# Physics and Education Majors

There is this course.  It’s called Physics for Elementary Education Majors (PHYS 142) – maybe that’s not surprising.  Anyway, I really like this course – it’s awesome.  Let me tell you a little about the history and future of this course.

According my email archive, I think this course was created in 2003.  Ok, technically it was created before that but 2003 is when we started offering the course again.  Actually, the fact that the course already existed made it much easier to get it going.  If you have ever been part of a university curriculum committee, you know what I mean.

We created the course for the College of Education.  They needed a science course for their elementary education majors that satisfied some particular component of NCATE (the accrediting agency for Colleges of Education).  I honestly don’t know (or can’t remember) what specific thing the course was supposed to do – but there it was.  This course was perfect for them.

The first semester I taught this course, I used the Physics by Inquiry (McDermott) curriculum.  This curriculum was especially designed for education majors – and it’s quite awesome.  However, there was one problem – maths.  There isn’t a ton of math in PBI but there is enough to make students panic.  I think they should indeed work through their issues with math, but it was causing problems with the course. Note: I tell students that they shouldn’t say “I’m not a math person”.

After math troubles, I decided to switch to a new curriculum.  At the time it was called Physics for Elementary Teachers (PET) but was later changed to Physics and Everyday Thinking (also PET – by Goldberg, Otero, Robinson).  Here are some of the awesome features of PET.

• Student learning based on evidence collected (not authority learning from the textbook or instructor).
• Explicitly includes ideas about the nature of learning.
• Emphasis on model building and the nature of science.
• Includes children’s ideas about physics.
• Math isn’t a barrier.
• OH, the best part.  The new version of the curriculum is called Next Gen PET.  This version explicitly aligns with the Next Generation Science Standards.  This should be a huge win for the College of Education.

Honestly, it’s great stuff.  Oh, there are still problems.  Students get caught up in the whole “why don’t you just tell us the answer?” thing – but I can work around that.

But like I said – this is the course that we have been teaching for 15 years (wow – even writing that is incredible).  This course was designed for the College of Education.  We typically have been teaching three sections of the course each semester with an average of about 25 students per section.

PHYS 142 Today

I accidentally discovered something recently.  The education majors informed me that PHYS 142 is no longer required in the curriculum.  What? How can that be?  Yup, it’s true.  The new science requirements for elementary education majors have the following three courses:

• Biology 1
• Biology 2
• Earth Science

That’s it.  I’m sure those are fine classes – but they miss a big thing.  They don’t emphasis the nature of science.  In fact, I suspect that these three classes might actually decrease the students’ understanding the nature of science.  Since these three courses have quite a bit of memorization elements in them, students might come away with the belief that science is about facts and not model building.

Yes, I’m not too happy about this.  Not only do I think this course is perfect for education majors (who will be the first to introduce science to children in many cases).  I also genuinely enjoy teaching this class. It’s great to interact with students and see them increase their understanding.  There’s nothing quite like being there when a student starts putting different ideas together.  It’s great.

On a logistical note, this course as some other huge impacts.  First – teaching load.  If we have 3 sections of this course, that would be 15 hours (it’s a 5 contact hour course).  Getting rid of the course will lose 15 contact hours for the department.  That’s one instructor position.  That sucks.

Oh, also I usually teach this course during the summer session. That’s going to suck to not have this.

# MacGyver Season 3 Episode 5: Dia de Muertos + Sicarios + Family

Battery and Solar Powered Fridge

It’s not a big “hack” in the episode, but we see MacGyver walking through the build a refrigerator.  You could use solar power and a battery with a normal fridge, but there is also the peltier cooler option.  The peltier is a small solid state device – when you run current through it, one side gets hot and one side gets cold.  You can use this device to cool the inside of a fridge.  It’s not super efficient, but it’s very simple.

Actually, I started to build one (but I haven’t finished).  Here is my progress so far.

Modify boom box to pick up beacon signal

So, Oversight builds a homing beacon.  He sets the frequency to 457 kiolhertz.  MacGyver needs to modify the radio to pick up this frequency.  He needs to make the modification because a normal AM radio only goes down to about 540 kHz.

So, how do you change the tuning frequency of a radio?  Let’s look at a super basic radio (a crystal radio).

There are two parts to tuning a radio – there is the capacitor (above that would be the tube with the aluminum foil) and the inductor (the tube with the wire wrapped around it).  The radio will amplify the signal with the frequency that matches that of the inductor plus capacitor combo.  So, just change on of those and you can change the minimum frequency of the radio.

