Need help on some questions - Moments, Bearings & Trusses

[princejan7]

Need help on some questions -- Moments, Bearings & Trusses

Hi guys, just have some general queries on a couple of problems :

1) http://i1097.photobucket.com/albums/g357/princejan7/reactiondirection_zps85d28093.png

They got the moment of the force BC about A by splitting into components at B and then summing up the individual moments.

Would you get the samet magnitude for the moment if you split the force at some other point on BC's line of action, eg. at C?


2) http://s1097.photobucket.com/user/princejan7/media/bearings_zps342d5a2a.png.html

Don't the thrust and journal bearings prevent rotation about the x and z axis?
The solution doesn't seem to have included these reaction moments. Why is this?


3) http://i1097.photobucket.com/albums/g357/princejan7/truss_zps5c6d8f88.png

In general, when considering the forces on the joints of a truss, how should I know which direction the forces act in?
At the point C, why do BC and CG act in opposite directions?

thanks for any help!

 

[Maiq]

1) You wouldn't be able to use point C for this problem since you don't know where it is exactly and with problems that are in static equilibrium the moment about any point is 0. For this problem you would need to first take the moment about A because that would give you an equilibrium equation with one unknown.

2) Those Bx, Bz, Ax, and Az reactions are preventing the rotations about the x and z axises.

3) All those joints on the truss are in static equilibrium. So at joint B, draw a FBD but have your y-axis go along AC and x-axis perpendicular to AC. Sum up the forces and what do you get?

 

[princejan7]

1) You wouldn't be able to use point C for this problem since you don't know where it is exactly and with problems that are in static equilibrium the moment about any point is 0. For this problem you would need to first take the moment about A because that would give you an equilibrium equation with one unknown.

2) Those Bx, Bz, Ax, and Az reactions are preventing the rotations about the x and z axises.

3) All those joints on the truss are in static equilibrium. So at joint B, draw a FBD but have your y-axis go along AC and x-axis perpendicular to AC. Sum up the forces and what do you get?

http://s1097.photobucket.com/user/princejan7/media/reactiondirection_zps85d28093.png.html
Is there a way to find out if the force acts along the direction BC or CB using equilibrium equations?

I tried by taking the moment about A:

M(A) = [moment of BC about A] -125[4] -600[9]

+5175 = [moment of BC about A]

So I think the moment of BC should be positive, but how do I know which direction the force acts in, BC or CB?
Thanks

 

[Maiq]

Yes it is positive. If you look at the FBD you can see that all the forces besides BC are in the negative direction. So what would that mean for BC?