Teaching Introductory Astronomy

Through an odd sequence of events—actually, not odd but academically sad, I am going to be teaching the intro astronomy course this semester. OK, if you MUST know the reason, here it goes.

You know I love teaching this Physics for Elementary Education majors course, right? Yes—it uses the Next Gen PET curriculum (which is AWESOME). I put this course together sometime around 2005 for the College of Education. They needed a hands on science course to satisfy their accreditation requirements and this course fit that need. It worked PERFECTLY.

Flash forward to today. Apparently the College of Education decided to drop this course from their curriculum without even telling us. Oh, am I bitter—maybe a little. But since the course isn’t required, I only had 2 students registered. The class was canceled and I picked up an astronomy course. The end.

Now for the astronomy class. This is a class for non-science majors, so essentially no math. I have taught it before, but that was maybe 10 years ago. I want it to be a great course, but I don’t have a lot of time to find some resources. That means I ask twitter for help.

Here are some of the suggestions.

I’ll post more stuff as I find them.

MacGyver Season 2 Episode 18 Science Notes: Riley + Airplane

Carpet body restraint

There’s really not too much to explain here. MacGyver rolls a guy up in a carpet and uses that to keep him in place. Seems like this should work.

Reminder—even though there’s not any great science to talk about here, I love these kinds of hacks.

Pick a lock with an antenna

MacGyver pulls a metal antenna off a motorcycle and then uses it to pick the lock on the same motorcycle. Oh, this is a police motorcycle—he uses it to turn on the siren.

Is this possible? Possible, yes. Normally you need two things to pick a lock—something to move the lock pins and something to provide torque. Theoretically, you could do it with one—but it would be tough.

Radiator fire extinguisher

There’s a car on fire and they need to get something out of it. MacGyver improvises a DIY fire extinguisher by using the cooling system of another car. He grabs a pipe and then pokes a hole in the radiator. When the engine is revved up, the coolant shoots out through the pipe and onto the burning car.

Again, the theory here is solid. You could probably even get the coolant to shoot fairly far. However, it would take a significant amount of coolant to put out a fire.

Statue battering ram

This one is good. MacGyver and Jack take a marble statue and support it from some rope and stuff such that it can swing. Then they swing this statue into a door. Boom—battering ram.

Hacking airplane wifi

OK, not hacking to get free wifi but using the wifi to control the whole plane. That’s what Riley does as she is stuck on a plane with a virus. Is this even possible?

Sadly, this might be true. Check this out. https://www.wired.com/2015/05/possible-passengers-hack-commercial-aircraft/

MacGyver Season 2 Episode 17 Science Notes: Bear Trap + Mob Boss

Radioactive Stuff

This is not a MacGyver-hack. But he is talking about radioactive stuff. Here is my super short explanation.

An atom is made of a positive nucleus plus some electrons. Normally, there will be the same number of electrons as protons in the nucleus. This makes the overall charge of the atom zero. But wait! There are also neutrons in the nucleus. Let’s just look at an example.

Strontium-90 is an atom with 38 protons and 38 electrons. It also has 52 neutrons in the nucleus—oh, and it’s radioactive. Strontium-88 also has 38 protons and electrons, but it only has 50 neutrons. Since these two atoms have the same number of protons, but different neutrons—they are isotopes of each other.

Sr-90 is radioactive. That means its nucleus is unstable. It goes through radioactive decay and produces Yttiruim-90 which has 39 protons. So, one of the neutrons in Sr-90 turns into a proton and also produces an electron (so that charge is conserved).

The half-life of Sr-90 is 28.8 years. Since the decay process is random, there are more decays when you have more atoms. This means that the decay rate depends on the number of atoms. So, you can’t say how long it will take for all the material to go through radioactive decay. Instead, we say how long it will take for half of it to decay—that is the half life.

So, does that mean that all of the material will decay in 2 times 28.8 years? Nope. It means that after 28.8 years, you will have half as much stuff. After another 28.8 years you will again have half of what you started with. Every 28.8 years, you will have half as much.

What about Plutonium? There are several different isotopes. Plutonium-238 has a half-life of 87.7 years. Plutonium-239 has a half life of 24,000 years. The shorter the half-life, the more “radioactive” it is. But for longer half-life stuff, it’s still going to stick around for a long time.