Shock vest

I love the parts where MacGyver and Oversight argue about physics stuff.  Here are some notes about their discussion.

• Does shortening the wires reduce the resistance?  Yes.  That’s true.
• You can get a shock by storing electrical charge in a capacitor – that is true (but most taser type things don’t do it that way).  More capacitors means more charge and more shock.

50 foot drop calculation

How fast would you be moving after a drop of 50 feet?  Let’s go over this calculation really quickly.  I hate imperial units, so I am going to switch to metric.  Since it’s about 3 feet per meter, 50 feet would be about 15 meters (rough approximation).

When an object falls, it has a constant acceleration of – 9.8 m/s^2 (assuming no air drag).  That means that for each second that it falls, it will increase its speed by 9.8 m/s.  We can write this as:

$-g = \frac{\Delta v}{\Delta t} = \frac{v_2 - v_1}{\Delta t}$

Since the falling object starts from rest, the initial velocity is zero.  We can then solve for the final velocity.  Oh, this is all in one dimension.

$v_2 = -gt$

Oh, I am assuming the initial time is also zero. But we don’t know the time the object falls.  Let’s look at the two definitions of average velocity:

$v_\text{avg} = \frac{v_1+v_2}{2} = \frac{\Delta y}{\Delta t}$

I can use this with the previous equation to eliminate time.

$v_2 = \frac{2\Delta y}{t}$

$t=\frac{-v_2}{g}$

$v_2 =\sqrt{-2g\Delta y}$

So, the change in y is -15 meters and let’s just say g = 10 m/s^2.  That puts the final velocity at the square root of 300 or about 17-18 m/s.  That’s like 40 miles per hour.

Fan motor as a brake

MacGyver lets a rope wrap around a spinning fan motor.  He then uses that to go down a building (not slow – but slower than falling).  Yes, this is plausible.  It would be better if the motor is on since then there will be a resistance to spinning it.

I could probably say more about this – but it would get complicated quickly.  Oh, how about this – a motor is the same as an electric generator.  It just depends on how you use it.

# Trip Report: Texas AAPT/APS Section Meeting

Since this is just a normal plain blog, I can do silly things like this report on my recent trip.  Why not?

Where and Why?

I was invited to give the keynote address as well as a workshop on python at the AAPT/APS section meeting at the University of Houston.  Since this isn’t too far away, I decided to just drive there – it’s about a 5 hour trip.  Not bad, plus I can bring as many pairs of shoes that can fit in my car.  I brought one pair of shoes.

I drove in on Friday and arrived Friday evening – I stayed at hotel on the outskirts of Houston.

A note regarding section meetings.

I really like section meetings.  They are smaller, cheaper, and it’s easier to get around and see everyone.  Oh, national meetings are cool too – but sometimes they are just too big.  Also, who likes paying 500 dollars just for registration?  Not me.

Python Workshop

For the workshop, I used my python material.  This is essentially the same stuff I used at the Chicago Section of AAPT.  Here are some notes.

• The material basically this stuff on trinket.io.
• I also have instructor materials and other files posted on the PICUP site.
• It seems there were about 15 participants. The room had computers for people to use – that helps out a bunch.
• There was an issue with the projector – it wasn’t quite working.  Someone brought in a backup, but it wasn’t bright enough.  It’s funny how small problems like this can make a big difference when people are learning.
• Another issue for python workshops – variety of people.  Some people have never used python and some have experience. This makes it slightly difficult.
• Other than that, I think the workshop went well.  I had one person ask me afterwards how to become an expert with python.  My response was to just keep practicing.  The best way to learn is to learn python to solve particular problems.  It’s pretty tough if you try to learn stuff without a purpose.  Oh, also – sloppy code is fine.

Keynote: Science Communication with MacGyver and MythBusters

Normally, I give a talk that focuses on physics of science fiction or video analysis or something like that.  I’ve talked about science communication before – but in this case I wanted to include a bunch of examples from MacGyver and MythBusters – so I had to make a new talk.

Check out the venue (maybe it’s difficult to see from this pic though):

This is the “club level” of the University of Houston football stadium.  No, there wasn’t a game going on at the time (but that would have been funny).  It was a nice place – the screens were in a weird position, but still it was nice.  Oh, I did make one fairly big mistake.  I was having trouble with the projectors and I ended up with “mirroring” on my computer.  This means that I didn’t see the next slide and and I didn’t have a clock. I really like seeing a clock.

