick in place a grenade pin. It only works for a little bit before the grenade explodes. However, the physics discussion is pretty good. Let’s go over some of the terms.

• Tensile strength. This is essentially the maximum force a material can withstand when being pulled apart. Just imagine a rope—how hard can you pull on the rope before it breaks. That would be the ultimate tensile strength. Yes, wood has a pretty high tensile strength.
• Compressive strength. How hard can you squeeze the thing before it fails? Something like concrete has a very high compressive strength, but not so much with tensile strength. Wood could have a good compressive strength if it’s wide and short. Long skinny boards of wood tend to buckle.
• Sheer strength. This is the maximum force an object can withstand when two forces are pushing in opposite directions but not directly at each other. Think of scissors.

What is a dirty bomb?

This is another non-hack. However, I just want to describe the difference between a nuclear bomb and a dirty bomb. A nuclear bomb uses a nuclear reaction (usually started with conventional explosives) to make a massive boom.

The dirty bomb is NOT a nuclear explosion. Instead, it uses conventional explosives to spread radioactive material around. It’s dirty.

Electromagnet

MacGyver builds a strong electromagnet to move a bolt inside a locked door. Yes, this is possible. You would need a strong electromagnet—that means high electric current and thus thick wires. You also need a fairly beefy battery to get this much current.

Oh, one possible problem. If the bolt is ferromagnetic (steel) and so is the door, then it’s going to be difficult to get that bolt to move. However, if the door is aluminum or some type of non-ferromagnetic material then this would work.

Wall walk

There are two methods to get over the pressure sensitive floor (they end up not using this though). There is a wall-walking stilt method and a rolling sled method. Both have the contact point with the wall at an angle—this is needed in order to work (because of physics).

Let me just start with a setup that would only barely work. Here is a view of a person using completely horizontal stilts along with the forces on the person.

The first problem is that the stick the reaches across the hall would have to fit perfectly. The harder it is pushed against the wall, the greater the frictional force. And it is this upward frictional force that balances the weight pulling down.

The second problem is with these horizontal arms. When they attach to the person, there is no upward force. This would be like trying to hold a rope with a weight in the middle perfectly horizontal. It won’t work.

Here is a better option.

This setup fixes both of the problems. The sticks can be longer than the hallway (and not fit perfectly) and there is now an upward component from the wall that helps support the human.

Mercury Switch

What is a mercury switch and how can you build something similar? Here, I made a video for you.

MacGyver uses some smartphones (as usual) to detect radiation from the dirty bomb. This is essentially real.

A smartphone camera has a sensor that is normally used to detect light. However, this same sensor can be set off by other types of radiation—like the stuff that is produced by radioactive stuff.

In order to actually detect this radiation, you need to block the light from getting to the camera—electric tape over the lens will do the trick.

But wait! There is a real project that uses normal human smartphones to detect cosmic radiation. Check it out—the CRAYIS Project. https://crayfis.io/

Rolling Tire Bomb

Yeah, mixing stuff and make explosions. The end.

# MacGyver Season 3 Episode 11 Science Notes: Mac + Fallout + Jack

How do you break out of iron shackles?

So, Jack and MacGyver are stuck in a fallout shelter and they need to get out of shackles. Maybe they are just old iron or maybe they are old steel—either way, they need to get out.

There are several steel-busting options, but let’s go with something thermal. In this case he could do something similar to thermite (similar). How about glycerol and potassium permanganate?

Again, this is not thermite—this is the stuff that’s usually used to start a thermite reaction though.

Opening a door with thermal expansion.

MacGyver uses some butter knives and electric current to heat up a steel door. When the door expands it cracks the cement around it so that they can open the door.

Would knives heat up? Yes. When you short out an electric circuit, you get a bunch of current. Lot’s of electric current means that things get hot. This could theoretically heat up the door.

Now, if you are going to escape with this technique yourself you need to be careful. A steel door has a very large thermal mass. It would take some time to heat it up—but it would indeed heat up.

Do things expand when heated? Absolutely. Here, I made a quick demo for you.

Scuba shooting crossbow.

