# MacGyver Season 1 Episode 17 Science Notes: Ruler

Propane tank flame thrower

Take a propane tank and bicycle tube. Cut the bike tire to make it a hose and connect it to the propane tank. Use a road flare to light the gas—boom. There is your flame thrower.

Oh but wait. It’s just a dream. Bozer’s dream. The flame thrower wasn’t real anyway.

Listen in on a landline phone

Who uses a landline now anyway? Oh well. They want to use a landline then it’s possible to listen in. Actually, this isn’t even that difficult. Check it out.

Here is another version.

You just need a capacitor and maybe an inductor. You could grab these from a radio or something like that.

But wait. I made a mistake. While going over this hack, I said something like this:

“Yeah, this is pretty easy. Just get the capacitor and earpiece (or radio) and then tie it into the wiring box”

Here’s what that looks like.

I just want to point out this small mistake (that you would never notice) just in case you saw it. You don’t actually “tie” the lines—that’s just a term we use in circuits to mean “connect”.

There’s a bomb in the truck. Where should you park it so that no one gets hurt? Yeah, this is a tough calculation. However, tough has never stopped MacGyver before and it won’t stop him now.

Here is my rough calculation and explaination.

Bombs are complicated. But usually it is the pressure produced by the explosion that will get you. We can come up with some pretty useful models to calculate their impact. First, there is the Hopkinson-Cranz Scaling Law (this is a real thing). With this law, the acceptable distance can be calculated based on the explosive weight.

$\mathrm{Range} = (z)(\mathrm{weight})^(1/3)$

In this expression z is a factor that depends on the type of distance with 14.8 being the distance factor for a public traffic route. That means that 2 kilograms would need 18.6 meters (60 feet).

Infrared face jammer

OK, it doesn’t actually jam your face. That would be weird. MacGyver wants to prevent the security cameras from recognizing their faces. So he takes some infrared TV removes and pulls out the IR LED lights. Normally these flash on and off so that the sensor on the TV can “see them” but humans can’t.

He mounts these IR LED lights on some sun glasses with a battery to power them. When a security camera sees the face, it just gets blinded by the IR light since many video cameras can also detect IR.

If your phone camera doesn’t have an IR filter (most now do) then you can actually see the light flashing on a TV remote by pointing it at your phone.

Oh, so this could really work. It just depends on the type of video cameras. Some people even put stuff like this on their car license plate so that police cameras can’t see them.

Car jacking

How do you open a locked car door? One way is to jam a wedge into the door. This will pull the door out just a little (by bending it) so that you can get a stick in there. The stick then can be used to push the “lock” button.

In this case, MacGyver uses something for the wedge—maybe a shoe horn or a door stop. Then a monopod is extended to click the lock button.

DIY soldering iron

You might have missed this one. But as MacGyver is building his stuff for the last mission, he needs a soldering iron. He takes the heating element out of a hair dryer and connects it to some stuff. That works.

Fake noses

Need a disguise? How about DIY latex to make a nose? Yes, this seems plausible. Here’s how to do it.

MacGyver makes a quick circuit board that can crack a keypad by using a brute force method that goes through all the combinations. This is from a different episode, but it’s the same idea.

If you want to play with one yourself, here is an online version of the code.

Well, it’s just a radio. MacGyver needs a speaker and a transmitter. Really, a radio transmitter is essentially the same thing as a radio receiver—OK, not really but sort of.

Instead of going over the way MacGyver did it, how about a real actual radio you could build yourself? Here is a spark gap transmitter from simple parts (and awesome).

Here is a more detailed explanation of the spark gap transmitter from one of my WIRED posts.

# Numerical Calculation Collection

The following are some of my best posts about numerical calculations.

# Working notes for my bouncing ball running model

I’ll be honest.  I had some problems getting my bouncing ball running model working.  Oh, here is the model.

https://www.wired.com/story/how-is-a-runner-like-a-bouncing-ball/

Basically, this models the speed of a running human by assuming they are bouncing ball.  When the human impacts the ground, there is some maximum impact force and an impact time.  The impact time decreases with horizontal velocity such that eventually, all the force is used in the vertical direction to keep the human off the ground long enough to switch feet in the air.  The end.

As I was making this model, I took some notes because I couldn’t get it to work.  Here are my notes.  Hopefully you can use this to see how to troubleshoot a program.

Running model notes

I think I mostly have it working:

http://www.glowscript.org/#/user/rhettallain/folder/blog_posts/program/runningbouncemodel/edit

Here is basically how it works.  Two big ideas:

• Humans can push off the ground with some maximum force.  This force does two things – gets them off the ground and in the air so legs can move and pushes them forward
• The contact time with the ground is small and gets smaller as horizontal speed increases
• This means as the human speeds up, the ground force eventually gets to where it can only push up and not forward

Here is what it looks like so far

Here is a graph of speed vs. time

• This model reaches a max speed of about 3.5 m/s in just a couple of strides – that doesn’t seem right
• I think my Fv calc is wrong – it gives back the same speed not the needed vertical speed to get the stride time
• Need to recalcualte Fv based on pfinal
• If you want to be in the air for ts seconds, then your initial vertical velocity must be -g=dv/dt.  dv=g*dt dv = 2vstart. start=(½)gdt
• Now to calculate the force. I know tc (contact time) so F = dp/dt = m*(vy2-vy1)/tc – this is the total force = Fv-mg so Fv = that stuff +mg

Something isn’t right.  Here is a plot of position vs. time

It’s getting higher and higher (and going lower – weird)

• I’m getting stride (in air) times of 0.09 to 0.13 – that’s wrong

Ok – I think I know the problem.  I need to set the force push time loop and forget about while human.pos.y<R – I think that’s my problem

1. Once human hits the ground – calculate Fv, Fx, and tc set tcount = 0
2. While tcount < tc – set human.pos.y = ground. And set the forces
3. When tcount = tc, turn off the forces and stop holding the person

It appears there is something wrong with my Fx.

• Fx is some value for the first push – but after that it goes to zero and the Fv is maxed out.
• Werid
• There is a problem with both Fv and Fx

The problem is the time of impact – it gets too small such that the required force is HUGE

• How about a min time – and it can’t go lower?

Fmax = m*2v/t

t=m*2*v/Fmax

I think the problem is that during the contact time, the horizontal force is too much so that the human ends up going faster than the theoretical speed.

I can use the time and force and velocity to estimate the average velocity and then recalculate the time

This is the paper

https://www.physiology.org/doi/pdf/10.1152/japplphysiol.00947.2009

It has this plot.

This shows a decrease in contact time with speed

Here is what I get for a fit

This gives a contact time function of

Although this “blows up” at v= 0.  Maybe I should say tc = 0.3612 for v < 2 and this expression for v>=2

End of notes – it finally worked.