For the talk, I focused on 4 “rules” of science communication:

• You can’t be 100% correct, but you can be 100% wrong
• Build a bridge from the science to the audience (complicated, conceptual, or shiny physics).
• Science fiction is still fiction.
• Use mistakes as a foot in the door to talk about what you want.

Overall, I think it went well.  Oh, there was one great question at the end.  “How do we use science communication to help people understand climate change?”  My response: we need to focus on the nature of science and understanding of what exactly science is all about.

Finally, here is another picture. This is me on the football field (which was kind of cool).

# MacGyver Season 3 Episode 4

Pressure Lift Bag

This one is pretty awesome.  MacGyver needs to lift up a truck to get it un-stuck.  So, he takes a rubber bladder (not sure where he got it – it could be part of a shock) and connects it to the exhaust (or maybe he connects it to the liquid oxygen).  Anyway, he fills the bladder with an expanding gas.  The bladder fills up and lifts the truck.  This would totally work.

Check out this version you can do yourself.

How does this even work?  Ok, so you have a trash bag.  When you blow air into it, you can approximately get a pressure of 2 atmospheres (just a guess).  The force from this pressure depends on the surface area using this formula:

$F = PA$

If you want to lift a human (mass of 75 kg) with a gauge pressure of $10^5 \text{ N/m}^2$, how big of an area would you need?  Solving the above equation for A:

$A =\frac{mg}{P} = \frac{(75\text{ kg})(9.8 \text{ N/kg})}{10^5 \text{ N/m}^2} = 0.00735 \text{ m}^2$

That might seem like a tiny area – but that would be a square about 9 cm on a side.  So, this is clearly possible (as you can see in the video that I actually did it).

Liquid Oxygen

We normally think of oxygen as a gas – and at room temperature it is indeed a gas.  Actually, it’s a molecular gas of O2 – two oxygens bound together.  I guess we should first talk about air and oxygen.  Yes, we need air to breath – but air is more than just oxygen.  It’s approximately 21 percent O2 and 79 percent nitrogen gas.

If you decrease the temperature of oxygen gas – it will turn into a liquid.  Yes, it has to be super cold at negative 183 C.  How cold is this?  Here is a video that shows how cold this is (and liquid nitrogen) along with some of the cool things you can do with super cold stuff.

High Pressure Air

Humans can survive under very high pressures.  However, there is a problem with breathing high pressure air.

The nitrogen in high pressure air can be absorbed into your tissues and stuff.  When the human then goes back to a lower pressure, this nitrogen comes out of the tissues.  If the change in pressure happens too fast, this nitrogen can bubble and cause all sorts of problems.  This is basically what we call decompression sickness.

The other problem is oxygen.  At 21 percent oxygen at normal atmospheric pressure, everything is fine for humans (since we live in this stuff).  However, as the pressure increases, the partial pressure of oxygen also increases.  At normal cases, the partial pressure of oxygen is 0.21 atm (atmospheres).  If you have 50 percent O2 at atmospheric pressure, this would be 0.5 atm.  The partial pressure is the current pressure multiplied by the fraction of gas.

Here’s the deal.  If the partial pressure of oxygen gets over 1.6 atm, bad stuff happens.  Stuff like convulsions.  Oxygen is bad stuff.  How do you get a partial pressure of 1.6 atm?  If you increase the pressure, the partial pressure of 21 percent O2 is 1.6 atm.

OK, now back to the show.  MacGyver can survive in high pressure one of two ways.  Method number 1: don’t breath air.  If he breathes a gas mixture that has a lower concentration of oxygen, This is what deep divers do when they breath mixed gasses like trimix.  Method number 2: use a constant atmosphere suit so that he stays at 1 atm pressure.  That’s what he does in this case.

What happens if MacGyver pulls out his air hose? Yup.  That would work.  Even at super high pressures.  Oh sure, his lungs would get super small because of the external pressure – but that’s just fine.  This is exactly what happens when a free diver goes deep (breath holding).

Oh, he would have to equalize his ears just like a free diver.

# MacGyver Season 3 Episode 3

Transparent Explosives

Yes, this is probably real – http://www.guns.com/2016/10/21/army-working-on-high-tech-see-though-explosives/

Liquefaction of Sand

This is real.  You can make a simple version of this yourself.

