# Homework Help: Simple truss problem

1. Feb 8, 2016

### Dave1357

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.

2. Feb 8, 2016

### PhanthomJay

You can't always tell by inspection, but in this simple case, does the leg on the right get pushed down or pulled up when the load is applied as shown? How about what happens to the other leg?

3. Feb 9, 2016

### Dave1357

I'm sorry I can't get what you mean. How does the compressive member come?

4. Feb 9, 2016

### PhanthomJay

Compressive forces are 'pushing' forces and tensile forces are 'pulling' forces. Take 2 popsicle sticks and fasten them at one end in the shape of the upside down V as in your problem ( ^ ). Set the legs on a table and apply the rightward horizontal force at the top. One of the legs will pull off the table, while the other leg will push on the table. Which is which? The one that pulls would be in tension if anchored to the table, and the one that pushes will be in compression.

5. Feb 9, 2016

### Dave1357

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.

6. Feb 9, 2016

### PhanthomJay

They can be a bit more complex, but, yes, that is best. Try to visualize when you can, then run the numbers.