DIY Geiger Counter

A Geiger counter is a device to measure radioactivity. It consists of an outer conducting tube with a conducting wire running down the middle. The wire and tube are at different electric potentials—usually a fairly high voltage.

When a charged particle (the result of a radioactive decay) enters the tube, it interacts with the gas molecules to ionize them—knock out an electron. This free electron then accelerates in the high potential and collides with other gas molecules to produce even more free charges. More and more charges are produced—it’s called an electron avalanche. This avalanche is then detected as an electron current—usually to produce an audible click. That’s the clicking sound you hear.

Here is MacGyver’s build.

He uses a motorcycle muffler as the tube—you can’t see the wire in the middle, but it’s there. The battery provides the voltage and then there is an audio speaker to convert the electron avalanche into sound.

Oh, I forgot—here is a video demo.

Bonus. Here is my sketch for the DIY Geiger counter.

DIY Ice Climbing Gear

There’s really not much to explain here. MacGyver uses some cut up chain link fence and some rebar to make spike-shoes and a type of ice pick. He uses these to climb up an old brick wall—sticking the metal into the old mortar. It seems like it would work.

Disabling Trucks

MacGyver builds one of those rolling things that mechanics use to get under cars (I forget the name). With that, he disables the trucks (all but one). Really, once he is under the truck he could do a number of things. He could cut the fuel line or cut an electrical line—let’s just say he does it.

Simulated Radioactivity

In order to convince the mob that the key guy they are trying to catch isn’t worth it. They make it look like he is super radioactive. The mob guys have a Geiger counter—so MacGyver needs to remotely set that off.

My original idea was to have him build an electron gun—like this https://www.instructables.com/id/DIY-Electron-Accelerator-A-Cathode-Ray-Tube-in-a-/

The electron gun would shoot electrons into the Geiger counter and set off an electron avalanche in the same way that a radioactive source would. Unfortunately, it was too complicated of a build.

What about a super high amplitude electromagnetic wave? The idea is that when the electric field part of the wave hits the detector, it will increase the field inside and make it more sensitive to ionizing radiation. This would make it seem like something is more radioactive than it is. Yes, I know this is a stretch—but it’s based on some real idea.

Here’s how he could build it.

  • Start with a radio – it has to be a transmitting radio.  Maybe a CB, maybe a portable radio?
  • Boost the power to the radio.  I think plausibly connect the power source from the radio to the 12 volt car battery?  
  • Get a bent piece of metal to put around the radio antenna to make a focusing dish.  Alternatively, you could find some pre-existing dish- aim the dish-radio at the Geiger counters.  boom.

MacGyver Season 2 Episode 16 Science Notes: Hammock + Balcony

Sorry for the delay in science notes. There were things that got in the way.

Block and Tackle

I don’t really like calling it that. A better term is a compound pulley. But the key to all of the simple machines is force vs. distance. If you increase the distance over which you apply a force, you can get a larger force output of the machine over a shorter distance. That’s exactly what happens with a “block and tackle”.

Here is a more detailed post about compound pulleys. https://www.wired.com/2017/01/physics-of-a-compound-pulley/

If you want to see a DIY pulley from an earlier MacGyver episode, here you go:

Drinking Without Getting Drunk

This one is pretty good. Yes, you can drink without getting drunk. Check this out (actually, don’t read it because you don’t have access) —https://www.nature.com/articles/nnano.2012.264

But basically it’s alcohol oxidase with some other stuff added. When mixed with alcohol, this makes aceteldehyde—the main thing that causes hangovers. MacGyver could get alcohol oxidase from alcohol test kits.

Note: I’m not a biochemist, I got this info from my brother (a biochemist).

DIY Chloroform

Don’t make this stuff. But it is possible. Actually, I’m not even going to include the link.

Mechanical Stuff

Using a hammock as both a ladder and a body sling? Yup, that’s good. Using a chain as a DIY car boot. That works too. Not much to explain, but I think both of those hacks are great.

Chloroform Bomb

In order to knock out everyone in the room (including himself), MacGyver throws some chloroform in a container into a fire. It explodes and everyone gets knocked out. Sure, this would be tough to do in real life—but it’s a least plausible.

