# 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.

# 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.

# The worst high school physics question EVER

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

It goes something like this:

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

1. A dry bean
2. Toast
3. Water

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

Water

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

Of course the evaporative cool depends on several things:

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

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

Toast

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

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

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

Dry Bean

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

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

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

Writing questions isn’t so simple

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

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