Yup. Basically this just takes some springs to store energy and then uses that energy to propel an empty scuba tank to break a door.

Zip gun

Jack has a small projectile in a tube with a spring—a zip gun. It’s basically a deadly sized nerf gun.

# MacGyver Season 1 Episode 9 Science Notes: Chisel

There is no introduction, just science.

Lighter and spray can stun thing

MacGyver gets a broken spray can of something (it really could be any aerosol can) and attaches a cigarette lighter to it. He then makes it so the lighter burns while the spray sprays. When he throws it, boom.

Yeah, these spray cans can ignite stuff. This is plausible.

DIY hot air balloon for Jack’s phone

Someone needs to make a super clip of all the times Mac says “Jack, I need your phone”. I think that’s funny.

In this case, the idea is to build a mini hot air balloon to lift up the phone so that they can see a “bird’s eye view” of the city. Here’s how it works.

• Get a thin plastic trash bag.
• Get some fuel—in this case it’s that stuff that burns to keep food hot for a buffet or something. Oh, they put it in aluminum foil.
• Hang the phone and light the fuel.

Boom. That’s it. Yes, it’s real. The basic idea is that the fuel heats up air that fills the bag. Hot air has a lower density than cold air—this means that the weight of the air inside the bag is less than the weight of air outside of the bag. This gives a net upward buoyancy force on the bag.

Here is a more detailed explanation of buoyancy, if you need it.

OK, but would this be enough to lift a phone? It would be tough, but it’s at least plausible. It depends on the weight of the phone and fuel, the size of the bag, the temperature of the inside and outside air. So, it’s possible.

Here is one you can make yourself.

Bullet proof paper

OK, it’s not bullet proof paper. It’s a calculation of how much paper you would need to stop a bullet. I love how well this turned out.

mythbusters bullet phonebook

Bullet proof shield

This one is simple. Yes, if you tape a bunch of kevlar vests to a door it will be fairly bulletproof. MacGyver’s calculation is great (I should know). OK, it’s not perfect—but it’s a good example how to make a basic estimation.

Personally, the dialogue gets to the basic point and the animations and graphics are really nice. LOVE IT.

Let’s go over some of the details.

• You need some basic values—like the speed and mass of a bullet from an AK-47. I googled this, but maybe MacGyver just knew it.
• From there, you want to somehow model the interaction between a bullet and paper. The first idea is to think of it like a drag force (just like a bullet going through air or something). Of course this causes a problem because that makes it a velocity dependent force and therefore VERY difficult to deal with.
• But what if there is a constant force on the bullet during the interaction with the paper? In that case, we can use the work-energy principle (which MacGyver says—YAY!).
• With a constant drag force, you can then find the distance over which this force needs to do work to stop the bullet.
• For the constant drag force, I estimated the density of paper (a little bit lower than the density of water) and assumed this was the constant force. Of course this is wrong—but it’s just a place to start. You have to start somewhere.
• Really, the rest is just calculations.

Here is my original estimation.

Oh, I guess there are a few things to point out. First, the MythBusters also looked at using paper to bulletproof a car. It sort of worked. Second, in the end MacGyver reports the paper thickness in inches. I hate imperial units—but I guess that’s just the way things are.

Still, super pumped at the way this turned out.

Improvised weapons

In order to fend off the attackers, MacGyver makes some improvised explosives to shoot marble cannon balls. I don’t want to go into the chemistry of explosives so I will just put my normal explanation.

Any time you mix two or more chemicals, it is plausible that it could make an explosion. The end.

MacGyver wants to figure out what the bad guys are saying on their radios. He uses a yaghi antenna to get a directional signal and then he connects it to an AM/FM radio and picks up the signal.

OK, they probably don’t broadcast on the AM-FM frequency range. However, it’s possible he could modify the tuner in the radio to pick up their frequency. It would help if he knew their frequency. Also it’s hopeful that they aren’t using encrypted radios.

Dish soap to slide a safe

This is basic physics. Dish soap can indeed decrease the frictional force—especially for smooth surfaces. This would make a great physics problem.