DIY Arc Lamp

Wait. There wasn’t an arc lamp in this episode was there? Nope. It get cut out of the beginning. However, I made an arc lamp anyway as part of my DIY videos. It’s awesome.

Notes and Comments on AAPT Summer Meeting 2019

Here are some things I need to share regarding this meeting. Overall, it was great to see everyone. As usual, the conversations were the best. I regret that there were some people I did not get to meet or talk to—maybe next time we will meet up.

Blog vs. WIRED

One question that came up multiple times was about this blog vs. my posts at WIRED. How do I decide where a post goes? OK, here is my explanation.

In the beginning, there was a blog. A blog was super informal and free form and alive with comments. It was like the 60s and I was a hippie. A physics hippie. I don’t know if this early blogging era was like the 60’s or maybe the wild-wild west. Well, the comments eventually turned into the wild west with a shoot out at the O.K. Corral.

When I moved to WIRED, everything was the same except it was at a different site. But I’ve been there for a LONG time (9 years?) and things evolve. My posts at WIRED are more edited and geared for a specific audience. That’s not bad, it’s just different. I don’t think I can just write whatever I want like I did in the old days. No more random posts that just talk about my cat (I don’t even have a cat).

So, that’s where this blog comes in. It’s a place where I can post whatever I want and no one can stop me. These are the kinds of things you will find here.

  • Random posts (like this one) that are just an outlet for me to write stuff and tell stories.
  • Explicit educational material. If a post needs too many equations, I would rather put it here. Many WIRED readers (while very education) don’t really get into all the equations. Also, since WIRED is paywalled it makes it more difficult for educators to access the stuff (in the off chance that they might find it useful).
  • MacGyver science notes. Oh sure, I post some MacGyver stuff on WIRED—but I really don’t think they want to see 50 posts on different episodes. So, those are here.
  • I think that pretty much covers it, so I don’t even need this last point.

In the end, I apologize for the confusion with the two blogs. Oh, actually there are three. I recently wrote a post on OneZero Medium (analysis of a car crash from Stranger Things) also. Not sure how much I will write there—but it’s still me.

What’s up with all the drone videos?

Yes, I have a drone. I love my drone. I can only hope my drone loves me as much as I love it. Honestly, I am honored that you even noticed my drone videos.

Oh, wait. You haven’t seen them? I can fix that.

In case you are curious. This is a DJI Spark. Great drone.

More Comments

Here are some more short comments.

  • Meeting with Bruce Sherwood and Ruth Chabay was great. I wish I had a picture with both of them (I did get one with Bruce though).
  • Bruce made this epic comment in regards to numerical vs. analytical calculations. People claim that analytical solutions are better because you can solve a problem in terms of known functions like sine and cosine. But how do you find the value of the cosine function? YUP – numerically or in a table or in an infinite series. So, in a way all solutions are numerical. Win for numerical.
  • The other deep thought by Bruce was a discussion on his AJP on energy. Read that paper. This sums it up. You can not find the work done by friction. Friction is crazy hard. I think I might write a WIRED post on this.
  • Eric Ayars had an excellent presentation on chaotic systems. One system was a bouncing ball on a moving floor. I wonder if there is a case where the ball just stops—this could happen if the relative collision speed of the ball and floor is zero.
  • The 30 demos in 60 minutes was pretty good. I love these things. Even though I’ve seen many of these demos before, I always find something new. Here is their site http://30demosin60minutes.com/
  • I went hiking. It was super hot, but I had a great time.

Using video games to build models. AAPT AD05

Here are the resources and links for my AAPT talk at the Summer Meeting. These are probably in chronological mass (unless I change my mind at the last minute).

How Do you Put Python in your Introductory Course? EN 01

This is just a resource page for my talk at AAPT. Here are some links (in order of appearance)

The physics of Michael Jackson’s moonwalk

Note: Originally posted on June 2009

Was the moonwalk fake? No, not the Apollo landings. I am talking about Michael Jackson’s moonwalk. You got to admit, he had a big impact on a lot of stuff and this is my way to give him respect – physics.

I am sure you know about the moonwalk. Maybe you can even do the dance move yourself, but how does it work? First, here is a clip of MJ doing his stuff.

