Re: [Phys-l] rich context luncheon meat

[John Denker <jsd@av8n.com>]

On 01/29/2011 11:02 PM, Stefan Jeglinski wrote:

> Of course, after considering all that, it occurs to me that the wires 
> better be able to hold the performer's full weight momentarily just 
> as the fall starts. 

Quite so.

Real-world engineers speak of the "live load" in contrast
to the "dead load".  (Live refers to dynamic, not to living
persons.)

To say it in physics language, in the lab frame there are 
inertial terms (involving ma) not just gravitational terms
(involving mg).  And then you need a safety factor on top
of that.

==========

Another technical flaw is that it is hard to achieve safety 
using two separate strands, because it is hard to ensure that 
both strands share the load.  The failure mode is that one 
strand fails, then the other.  Making your own multi-strand 
cable from scratch is possible but requires detailed engineering 
that would be an inappropriate use of the prop master's time
and resources.  It would be safer, cheaper, and simpler to
go buy better cable.

Because of the safety issue, in real life I would never allow
homebrew cables for lifting a person, or for lifting stuff
that could fall on a person.

===========

The device in question has a name:  It is called a Peter Pan.
It is very, very tricky to get right.  There are engineering
companies who specialize in providing such things, so that
theater companies don't need to reinvent this particular wheel.

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

> Perhaps the problem is fatally flawed after all.

I wouldn't go that far.  The technical flaws don't make the
problem unsuitable for a _guided_ discussion.

In the real world, deciding it can't be done with the materials
on hand is very commonly the right decision.

Building a Peter Pan is definitely a genuine real-world problem.
You might decide that the problem is too complicated for this-
or-that group of students, but that's a separate question.