Sugar putty bomb

Again with the bomb thing—using sugar for an explosive. Well, you can make a rocket from sugar (again—from the MythBusters)

In order to detonate the explosives, MacGyver takes apart one radio such that it makes a spark when receiving a signal (instead of making a noise). This is fairly plausible.

There are a couple of other things, but I will stop here.

# MacGyver Season 1 Episode 8 Science Notes: Corkscrew

Remember, I’m just going over the MacGyver hacks with science stuff in them.

DIY Blacklight

This one is fairly legit. MacGyver is in an escape room and needs to find a blacklight to read some hidden words on the wall. He says it would be easier to build a blacklight than it would be to find it.

Here is MacGyver’s build. Use a smart phone LED light and an old floppy disk. In theory, this could work. Here is the short answer: most white LED lights work by having an ultraviolet light with a fluorescence coating to produce white light (which is the way the old school tube-like fluorescent lights work). This means that the white LED also produces UV light (also called blacklight). You just need to block out the visible light—and that’s where the floppy disk comes in. If you take the actual disk out of the floppy, some of them block visible light.

I actually wrote a WIRED post on this—here it is.

https://www.wired.com/2016/12/make-uv-light-phones-led-flash/

Fluorescence of stuff on the wall

The second part of this hack is to use the DIY UV light to read the stuff on the wall. Here’s how that works.

Electrified stair rail

A bad guy is getting away and running down a stairwell. MacGyver pulls some wires out of a wall light and touches one of the wires to the rail and the guy gets shocked and falls.

Would this work? Maybe. In order for the guy to get shocked, there has to be a complete electrical circuit that passes through the dude. That means the current would come out of the wall, go to the rail, go to the guy, go OUT of the guy, and then back to the wall.

In order to get through the guy, he would have to be grounded and the rail would have to NOT be grounded. I suspect that building code requires a rail to be grounded for safety—but you never know. In order to get the guy grounded, he would have to stand on conducting ground (like metal) and have terrible shoes.

But still, it’s at least possible.

Hacking magnetic lock

MacGyver is trapped in another room—with essentially nothing in it. He grabs some wire out of the ceiling panels can fishes out the wires for the security pad. Then he manually enters the keypad code by connecting wires.

OK, this could work. However, it if it’s a legit security pad it would probably be harder to hack.

Wine bottle rocket

MacGyver takes some wine bottles, dumps out some of the wine and recorks them. Then he pumps them up and let’s the cork pop out. Now it’s a water bottle rocket.

Here is the launch in slow motion.

Of course like many MacGyver hacks, this is real. The only problem is that it would take a normal person a few minutes to set up and not a few seconds.

MacGyver needs to take out some remote controlled guns. He grabs a CB radio from a truck and hooks it up to a large power supply. This broadcasts enough static to jam the radio signal to the guns.

Let’s go over the details.

• Could he get a CB out of a truck? Yes. Easy (but it wouldn’t be as quick as he does it).
• Could he hook it up to a power supply? I think he used the power lines to some metal crusher. This probably wouldn’t work. The CB runs on DC current and the big power is probably AC. Also, it probably expects 12 volts.
• Would this jam the signal? Here’s where he might get lucky. If the guns run on the same channel as the CB —it would work. If the power supply messes up the radio so that it just somehow broadcasts on a bunch of frequencies—it would work.

So, it’s possible.

# MacGyver Season 1 Episode 7 Science Notes: Can Opener

Let’s do it.

Ring of fire

MacGyver uses a fire hose to spray gasoline on the ground and then Jack ignites it with a bullet. It looks cool, but there are two problems. One small problem—bullets don’t make sparks (usually)—http://www.cracked.com/article_19781_6-stupid-gun-myths-everyone-believes-thanks-to-movies.html.

Oh, also gasoline doesn’t always burn that well.

OK, but gasoline CAN burn and bullets CAN make sparks—so this isn’t completely crazy.

Fixing an inhaler

MacGyver uses part of a plastic force to fix the part that pushes the release valve of a broken inhaler. This is exactly the kind of thing MacGyver would do.

Battery hydrogen bomb

OK, this is mostly real—but very awesome. Here’s how it works. If you put an electric current through some water, you can break the water into hydrogen and oxygen. In order to do this, you need the water to be able to conduct electricity. NEWS FLASH: pure water doesn’t do this. If you add some salt, water conducts just fine.