As a side note, I can’t remember where I saw it but there was a great discussion of the history of the moonwalk. If I recall correctly, some were saying Michael didn’t create this move. One thing is for sure, he made it popular. Now for the physics.

The key concept here is friction. Friction is actually uber-complicated, but a simple model works for many cases. Static friction is a force exerted on an object when it is in contact with some surface but those two surfaces do not move relative to each other. Kinetic friction is a force exerted on an object when the two surfaces are moving. Suppose I have a block at rest on a table and I pull it with a slowly increasing force. This is what it would look like:

Two key things from this graph. As you pull on the stationary block, the block doesn’t move. If I pull with 1 Newton, and it doesn’t move then the frictional force is 1 Newton. If I then pull with 2 Newtons and it still doesn’t move, the frictional force is 2 Newtons. The static frictional force does what it can to make the thing not move – but not more than it can. This leads to the static friction model of:

F_\text{static} \le \mu_s N

In this model, the force is less than or equal to the product of some coefficient (that depends on the two types of surfaces) and the normal force (how hard the two surfaces are pushed together). The direction of this frictional force is parallel to the surface in the direction that prevents the object from sliding.

The other key feature in the graph is the small jump down when the thing starts to slide. This is because the coefficient of kinetic friction is typically smaller than that for static friction. Also, if the object is sliding, the frictional force is constant.

F_\text{kinetic} = \mu_k N

Back to Michael and the moonwalk. The key here is: how do you make one foot slide and the other not slide? If both feet are stationary, then this is dealing with static friction. I could make the frictional forces on these two feet different by changing my center of mass. Here is a free body diagram:

Since he is not accelerating up and down, the following must be true:

N_1 +N_2 = mg

These are the forces in the y-direction. They must all add up to zero so that:

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

There is another condition that must be satisfied. Since he is not rotating, the total torque about any point must also add up to zero. If you want more info on torque, check out this post (Note: link doesn’t work). But for this post I will just say that torque is like the ‘rotational force’. It depends on the point about which you want to rotate and is essentially the force applied times the perpendicular distance to the point of rotation. For the free body diagram of Michael, I have chosen one of his feet to be the point about which he is not rotating (I could chose any point). This makes 3 of the forces have zero torque (N2, F2 and F1 have zero torque because the perpendicular distance to point O is zero). Here I labeled the other important distances:

The only two forces that exert torque about O are the weight and the N1 force. They have opposite directions of torque because they would cause rotation in different directions. This along with the previous equation gives:

mgr_1 = N_1r_2

N_1+N_2 = mg

Eliminating mg, and solving for N1, I get: (I know the indices for the forces and distances don’t match)

N_1 = N_2\left(\frac{r_1}{r_2-r_1}\right)

If his center of mass is in the middle, then r2 – r1 = r1 and the two normal forces would be equal (as you would expect). If the center of mass is more towards the foot on the right, then r2 – r1 is less than r1 and N1 will be larger than N2. This will make the frictional force on the foot to the right greater and the other foot slide.

Well, what if r1 is greater than r2? One of two things would happen. Either he would fall over, or there would have to be a force pulling the foot on the left down. This is similar to Michael Jackson’s trick in “Smooth Criminal”.

Here he used special shoes that connect to the floor so that he could do this. More details on this page.

Ok. So that is how Michael gets one foot moving. How does he keep one foot sliding and the other not sliding? It is really the same thing as above except that he can increase the force on the moving foot a little bit more since it is sliding. Sounds easy, but Michael could really make it look cool.

Finally, I just want to show another demo that is essentially the same idea.

You can find more details on the meterstick demo in this blog post.

MacGyver Season 2 Episode 15 Science Notes: Murdoc + Handcuffs

Non-science note. The more I see David Dastmalchian as he plays Murdoc, the more impressed I am. That dude is awesome. Also, this episode has a great “Midnight Run” type of feel (great movie, btw).

Now for some science stuff.

Portable Gas Chromatagraph

This part of the script is pretty nice:

Jack: “You brought something too?”

MacGyver: “It is a portable gas chromatograph. So, It takes samples from the air and it scans them for explosive particular matter.”

Jack: “So, it’s a bomb detector? Why didn’t you say that?”

MacGyver: “I thought I did.”

Perfect.

A gas chromatograph is sort of complicated. But yes, it could be used to detect chemicals. Yes, you can make them portable. That’s enough.