Or you could make a crazy huge version like this.

Weather balloon pop

MacGyver needs to get a thermal camera down from a balloon.  The balloon (it’s not actually a weather balloon) is tethered down by multiple lines.  So Mac uses the jumper cables from the car and connects them to the car battery.  Then he connects ONE cable to the wire and the current causes the balloon to burst.

OK, let’s step back for a moment.  Remember that this is a show – this is not real life.  I just want to make sure we are all on the same page there.  So, there’s a small mistake here (you can blame me if you like).  In order to get an electric current from the car battery to go through the balloon, you would need to make a complete circuit.  One jumper cable connected to the line is a start, but there needs to be a path for the current to get back to the battery to make a complete circuit.

One way you could get this to work is to take another line going to the balloon and connect the other jumper cable to that one.  If you look close, it seems like the other cable isn’t connected to anything (in the show).  Of course, that mistake is better than connecting both wires to the same line.

This is sort of the same problem as this double spark in Iron Man 3.

Thermal camera

Yes.  Thermal cameras are indeed real.  Yes, the heat signature of an electric car would be different than an internal combustion engine car.  Actually, I need to see how hot they get in real life (electric cars).  I’m going to test this the next time I see a Tesla.

Oh, and here is an overview of seeing stuff in infrared (also called “thermal image”).

Just for fun, here is a visible and infrared image of me with a bag over my head.

X-Rays from a Vacuum Tube

MacGyver needs to find the transparent explosive.  One of the tools he needs for this is a source of x-rays.  This seems to be real – but it appears you can make x-rays from a vacuum tube, a lighter (the long kind) and a diode.

Here are the instructions from hackster.io (I need to build one of these).

There are so many cool parts of this hack, I could probably write a book on just this one thing – maybe I will write a separate post.  This x-ray device does the following:

• Uses a vacuum tube from an old radio.  Historically, the vacuum tube was used where transistors are used now.  These things are awesome.
• The lighter has a piezoelectric in it.  When you apply a pressure to these devices, it produces a voltage – the voltage can get high enough to make a spark in air which lights the gas from the lighter.
• When you connect the piezoelectric to the vacuum tube, you can make a super high voltage inside the tube.  This can accelerate electrons such that they crash into the other electrode.  This crashing electrons is exactly how you create x-rays.
• X-rays are just like normal visible light except that they have super small wavelength.  This can make them interact differently with matter.  For instance, they can pass through some materials (like human skin).
• What is the x-ray used for in this hack?  X-ray fluorescence.  This is essentially the same as glow in the dark (kind of) material except get’s “activated” with x-rays instead of other visible light.

Oh wait! I already have a video on x-ray fluorescence.

One final note.  In the show, MacGyver says something about shooting ions.  That’s not really what happens here.  X-rays are not ions.

Hydrogen balloon from a trash bag.

Can you fill a trash bag with hydrogen?  Yes.  Will it lift stuff?  Yes.  Could it lift a trash can?  Maybe…just maybe.

Here is my super short introduction to buoyancy.

Suppose you take a box of air – the box is 1 meter on a side such that the volume of this air is 1 x 1 x 1 = 1 m3.  Assuming there is no wind or breeze, this “box of air” will stay in the same location.  Since the box is at rest, the total force acting on the air must be zero.

OK, there is obviously a downward gravitational force on the air puling it down.  Yes, air has weight.  If something has mass, it has a gravitational interaction with the Earth.  Everyone likes to think of air as being weightless – but that’s probably because it has a low density and it’s normally “floating”.  But if there is a downward gravitational force on the air, there must be an upward force pushing to counteract the weight.  This upward force is the buoyancy force.

Since the box of air floats, we know the buoyancy force has to have the same magnitude of force as the weight of the air.

Now let’s suppose I take away that “box of air” and replace it with a sealed cardboard box (it could be a box made out of anything, but in my mind it’s a cardboard box).  The air around this box is going to interact with it in the same way as it did with the box of air (because air is dumb and doesn’t know any better). This means the cardboard box has the same buoyancy force as the box of air – it is equal to the gravitational weight of the air the box displaces – this is essentially Archimedes’ principle for floating stuff.

Oh, this buoyancy force is still the same no matter if the object is floating or not – it just has to displace air.  You can also do this with water or really any substance –  like pudding.  Not sure why you would float something in pudding.

