Re: [Phys-l] Statics conundrum

[chuck britton <cvbritton@mac.com>]

This is an excellent 'learning opportunity'.

Try it - with suitable spring scales on each string.
Have a screw adjustment at the top of each string so you can change 
it's length.

Now you can explore experimentally the 'solution space' that JD describes.
The middle string can certainly be slacked down to zero force.
What's the max that the middle string can exert before zeroing out 
the far end string?

How do strings 'know' what to do?
Because they are elastic. (The spring scale makes this more obvious.)

How does the floor of your classroom 'know' how much 'normal' force to exert?
It 'knows' because it is really a trampoline that 'gives' until the 
restoring force is just right.

Figgering out how to answer this Q from a student was a real learning 
experience for me.

You never understand something until you figure out how to teach it.


At 7:57 PM -0400 3/14/10, Bob Sciamanda wrote:
> Perhaps it will help to specify a particular situation:
> Let the beam be hung from three strings of identical properties (length,
> etc), at the two ends of the beam and at L/4.  Slow, smooth, simultaneous
> release all along the beam.
> Can we determine the tensions in the strings?  Why not?  How do the strings
> "know what to do"?  (I speak as the student that I am.)
>
> Bob Sciamanda
> Physics, Edinboro Univ of PA (Em)
> treborsci@verizon.net
> http://mysite.verizon.net/res12merh/