Re: [Phys-L] real-world physics

[John Denker <jsd@av8n.com>]

On 06/05/2012 07:17 PM, Kirk Bailey wrote:

> This is the problem I propose.  Every office in the world has so many
> electrical cords that lowering ropes could be constructed from them for
> rescues from high rise burning buildings.  The ropes would have to be long
> enough to get below the fire, hopefully on a side with no flames.
>
> What is the tensile strength of an electric cord?  How many would need to
> be placed in parallel or wound together to make a usable rope?  What knot
> is best to connect the cords end to end?  How difficult would it be to
> break a window from the outside in order to get in below the fire?  What
> readily available tool would be best for the job? Would it be best to soak
> your clothes in water first?  What sort of tether could be made in case
> your hands slipped off the rope?  Are there other better options (makeshift
> parachutes, etc.)?

Very nice!

It is very common for real-world scientists to get called into a brain-
storming session to attack problems like that.  Usually this is only a 
side-line, not their "main" job description, but it is important.  By 
way of example, Feynman's main line of work was quantum field theory, 
but he also served on the Challenger investigation commission.

Things to notice about such work include:
 -- It requires being able to "switch gears" and get up to speed on a
  new topic quickly.  Not everybody has this skill.  Some people like
  to work on one thing, without distractions.  Others actually enjoy
  a certain amount of multi-tasking.  Feynman, for example, knew that
  he could not work on one thing for very long without getting stuck,
  and chose his career path accordingly.

  I reckon that 10% of the population is born with the multitasking
  skill, and another 80% can learn it, and the remaining 10% will
  never learn it, which is OK.

 -- Sometimes there is more than one good answer to a problem.

 -- Sometimes there is *no* good answer to a problem.  For example,
  there may not be any good ways of rescuing people from a burning 
  high-rise building.

 -- To be practical, you need more than a proof-of-existence;  you
  need a /scalable/ solution.  For example, if you work for the phone
  company, it is not sufficient to have something that works for one
  phone call at a time;  you need something that can handle millions
  of phone calls at the same time.


In the case of using electrical cords (or whatever) to rappel to 
safety, I reckon it would be possible to rescue a few people that 
way, but there are going to be hundreds of people who need rescuing, 
so I'm not convinced this approach is scalable.  Also it fails if
the building is surrounded by fire on all sides.

Improvising a parachute is unlikely to work.  The classic WWII
round parachute -- MC1-1C -- is 35 feet in diameter.  It seems
unlikely that you can find (or make) anything that big on short
notice, and even if you did, it would be too heavy and not quite
the right shape.

If I were brainstorming this, I would investigate rescue schemes
that involve a reasonable amount of planning and preparation, as
opposed to purely improvisational "MacGyver" schemes.

In particular, my first approach would be to investigate ways of
rescuing people from the roof.  I would start by understanding why 
the obvious techniques don't work.  For example, rescuing people
by helicopter is very problematic.  Flame and smoke may cause
trouble  Many buildings don't have a suitable helipad.  If there 
are a thousand people milling around on the roof, they could
interfere with landing.  Also they might mob the helicopter after 
it has landed, which means nobody gets rescued and you lose the
helicopter crew as well.

So ... how about this:  Have the helicopter fly over the building
and drop a "care package" containing parafoils ... not old-style
round parachutes, but the modern parafoil things, which are much
smaller and which allow you to fly /sideways/ away from the fire.
The physics here is simple and beautiful:  A wing develops *lift*
unlike a round parachute which relies on *drag* .... In other
words, it pays to visit a large amount of air and pull it down
gently, rather than visiting a small amount of air and trying
to pull it down violently.

Note that the same helicopter that could carry 10 people could
carry 200 parafoil packs.  This means you could rescue far
more people per unit time.

===

Another approach would be to set up ziplines leading away
from the roof.  This is a tried-and-true way of rescuing
people, used for about 200 years for rescuing people from
shipwrecks.
   http://en.wikipedia.org/wiki/Henry_Trengrouse

You could pre-position the equipment on the top of each
building, or you could drop it from a helicopter.  Even if
the bulk of the equipment comes in by helicopter, some
nontrivial preparation is required on the building, namely
installing some very strong attachment points.

This has the advantage that riding a zipline takes less skill
than piloting a parafoil.  Also the cost per person is less.

Note that this requires somebody to lash the bottom end of
the zipline to a tree or a truck or some such.  This seems
reasonable to me;  if you're going to postulate a helicopter
you can equally well postulate some first rescue personnel
on the ground, especially if they are not required to endanger
themselves by getting too close to the building.

I worry about the possibility of too many people trying to
ride the zipline at once, overloading it.

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

I'm going to stop here.  The point is not for me to solve the
problem, but for the *students* to solve the problem as best 
they can.

I think that analyzing real-world problems like this is super-
important.  If done right, it turns the whole "motivation" 
issue 180 degrees;  rather than resenting the requirement to 
learn a bunch of useless physics, they now wish they knew 
*more* physics.

The ideal situation is where the students *want* to think
about physics.  They turn off the TV because the physics
homework is more interesting.  We can't always achieve this 
ideal, but we ought to try.