Re: [Phys-l] Circus Physics

[Stefan Jeglinski <jeglin@4pi.com>]

> My school's librarian sent me this link that I thought some of you 
> might find interesting.  It's a collection of videos that explains 
> some of the physics during various circus acts.
>
> http://www.pbs.org/opb/circus/classroom/circus-physics/


I didn't look at very much of it but if the rest is like the example 
of the dog tricks and Newton's Laws then we have a problem, Houston. 
I believe that would be this link specifically:

http://www.pbs.org/opb/circus/classroom/circus-physics/newtons-laws/

I felt that their examples were straining the concept to begin with, 
but aside from that we have, to wit:

The vector diagram (net force on the dog down the slide) is drawn 
wrong. If you sum up the vectors shown (friction, normal force, 
gravity), they do not add up to the given resultant vector down the 
slide. You could argue that this is just sloppy artwork, but IMO it's 
not even close and if there really are any children or others trying 
to use this to further their knowledge of physics, they're going to 
be confused at best, wondering if there is a missing mystery vector.

Worse is the last sentence of the paragraph above the vector diagram: 
"To find out how fast the dog will slide, we need one other piece of 
information, his mass."

This is an abjectly incorrect statement. The acceleration down the 
slide is g(sin(theta) - mu*cos(theta)). Not only is mass not needed, 
but the other piece of information needed is mu (and of course theta).

I understand of course why they said the mass is needed - they 
suppose they have drawn all the forces, and then a = (1/m)*F. On the 
level of their presentation, I guess they do need mass, but all in 
all I'm left with a bad taste in my mouth after the vector diagram - 
enough so that I fear looking at the rest. Just to whet your 
appetite, the unit on centripetal acceleration has this attractive 
lead-in statement: "Trick riders depend on horses that can run in 
circles without getting tired." (I really was afraid to look - maybe 
it's not bad).

So what say we? Do we let certain things go in an educational context 
(e.g., trying to garner interest using the circus), believing that we 
have at least imparted *something* to those who are otherwise 
unlikely to ever consider learning it? Or do we demand precision that 
others might argue is nitpicking, despite the ease with which a fully 
correct presentation could be made?


Stefan Jeglinski