Science of CSI

Delphi91

Just watching CSI on LivingTV+1. I was alwasy impressed with it but tonights episode has disappointed me because of two errors (so far).

At one stage, after a guy falls from a construction site, Grissom says "Terminal velocity is 9.8m/s^2, ...." Velocity isn't measured in m/s^2!!

Checking to see if the corpse had been electrocuted, one of the CSI's says that he couldn't have been electrocuted because he was wearing construction boots with rubber soles. She then goes on to say that "that's why you won't get electrocuted in a car if it's hit with lightning - because of the cars wheels". I was always under the impression that the metal in the car acted as a Faraday cage and that the rubber tyres had nothing to do with it. After all, if lightning can jump from a cloud to a car then surely the gap from the car to the ground would not pose a problem?

Mike

Professor_Fink

Hi Mike,

I find CSI rather disappointing too. They seem to be more interested in sounding authorative, than actually checking the facts, which I suppose is understandable give that they are out to entertain rather than educate.

The terminal velocity thing is obviously wrong, and the actually terminal velocity will depend on things like cross-section and mass (also if they are wearing loose or tight clothing etc), and so won't be the same for any two people. Also, 9.8 m/s (I'm dropping the second s^-1 here, since from your post they called it a (terminal) velocity) works out at roughly 36000 meters per hour, (36 km/h or about 24 miles per hour). Consider being hit by a car going at this speed. You'd survive, almost certainly, and probably without serious injuries.

What would happen if you jumped out of, say, at 30th story window. You'd die. So the terminal velocity is much greater than 9.8 m/s in any case, so its not just a case of getting their units mixed up.

As regards the rubber boots, the reason you are less likely to get electricuted is because current will no be able to pass trough your feet to the ground. The current takes the path of least resistance (or at least the bulk of it does), and so if you touch a live wire, but there is no path through your body to ground it, then it shouldn't do you as much harm. If you're touching something else with your other hand though, all bets are off.

And yes, the car acts as a Faraday cage.

Tar.Aldarion

Heh, it's just a show, not meant to be perfect, although if I was making it I would make sure it was. Those two errors are quite obvious,"terminal velocity is 9.8m/s^2", heh.
I like the idea of this thread, let's watch CSI and point out more errors.

ianobrien

Don't get me started on CSI!

1. Analysing a wax via GC MS. How did they get the wax into the machine without major derrivisation? And reconstruct it after for MS!
2. FTIR machines dont tell you the brand of lighter fuel, all they tell you is the functional groups in the fuel constituents.
3. Talk about possible sample contamination in the show. (I suppose having enerybody suited up isn't glamorious)
4. MS machines the size of laptops? I wish I could get them!
5. No body seems to run instrument calibrations, prepare calibration curves, error analysis, etc.

seinstein

Delphi91 wrote:

Just watching CSI on LivingTV+1. I was alwasy impressed with it but tonights episode has disappointed me because of two errors (so far).

At one stage, after a guy falls from a construction site, Grissom says "Terminal velocity is 9.8m/s^2, ...." Velocity isn't measured in m/s^2!!

Checking to see if the corpse had been electrocuted, one of the CSI's says that he couldn't have been electrocuted because he was wearing construction boots with rubber soles. She then goes on to say that "that's why you won't get electrocuted in a car if it's hit with lightning - because of the cars wheels". I was always under the impression that the metal in the car acted as a Faraday cage and that the rubber tyres had nothing to do with it. After all, if lightning can jump from a cloud to a car then surely the gap from the car to the ground would not pose a problem?

Mike

Let's do a little detective work here ourselves. If Grissom's actual words were "Terminal velocity is 9.8m/s^2..." then everyone above is correct, it's about 24mph, wouldn't kill you.

But the figure he chose to use is a bit confusing, not to mention coincidental, esp. if he used the units m/s^2. As it happens, 9.8m/s^2 is the acceleration due to gravity of an object. So whether I drop a feather or a stone or anything from a given height above the earth, they will all accelerate at 9.8m/s^2 (assuming we neglect air resistance).
So I am led to believe that maybe he didn't say those exact words; maybe he was saying that the person falling from the apartment accelerated with that speed. Correct, m/s^2 is the unit of acceleration, wheras velocity is measured in m/s (and usually a direction, but seeing as down is the only way to go when you fall from a building then it's understood!).
He could have even said that the person reached 'terminal velocity' ie they hit the ground after 9.8 seconds, but then I don't understand why he would have chosen that number, 9.8, or even have used the term 'terminal velocity'; that term is the moment when an object's acceleration ceases due to air resistance. I doubt that even from the 30th floor of an apartment block someone could reach this stage.
Therefore someone must have misheard him, or it was a terrible error let go by the director/producers.

Delphi91

seinstein wrote:

Let's do a little detective work here ourselves. If Grissom's actual words were "Terminal velocity is 9.8m/s^2..." then everyone above is correct, it's about 24mph, wouldn't kill you...

How could 9.8m/s^2 be 24mph?????? They are two totally different quantities - one is acceleration, the other is velocity.

The reason I posted the original post was because he said m/s^2 and then used the words "terminal velocity". I didn't mishear.

breadmonkey

The bottom line is they don't have a clue what they're on about with the terminal velocity. One of the writers probably said, "Oh yeah, 9.8m/s^ is terminal velocity, I remember that from my high school physics."

Aside: "Which I FAILED...bwahahahahaha!".

I'll get out

seinstein

Delphi91 wrote:

How could 9.8m/s^2 be 24mph?????? They are two totally different quantities - one is acceleration, the other is velocity.

The reason I posted the original post was because he said m/s^2 and then used the words "terminal velocity". I didn't mishear.

I didn't say it's equal to 24mph. I was referring to Professor_Fink's statement, and I also put 3 dots after 9.8m/s^2 as I was quoting what Grissom said.

breadmonkey wrote:

The bottom line is they don't have a clue what they're on about with the terminal velocity.

I think you aren't bothering to read topics properly before critiquing.

Capt'n Midnight

terminal velocity is about 196mph in the head down position .
but it varies a lot , orientation and air pressure and stuff. If you hit water or ground at that speed it's pretty terminal.

they used to do an ad about a guy jumping off a building -5 or stories high - on to the glass proch below, and pointing out it was the same as hitting a windscreen at 30 or 40 mph if you don't wear a seatbelt.

9.81 ms-2 would be more like initial acceleration, as air resistance would reduce it rapidly,

seinstein

Finally, a voice of reason.

breadmonkey

seinstein wrote:

I think you aren't bothering to read topics properly before critiquing.

I think you are wrong.

seinstein

breadmonkey wrote:

I think you are wrong.

I think YOU are wrong.