But what if you want to calculate this buoyancy force?  In that case, you need to know the density of the air (which is around 1.2 kg/m3) and the local gravitational field (9.8 N/kg).  With that, the buoyancy force would be:

$F_\text{buoyancy} = V_\text{object} \rho_\text{air}g$

Finally, we are getting somewhere.  Now you can calculate the size (solve for V) of a balloon needed to lift a trash can.  If you want a simple estimate – you can ignore the mass of the hydrogen in the balloon (but it does indeed have both mass and weight just like the air).  I’m leaving the rest of this as a homework assignment for you.

# MacGyver Season 3 Episode 2

I’m going to change up my posts on MacGyver hacks.  I’m going to limit the focus on things that I can significantly talk about.  So, suppose there is some hack involving a belt that loops around a pole and something happens.  It might be a great “hack” – but if there’s no fun science to discuss, I will just skip it.

Also, I’m not going over any of the chemical explosions.

MacGyver does something to a radio to such that it creates a high pitched squeal – a type of sonic weapon.  Is this plausible?  Yes.

Since a radio has both a microphone and a speaker, it’s possible to set up an audio feedback loop.  Here is a short video showing this.

Actually, this demo leads to some interesting questions.  In particular, what does the feedback frequency depend on?  I think that the frequency of the squeal depends on both the audio properties of the speaker AND the mic.  If you change either one of these, the frequency should change.  This would make a great science fair project.

# MacGyver Season 3 Episode 1 Hacks

I’m way behind, but I figured it would be best to go over the new stuff while it’s still fresh.  Don’t worry, I’m still going to do the old stuff.

Episode 301: Improvise

A whole bunch of DIY stuff

(various, mostly real)

I don’t know how much stuff to go over at the beginning of the episode.  Clearly MacGyver has been working hard in this village and there are all sorts of DIY stuff.  Let’s just leave it at that.

Water pump

(mechanics, real)

So, you need water from a well – right?  That’s where a water pump comes into play.  Let’s just talk about pumps in general.  There are really two types of water pumps. There is a suction pump and a spinning pump.

For the suction pump, air is removed from a tube above the water.  This reduces the pressure on top of the water.  There is still air pressure pushing on the rest of the water – and this air pressure (from the atmosphere) pushes the water up the tube.  Once the column of water produces the same pressure as the atmosphere, the column of water stops rising.  This means you can pump water up 33 feet.

For the spinning pump (not its actual name), there is something that spins and pushes the water.  Think of this as a water fan blade.  The blade spins and pushes the water up the tube.

Quartz and steel to make a spark

(chemistry, real)

Note: you can barely see this hack – it’s fast.  But it’s there.

Yes, you can make a spark with quartz rock and steel.  The key thing is that the quartz is strong enough to chip off super tiny pieces of steel.  These super tiny pieces are hot enough and small enough to react with the air and get super hot.

Here is a video.

# MacGyver Season 1 Episode 6 Hacks

Episode 106: Wrench

Using an air bag to break through a bullet proof car window

(physics, explosive, car hack, fairly plausible)

So the heroes are stuck in a car in the water.  The windows won’t roll down.  MacGyver takes an air bag from the car and connects it to the window to blow it out.

Yes, this has a fairly decent chance of working.  Air bags are basically explosives – ok, they are.  There is surely enough explosive power to crack open a window.  However, there is one problem – forces.  As the air bag expands, it pushes in all directions.  If you don’t have it attached to the window, it will just shoot off.  Even in that case there is still a chance it would break the window.

Related: have you seen those crazy videos of people sitting on air bags and then setting them off?  Don’t do that.

Covering a bomb.

(explosive, plausible)

Mac puts a bomb in an upside down dumpster and then pins it with a car.  That has a good chance of helping with the explosion.  Oh, but hot wiring the car isn’t going to be easy.  Most cars made after 1997 have those chips in the key – no more hot wiring.

Smart Phone Camera Extender

(electronic, based on real stuff)

MacGyver takes the camera out of a smart phone and attaches it to a blade so that he can look on the other side of a door (by sliding the camera under the door).  Let’s be honest – there is nothing technically wrong with this hack except for one thing.  There is no way you are going to take apart that phone.  Have you tried taking apart one of these things?  They are glued together and the camera is usually part of the main motherboard.  There is rarely a cable running in there that could be extended.

