When can I treat a joint as a fixed conection?

[John Mcrain]

Question is : is pivot point is both case at same position...

Useing only external forces (100kg,10kg and 30kg) I can treat joint as fixed conection and still get correct results,in both case pivot point is at same positon,even right case has joint at right side of beam..
How is possible that I treat joint as fixed conection and still get correct result,isnt this confusing becasue joint can't trasnfer moment ,only forces?When I can't treat joint as fixed conection?
Because joint can't trasfer moment only forces,so I can use internal forces inside joint and still get same results,pivot point is at same position in both case..

 

[A.T.]

Question is : is pivot point is both case at same position...
View attachment 278592

Define "pivot point " mathematically.
Is the whole thing floating in space?
Is the 100kg box free to separate from the 10kg box?

 

[John Mcrain]

Define "pivot point " mathematically.
Is the whole thing floating in space?
Is the 100kg box free to separate from the 10kg box?

no in space,yes it can separate..
Pivot point is point below 10kg beam so beam is balance,stay horizontal..