Now with the current going through the water, you get tiny bubbles. Bubbles on one electrode (I can’t remember which) are bubbles of hydrogen gas. The other set of bubbles on the other electrode are oxygen.

You can seriously do this on your own. It’s really not hard.

But how would you make an explosion? You can let the hydrogen and oxygen gas mix together—nothing will really happen until you get a spark to star the reaction between the two gases at which point BOOM.

So, in the case of MacGyver he wants to let the hydrogen gas collect at the ceiling. Yes, the hydrogen gas would rise but it’s also difficult to contain. Any tiny crack and it’s gone—but still, it’s a good idea.

What about the ignition? A spark will do. Yes, you can indeed get a spark from a fluorescent light. Especially from the ballast since it makes a high voltage.

Oh, one more comment. What about the 2 psi pressure difference? Is that possible? Probably. Would it knock the door off? I think so—especially if the pressure change is quick. Would it be bad for humans? Could be.

Morse code with the tail lights?

Totally legit.

Final breakout part 1: making liquid nitrogen

OK, this is a stretch. MacGyver uses a carbon dioxide fire extinguisher and some alcohol to make liquid nitrogen. It doesn’t say liquid nitrogen, but that’s what it is (from the air). Let me just show this video:

See. It’s possible. It’s not easy, it’s possible. What about the dry ice? Well, that’s another stretch. Expanding carbon dioxide from a compressed tank CAN INDEED make dry ice. In fact, that’s how we do it in chemistry sometimes.

Final breakout part 2: breaking steel

MacGyver takes the liquid nitrogen and puts it on some steel bars. The get super cold and breakable. The first problem: you would need a lot of liquid nitrogen to cool off the metal. But OK. The second problem—there really isn’t another problem. If you get it cold enough, I think it would work.

# 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.

# All About Rhett: A Resume

Who is Rhett? Where is he from? What are his super powers? Why do you want to contact Rhett for your project? Let’s answer these questions.

Super Brief History

How about just the highlights? Yes.

• Born in the past, not the future.
• B.S. in physics from Benedictine University (although it was called I.B.C. back then).
• M.S. in physics from the University of Alabama (including work at CERN).
• Ph.D. in physics from NC State University.
• Some other stuff here.
• Associate Professor of Physics at Southeastern Louisiana University.

Stuff I Do: Blogging

Yes, I like to write. Blogging is the best (for me). It’s quick and free form. You can use a variety of tools (video, gif, graph, python). It’s just the best. THE BEST!

I should probably include some of my favorite posts—but there are just too many that I love. If you want a post on a particular topic (like video analysis)—just google “rhett allain video analysis” and that should get you what you need.

Here is my WIRED blog stuff.

Oh, I also blog here—but I guess you already know that.

Stuff I Do: MacGyver

I am currently the technical consultant for the CBS show MacGyver. It’s a super fun and awesome job. But what do I do? Essentially, I help out with the “MacHacks” in the show. I look over the hacks and see if they are at least plausibly based on some real thing. Other times, I make suggestions for hacks. Finally, I offer suggestions for some of the science-type stuff that MacGyver says.

Oh, I also make DIY home-versions of hacks. Here is an example.

As a bonus, here is an interview I did with CBS KPIX out of San Fransisco on my work with MacGyver.

Stuff I Do: MythBusters

I’ve actually been working on MythBusters much longer than I have worked for MacGyver. I worked on about the last 4 seasons of the original MythBusters and then I did the reboot with Jon Lung and Brian Louden. Finally, I worked on MythBusters Jr.

So, what do I do? Really, there are two things. First thing is to do some background check on future myths. For some myths, I do some estimations and calculations to see if there is a chance of a myth working. You don’t want something that is either too easy or too hard, it should be just right.

The second thing I do is the science explanations. When there is a short explanation about how something works, I help with that.

Stuff I Do: Books

Writing books isn’t as much fun as blogging, but there it is. Here are the books I have authored.

I could talk about books more—but I’m just going to leave it at that.

