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[Phys-L] Re: water-rocket lifting a man?



On Fri, 23 Sep 2005 17:10:49 -0400, Scott Goelzer <s.goelzer@COMCAST.NET> wrote:

I saw this too and wonder why the flight looked so different in
height and range from the different camera angles. I also wondered
why he did not spin in place like an model rocket engine launched
without a rocket (experience and regrets).
I had not thought about there being a tow line above and wondered if
the guy was riding a tether or rod for a bit so I looked at little
closer for some evidence. Capturing to quicktime and looking frame by
frame:

-in the scene where a man throws a inflated ring, before he throws
there is a cable that appears to be coming up from the dock behind
his head at angle consistent with the launch. Maybe.
- about three to four frames after launch, the bottle array is turned
nearly at right angle to the direction of motion yet he continues to
ascend. Some external force must be providing guidance.

Based on my observations of launching this type of rocket (I run an entire
course on kinematics based on experiments with these things - but not with
people strapped to them), this change in direction and the continued upward
motion are not incompatible. The available thrust by the time he has
reached that height would be small, effectively he is already in a
projectile motion pattern.

The short thrust phase will allow an empty bottle launched in this manner to
go straight up, or a soup can ride an explosion (hydrogen or firecracker)
straight up, but a rocket motor (long duration thrust phase) will continue
to thrust as it rotates and will be totally unpredictable.

I'm not claiming that the video is true, but if someone were crazy enough to
try this stunt, the video is approximately what I would expect to see. The
last clip (from near the launch site) looked a little different to the
others the first couple of times I watched it, but after further watching, I
thought that my initial skepticism may have been misplaced. The lack of a
background makes this clip particularly difficult to analyze.

Gary Turner
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