DIY Noise Maker

How do you get the attention of a bad guy? House about a noise maker? This is essentially the same as this mouse-trap powered car. https://www.wikihow.com/Build-a-Mousetrap-Car —but instead of a spinning wheel, it

Oh, it has a built in timer—a leaking bottle of water. Here is the shot from the show.

And here is my rough sketch.

Speaker Microphone

Oh snap. That pay phone is broken. Wait! What’s a pay phone?

MacGyver fixes the missing headset by using two speakers from the car. One speaker is used for the speaker and the other speaker is used for the microphone.

It’s actually pretty cool, but most speakers can be used a microphone. The normal speaker is basically just three parts:

  • A magnet.
  • A coil of wire.
  • Some type of surface area to push air.

The coil of wire is connected to the speaker surface. When current is run through the wire, the coil makes a magnetic field. This magnetic field from the coil interacts with the other magnet to either push or pull the surface of the speaker. This in turn pushes the air into compressions—and it is these compressions in the air that make the sound.

For a microphone, the reverse happens. Compressions in the air push the surface. This moves the coil closer (or farther) from the magnet. This motion changes the magnetic flux (via Faraday’s Law) which induces a current in the coil. This current is then recorded as an audio signal.

Don’t believe me? You should try it. Oh, make sure you use a speaker like this:

OK, there are some weird things in these old phone head sets. I think they have to use super low resistance microphones and speakers since the power comes over the phone line. But still, this is very plausible.

Break a chain with handcuffs

Oh, and a steel rebar thing. MacGyver loops the handcuffs around the chain and then puts the rebar through the cuffs and twists.

With a longer metal bar, you can get a high torque on the chain. This should break the chain—at least as long as the hand cuffs are stronger than the chain.

Back of the Envelope Estimation Problem for Faraday’s Law

I told my students I would solve this problem for them. It’s a real life problem too.

Is it possible to use a Neodymium magnet and a coil of wire to get an LED to light up?

That’s the real version. But because I was afraid students would be overwhelmed, I added the following:

  • The magnet has a maximum magnetic field of 0.2 Tesla.
  • The LED requires 1.5 Volts and 10 mA to light.

I like this problem because I don’t know the answer. Also, the answer is useful. If you want to do a physics demo showing the voltage induced by a changing magnetic field—what better way than with a hand-held magnet and a small light?

But will it even work? Let’s get started. Here is a diagram.

By changing the magnetic flux through the coil, this will create a curly electric field and an electromotive force (a change in electric potential). The magnetic flux is defined as:

\Phi = N|\vec{B}|A\cos\theta

Where N is the number of turns in the loop, A is the area of the loop and \theta is the angle between the magnetic field and a vector perpendicular to the area. In the diagram above, \theta = 0 since the magnetic field is perpendicular to the coil.

A change in flux produces a voltage according to Faraday’s Law:

\Delta V = \frac{\Delta \Phi}{\Delta t}

Note: yes, I’m different. I think that the number of loops (N) is part of the magnetic flux and that the minus sign in Faraday’s Law doesn’t really mean anything.

Putting the flux into Faraday’s Law, I get (assuming \theta = 0):

\Delta V = \frac{NBA}{\Delta t}

Now for some estimates. I could just estimate everything and then calculate the voltage—but instead I’m going to estimate everything except the number of turns. I can then solve for N and see if it’s reasonable.

Here’s what I have.

  • B = 0.2 T
  • A: radius = 0.01 m
  • Time interval = 1 second
  • Voltage = 1.5 V

Solving for N:

N = \frac{\Delta V \Delta t}{BA}

This is the perfect case to use python for your calculator. You can put your estimates as variables so that you can easily change things up. Here is my code. I get the following output.

Umm….yeah. That’s 23 thousand turns. I’m not going to do that. Even if decreased the time to 0.1 seconds, I would still need 2000 turns. Arg.

Oh, what if I just make a HUGE loop? Nope. That wouldn’t work. In my estimation for the change in flux, I assumed a constant magnetic field—this is obviously not true, but good enough for a small loop. With a big loop, you would have some of the magnetic field creating a negative flux. It would just make things worse.

What if I put the magnet on a spinning stick (run by a motor)?