Stuff I Do: Python and Numerical Calculations

It’s not my fault. If you want to blame someone—how about Bruce Sherwood and Ruth Chabay. It is through their textbook, Matter and Interactions (Wiley), that I was introduced to python and numerical calculations. It’s awesome.

Once you start solving problems by coding, it sort of gets addictive. I’ve been creating python programs since about 2003 (just a guess). Later I started putting more and more of it in my classes—and here we are.

I’m not an expert coder, but I am an expert at implementing this stuff in introductory courses. That’s what I do. I have even held workshops for teachers and educators. It’s fun.

If you want to look at the stuff I’ve done, here are three versions (all online).

• Physics Python for Mere Mortals. This is an online type book with embedded code. Designed for intro students with zero previous programming experience and some physics. This is what I use in my lecture and lab courses.
• Introductory Physics with Python. This is more like a full book. It’s incomplete, but the idea is to teach physics using python. I want to work on this some more.
• Numerical Calculations in Physics. I wrote these as tutorials for students in the calculus based physics course. It uses more vectors and stuff than the over stuff.

Again, I could list a BUNCH more stuff on python but I won’t.

Stuff I Do: Talks

I’ve done quite a few talks. Instead of listing all my talks (or my favorite talks), I’m going to list my favorite topics:

• The physics of superheroes.
• The physics of Star Wars and science fiction.
• Video Analysis (real vs. fake videos).
• The best physics models (python and stuff) and estimations.
• Science communication (blogging, MythBusters, MacGyver).
• The physics of video games.
• Learning about physics learning (education stuff).
• Physics demos.
• Physics and python.

Other Videos

I’m not a huge video guy, but I do make videos. Here is one that turned out better than I expected. It’s a video made at WIRED.

Here is one more. I like to make videos that show physics solutions.

Wait. One more. This one is a more advanced video.

The End.

Contact me if you want to work on something. I’m always looking for extra jobs.

I wanted to build a spark gap transmitter—you know, for fun.  However, things didn’t start off so great.  Here is how it went down.

My first plan was to build this.

I like it, but it uses an ignition coil and some other thing.  However, check out the receiver.  That’s awesome.  It’s a coherer receiver (I think) and it basically detects a spark with those two bolts in the plastic sleeve.  There should be some iron filings or something in between the bolts.  When a spark is detected, the filings jump the gap and make it a conductor.  I’m not sure why the LED light is connected to a 9 volt battery though.

After that, I just did some google searches for spark gap transmitter and attempted to build the designs I saw.  None of them had capacitor values, so I just had to guess.  But they didn’t work.

I honestly thought I knew how to do this.  I tried a step up transformer with a capacitor.  Nope.  Actually, I was getting a spark on the battery side but not the step up voltage side.  How did I even pass physics courses?

Here is my attempt with a transformer.

Finally, I found a page that used an electromechanical bell.  That works.

I decided to build my own oscillator from scratch.

Homework (for me)

• Make this more solid (the connection to the steel plate is iffy.
• Could you replace the steel plate with a paperclip?
• Can you change the buzzing frequency by adding weights to the oscillating bar?
• Use a step up transformer to get BIGGER SPARKS.
• Build a coherer detector.

I like to solve physics problems.  Here is one for you (I just made it up).

A 4 meter ladder leans against a frictionless wall at a 30 degree angle.  The mass of the ladder is 10 kg.  A human stands 1 meter up the ladder and has a mass of 70 kg.  What is the minimum coefficient of static friction between the floor and the ladder so that the ladder doesn’t slip?

Here is the solution—in video form.

But wait! There’s more.  Let’s do another problem.

Suppose you have the same ladder and the same human.  However, in this case the coefficient of static friction between the ladder and the ground is 0.55.  How far up can the human move before the ladder slips?

I like this question because I don’t know how the answer will turn out.  That makes it fun.  So, let’s do it.

But what kind of problem is this?  I’ll make this a multiple choice question.  Here are your options.

1. A friction problem.
2. An equilibrium problem.
4. A work-energy problem.

Did you pick?  OK, I’ll tell you that there will be quite a few students that will say this is a friction problem because it has a coefficient of friction.  That’s not untrue—but it’s not a good way to classify the problem.  You could also say this is a “ladder problem”—again, not untrue but not helpful.

