Oh, I understand. Thanks a lot !
So, this kind of checking can only be applied on simple case like the above, right?
If there are complicated trusses, we must use method of joint/section to calculate.
I wonder how to determine a member is in tension or compression without calculation.
For the above video starting at 3:03, how to idenfify the last 2 members are in compression and tension respectively.
Hope some more examples and diagrams to illustrate.
Thanks a lot.