Figuring Out Forces in a Pulley System: What and Why?

[princejan7]

Homework Statement

I don't really understand how to determine the forces in a pulley system. Like in this picture :

http://postimg.org/image/4vxrf59ar/

How do you decide what forces are present for the pulley? Why isn't there a 375lb on the right side as well? What would be the pulley forces at H?

Also, why does that 375lb affect the member BFG?

 

[haruspex]

Why isn't there a 375lb on the right side as well?

There is, it's just not shown. You are probably expected to understand that the tension in the rope is everywhere the same. If it were not, some part of the rope would move.

What would be the pulley forces at H?

Draw a FBD for it. What forces do you see?

Also, why does that 375lb affect the member BFG?

There are two supports at H, one holding the pulley and one holding the jib. Together they provide the forces necessary to balance the system. You can calculate the force at the pulley in isolation, then determine what additional force is required at the other support. The position of the trolley does affect the force required at the pulley, but it does affect the total force from the supports. Therefore the 375lb has consequences for the member BFG.