Date: Sat Oct 17 10:27:24 2009

Author: Urs Lauterburg

Subject: Re: DANGER: Fire-extinguisher on a



What a nasty incident. Your report made me
remember something which happened to me about two
years ago. I actually had an accident with our
repulsion demo as well. I was lucky too because
it could have been much worse. In fact our cart
is a pretty potent vehicle driven by two rather
heavy and thick steel walled flasks that contain
pressurized air at 200 bars each. The two flasks
are mounted standing upright in a little distance
from each other, allowing some degree of steering
by individually operating the valves with both
hands. I stand on the cart upright to navigate
with the valves and would move backwards through
the entire front part of the auditorium. I have
to wear ear protection because it's very loud
when I first open the valves for the acceleration
to later reduce the air flow about midway to not
crash into the wall on the other side. I also aim
at an emergency exit door that I am occasionally
able to make, thus making the action even more
spectacular (we don't smash glass though ).

Anyway, that time all was good at the beginning
but for some reason the path got curved in a way
that I hit the support table of the overhead
projector. The momentum of me plus the cart
loaded with the flasks knocked the support table
away from underneath the overhead projector. The
inertia of the projector made it crash onto the
floor right afterwards. In a reaction to this I
somehow lost control of the whole vehicle and
fell to the floor along with the cart and the
flasks. More unconsciously than else I turned off
the valves and the whole thing came to a
standstill fortunately. I did receive a big
applause for my performance from the whole crowd
but my elbow bone must have gotten cracked a bit
in a ways I still feel pain upon certain
movements of my right arm now. However, the last
time I went to my doctor she made me take an
x-ray of my elbow with no real anomaly showing
up. Maybe the pain is just from the psychological
impact of that scary moment ;-).

I have done the demo since, but in a bit a more
prudent manner. It's definitely better to prevent
these kind of things with proper precautions. As
is common I learned the lesson by suffering pain.

Regards and happy physics demonstrations


Urs Lauterburg
Physics demonstrator
Physikalisches Institut
University of Bern

>Hi everyone,
>We did our demo show saturday to a standing room
>only crowd. All went great except for a really
>dangerous accident which very easily could have
>broken an arm, leg, or something like that for
>either a participant or even an audience member.
>We did two roller carts with CO2 extinguishers
>racing across the stage (with two people of very
>different masses). This worked fine. Both
>people sat on 24" x 24" platforms mounted on
>roller skate wheels.
>Each person held the extinguisher in front of
>them without the extinguisher strapped down or
>otherwise fastened. This did not have any
>problems, but I'm now thinking that perhaps we
>should have the extinguisher fastened to the
>The next demo was to use the Pasco hoovercraft with the extinguisher.
>The extinguisher was held (by myself) in my lap
>and over my feet so that it was directed
>tangentally. I pulled on the lever and started
>spinning in circles on a cleared stage. Since
>this was near the very end of the show, we had
>cleared the stage of everything so there was
>nothing to hit. We also had 'spotters' along
>both sides of the stage and off the front of the
>stage if the hovercraft drifted too far so I
>wouldn't fall off the stage or hit something off
>stage. But what happened was the the
>centrifugal force of the extinguisher somehow
>pressed against my hand, and I could not let go
>of the trigger. At least that is the feeling I
>recall. I spun faster and faster (all within
>what must have been just seconds). I could then
>not even hold onto the extinguisher any more
>(too great a centrifugal force), and it took off
>skipping across the stage with me just sitting
>on the spinning hoovercraft. Fortunately the
>entinguisher went straight into a barrel off
>stage which absorbed the impact. The
>extinguisher was not still spewing after it left
>my hands. If it had skipped off a moment
>sooner, it would have landed in the audience,
>and maybe (but doubtful) would have had enough
>speed to skip over the blocked off 1st row and
>then go into the second row where people were
>So I am thinking that this type of a spinning
>demo is inherrently unsafe unless the
>extinguisher is fastened down to the cart and
>positioned in the center of the cart so that it
>cannot obtain any translational kinetic energy.
>This is what I plan on doing for future years.
>I am also going to go a bit further and also
>remake the linear carts so that the extinguisher
>is fastened into them too.
>Richard F.