Statics example from Meriam ( SI improper )

[lakuzamalakuzam]

could not understand why it is partial constraints. Also, I don't understand how link works. Below here is what I am thinking i think it is totally wrong can someone explain what's wrong with my idea here.

 

[Baluncore]

Welcome to PF.

The blue rectangle is not fully constrained by the three links within the frame. The position of A is fixed by the first two links, but the diagonal link 3 may be stretched as the body is rotated slightly about A.

The top of the cosine function at zero, is horizontal. Because the line from A to link 3 is straight, very high tension forces could be present in link 3 if the body was rotated slightly.

 

[Tom.G]

As @Baluncore stated, the tension on link 3 goes to infinity when its attachment points are co-linear with point A. The situation is similar to a rope or chain stretched tight between two supports. Try calculating the tension when pushing on the center of the rope/chain span.

A practical use of the phenomena is retrieving a vehicle from a ditch.
Two people with a chain, a tree, and something to block the wheels, can pull the vehicle out. Providing there is a portion of the chain not being supported (by the ground):

1. chain taught between tree and vehicle
2. person 1 stands on the middle of the chain, deflecting it
3. person 2 chocks the vehicle wheels so can't roll back down into the ditch
4. person 1 steps off the chain and repeats from step (1)

Upon completion, they choose a different route.

Cheers,
Tom