This is an equilibrium problem.  We are trying to find the point at which the ladder slips—the point it leaves equilibrium, but it’s still sort of in equilibrium.

For an object in equilibrium, there are two main ideas—especially for a rigid object.  First, it must have zero net force.  Second, it must have zero net torque about any point.  In two dimensions, I can write these two conditions as the following three equations.

$F_\text{net-x}=0$

$F_\text{net-y} = 0$

$\tau_\text{net-o} = 0$

Let’s talk about the torque stuff.  I don’t want to get into a whole thing about torque, so let me just say that torque is like a “rotational force” and it can be calculated as:

$\tau = Fr\sin\theta$

In this expression, F is the applied force, r is the distance from the point at which you are calculating torque, and $\theta$ is the angle between r and F.  This equation is equivalent to F-perpendicular times r or F times r-perpendicular.

Oh, torques that would make an object rotate clockwise are negative.

One last thing about torques—especially the sum of the torques.  If an object is in rotational equilibrium about some point (point “o”) then it is also in rotational equilibrium about any other point.  When you set up the torque equation, you can pick whatever point you like to sum the torques—it’s your choice (but choose wisely).

Now we can start setting up some equations for equilibrium.  I’ll start with a force diagram for the ladder.

I know that’s a little busy—but it will have to do.  Here are some comments.

• There are two normal forces.  One from the wall (labeled 1) and one from the floor.
• There are two gravitational forces—and this is a cheat.  There is a gravitational force on the ladder and it is as though it is one force acting at the center of mass for the ladder. If the ladder has a uniform density, the center of mass is the center of the ladder.
• The other gravitational force is fake.  This force $m_2\vec{g}$ is there to represent the weight of the human.  But that gravitational force acts on the human, not the ladder.  The ladder pushes UP on the human the same as the weight.  Since forces come in pairs, an upward pushing force from the ladder means there is a downward pushing force from the human on the ladder.  It’s equal to mg, but not mg.
• The friction force is parallel to the floor.  The maximum magnitude of this friction force would be: $F_f = \mu_s N_2$ where $\mu_s$ is the coefficient of static friction.

Now for some equations.  First, this is the sum of the forces in the y-direction.

$F_\text{net-y} = 0 = N_2 - m_1 g-m_2g$

Just a quick reminder.  These are not vectors.  These are components of vectors in the y-direction.  That’s why the two gravitational forces have a negative sign.  I guess I can simplify this a little bit.

$N_2 = (m_1 + m_2)g$

Next is the sum of forces in the x-direction.

$F_\text{net-x} =0=N_1 - F_f$

Since we are at the point of maximum friction, I can include the expression for the frictional force in terms of the coefficient.

$N_1 = \mu_s N_2$

Notice that I know the value of $N_2$ since it only depends on the mass of stuff—but I don’t know $N_1$.  I’m going to need another equation. That’s where the sum of the torques comes in.

In order to write down the sum of the torques, I need to pick a point about which I calculate the torque.  I’m going to go with the bottom of the ladder.  At this point, there are two forces applied.  Since they are applied at the point about which the torque is calculated, they contribute zero torque and they won’t appear in the equation.  Winning.

Here is the sum of the torques about point O (at the bottom of the ladder).

$\tau_\text{net-o} = 0 = m_2gs\cos\theta +m_1g\left(\frac{L}{2}\right)\cos\theta - N_1 L\sin\theta$

What do I want to solve for here?  I want the distance the human goes up the ladder.  This is the value “s” in the expression above.  Really, I have all the values I need in that expression to solve for s.  I just need to set $N_1$ to the maximum frictional force.  But that would be boring.

Instead, let me make a plot of the frictional force as a function of human distance up the ladder.  That will be more fun, right?

Here’s what I get.

From this plot, the human can go up 1.16 meters before the required frictional force exceeds the maximum.  Oh, here is the code for that plot.

Now that you have the code, you can change stuff—like the angle of the ladder or the mass of the human or whatever.

The end.