Diesel fuel dissolving SEMTEX

(chemistry, explosive, plausible)

MacGyver uses diesel fuel to dissolve explosive.  Yeah, this has a good shot of working.  It wouldn’t dissolve right away though.

DIY Pulley System

(physics, mechanical, real)

In order to yank a bolt out of an explosive, MacGyver makes a pulley thingy.  Here is a video with something similar you could build yourself.

The key to a compound pulley is to arrange it such that one string gets pulled further than another string.  This means you can pull with a small force over a long distance to get a large force over a short distance (the key to all simple machines).

Here is a blog post that goes over this in more detail.

# MacGyver Season 1 Episode 5 Hacks

You know what would be great for these posts?  Images or gifs of the different hacks from the show.  Maybe someday I can add those.  For now, It’s just science and stuff.

Episode 105: Toothpick

Redirect a heat-seeking missile

I don’t like to use the “barely plausible” tag – but let’s be honest.  It’s going to be pretty difficult to redirect a heat seeking missile.  I guess it’s not impossible though.

A heat-seeking missile has an infrared receiver to look for “hot” objects.  Usually, this hot object is the jet engine exhaust for an aircraft.  So, for this hack, MacGyver wants to get the “attention” of this missile using a flood light.  Yes, these things get hot – but not jet engine hot.

Normally, a plain light would radiate energy in all directions.  Imagine this light as an expanding sphere with a particular radius.  Since the light must be even over the surface of this expanding sphere, the intensity of light decreases with distance.  The area of a sphere is proportional to the square of the radius – so this is where we get the “inverse square law” for light.

Of course the inverse square law means that a bright (and hot) flood light won’t do much if it is far away.  One way to fix this is to use a parabolic dish to refocus the infrared light – a spot light.  Yes, you can do this with a plain metal dish for infrared.

So, in the end MacGyver has an infrared source he can focus on the heat-seeking missile and perhaps get its attention.

Electric Razor Taser

(electric, plausible)

MacGyver makes a type of taser (not really a taser) out of an electric shaver.  The basic idea behind a taser is to shock a person with an electric current.  There are two types of current – there is direct current (DC) like the stuff that comes out of a battery.  This current just goes “one way” in a circuit.  Shocking people with DC current is a little bit more complicated.

The other current is alternating current (AC) in which the current constantly switches directions with some frequency.  In the US, AC current is at a frequency of 60 Hz.  Why does this matter?  It matters because if it’s a plug-in type razor then you already have AC current and it’s easier to shock someone with AC.

So, to make a plug-in razor taser you just need to take the main power lines coming into the razor and connect them to something on the front.  When a human victim touches both of these wires, the human becomes part of the circuit and gets shocked.

Don’t try that at home.

Toothpick gun jam

(mechanical, seems real)

If you don’t clean your gun, it probably won’t work.  A toothpick in the move-y part will probably mess it up.  Not a gun guy.  Oh, I don’t think this would make the gun misfire – just jam.  But you can see where the name of this episode comes from.

(physical, seems real)

So, MacGyver puts two smoke grenades in a jacket . Really, there’s not much to say here.

Metal Belt Buckle Attack

(physical, real)

In a classic MacGyver move – he uses his belt buckle as a weapon.

Fork Lock

(physical, real)

MacGyver uses a fork to lock a door.  This is a sliding door – but I was thinking of something like this:

Torpedo attack

(explosive, probably real)

MacGyver uses a train torpedo to attack a dude.  Not much to say here.

Thermite

(chemical, based on real thing, plausible)

MacGyver uses a pipe to hold the stuff – magnesium shavings and iron oxide shavings.  Boom – thermite.  Thermite is a chemical reaction that gets SUPER hot.  It’s actually pretty awesome but not easy to make.  You need to get the metal into a very fine powder.  But if you do that, here’s what happens.

Stopping a train

(electric, physics, partially plausible)

How do you stop a train?  One way is to mess with the DC motors.  Yes, most trains are diesel-electric.  They have diesel engines that power electric motors.  If it’s a DC electric motor, you can switch the polarity of the wires and the motor will rotate the opposite way.

This might not stop the train right away – but it’s a start.  Here is a demo you can try at home.  Oh wait – I can’t find the video.  Well, here’s what you do.  Get a battery powered toy car with a motor (a cheap one).  Switch the direction of the batteries and the car will drive backwards.

I found the video.  Here it is.