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Re: [Phys-l] rocket car -- detailed story



Hi all-
-> 1 lb of push = 2 HP ???
I'm always interested in knowing the genesis of such statements. I was never very good at arithmetic, but my take is that 1 lb generates 2 HP at a speed 1100 ft/s, ~ 600 Kts. This is a jet speed.
Back in '46, I was serving in the Navy's fighter design branch, and the first jets were being redied for the fleet. We would sometimes amuse ourselves by trying to find ways of comparing jet engine thrust with piston engine horsepower. One way to put the two on a comparable basis is to calculate the expended HP by a given thrust at some standard speed. We well might have adopted 600 kts. as the speed for all such comparisons.
All such comparisons would have been marked SECRET - BURN BEFORE READING, sealed in a water-filled practice bomb, and launched into Chesapeake Bay, of course. It is tempting to think that some outgrowth of such musings might have come to the attention of one of the script writers of the show in question.
Regards,
Jack


On Fri, 13 Mar 2009, John Denker wrote:

On 03/11/2009 08:46 PM, chuck britton wrote:
MythBusters did a show on this.

Actually at least two shows, including
*) Season 1, episode 1 "Rocket-Assisted Chevy"
*) Season 5, episode 21 "Supersize Special"



They occasionally do some good researching.

Surely you're not holding out the rocket car escapades as
examples of "good researching", are you?

If you're telling your students that they should pay attention
in class, so they can grow up and become Mythbusters, they are
going to be very disappointed.

If they manage to learn anything about physics, they will be
*disqualified* from having any role in the show. I'm not kidding.

Here's a quote from Jamie Hyneman:

I think if we actually knew what we were doing around here, it
wouldn't be near as interesting to people to watch, you know, we'd
be some scientist that is just reading stuff off a chart, you know,
it, people would find it boring.

That's from "special episode 6" at about the 6:31 mark.

And Jamie is absolutely correct. Anybody who knows anything about
physics would analyze the JATO Chevy myth for a few minutes and
conclude that it could not have happened as described, not even
remotely as described.
http://www.snopes.com/autos/dream/jato.asp
The analysis would take only a few minutes. It would involve "reading
stuff off a chart" and would be boring to watch.

TV (including Mythbusters) is for entertainment. There is a perfectly
good entertainment rationale for building a rocket-powered Chevy, but
no good scientific rationale.

In some cases, they even go so far as to pretend they know less physics
than they actually do. For example, when testing the Hollywood myth
that a person can be knocked down or shoved backwards by a bullet,
they knew in advance that it was physically impossible, but they made
sure not to mention this until after they had done the demonstration.

Usually, though their genuine ignorance of physics is so vast that they
don't need to pretend. They use terms such as force, momentum, and
pressure almost interchangeably. In the rocket-car episode, they announced
that 1 pound of thrust is equivalent to 2 horsepower. And they meant it!
They /repeatedly/ asserted that their 1500 pound-thrust rockets provided
3000 horsepower. How anybody could believe that for even a femtosecond
is beyond me ... yet apparently everybody on the show took it at face
value.

If you want to make a lesson out of this, assign the students to watch
an episode, and to make a list of things that make no sense at all
(such as the alleged thrust to power conversion). If they're paying
attention, the list will be long.

===============

Let's be clear: I have nothing against entertainment.
*) If you hire me to do research, I'll do research.
*) If you hire me to do entertainment, I'll do entertainment. (I'm
actually kinda good at it. I've earned more money designing toys
than I have doing research.)

But please, let's not confuse the two. If you let students think that
entertainment is research or vice versa, you're doing them a terrible
disservice.

There is an issue of long-term versus short-term gratification. A career
in science can be very rewarding in the long term. But if you are more
interested in the short term, blowing something up provides much more
immediate gratification.

Also, as previously discussed, the Mythbusters are highly skilled at
rapid prototyping and fabrication. These are serious, respectable skills.
Such skills are necessary for doing experimental physics research ...
necessary but far from sufficient.

Memo from the keen-grasp-of-the-obvious department: TV is entertainment.
It's not science. The Mythbusters blow stuff up because it's fun to blow
stuff up. It's not science.

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--
"Trust me. I have a lot of experience at this."
General Custer's unremembered message to his men,
just before leading them into the Little Big Horn Valley