Re: [Phys-l] pin-connected constraints

[John Denker <jsd@av8n.com>]

On 08/17/2007 07:24 AM, LaMontagne, Bob wrote:
> John - I am missing something crtical in the diagram. If the strut
> has negligible mass and the block is free to slide and is at rest on
> a horizontal surface, what is the "force in the direction of the
> strut"? Are any other forces beside gravity acting that I'm not
> seeing?

  http://www.av8n.com/physics/img48/pin-connected.png

OK.  Good point.  

(I had an applied force in mind ... but it works better if
I *say* what I had in mind ... and diagram it .....)

Two fixes:
  
  1) I just now added to the diagram an applied force F_a.  
   The applied force is purely horizontal.
   You may need to hit "reload" to see the new version of 
   the diagram.

  2) You could omit the applied force and just tilt the
   whole diagram a few degrees clockwise, so that the
   gravitational force would (if unopposed by the strut)
   push the anvil to the right.

   This option (2) makes the situation more complicated
   than it needs to be, which is undesirable in an
   introductory course, since the whole idea of pin-
   connectedness is already scary enough.