[Phys-l] October Physics Challenge

[Carl Mungan <mungan@usna.edu>]

How many of you look at the Physics Challenges in TPT? Well, anyhow, 
go look at the October challenge and its solution at:

http://scitation.aip.org/journals/doc/PHTEAH-home/challenges/oct2010.pdf

I find myself in a bit of disagreement over the parenthetical note at 
the beginning of Case 2 which is supposed to explain that the heavy 
block can never slip on the ribbon.

I agree that both blocks cannot continuously slip (unless we allow 
the ribbon to bunch up near the top of the prism!). But I think that 
for steep angles, either block could slip. For the sake of argument, 
suppose I initially force the light block to move with the ribbon (by 
some unspecified coupler) but allow the heavy block to slip on it. If 
I now release the light block coupling, there's no reason it'll 
suddenly start slipping. Once started with the heavy block slipping 
and the light block moving with the ribbon, that configuration is 
stable.

So what happens when the two blocks are "simultaneously" released 
depends sensitively on the exact details of the starting conditions. 
I think it's like putting a pencil on point and asking which way 
it'll fall. Who knows!

But I'm interested to hear what others think. Careful though: think 
about it first or this could turn into another endless buoyancy 
thread.... Carl
--
Carl E Mungan, Assoc Prof of Physics  410-293-6680 (O) -3729 (F)
Naval Academy Stop 9c, 572C Holloway Rd, Annapolis MD 21402-1363
mailto:mungan@usna.edu     http://usna.edu/Users/physics/mungan/