Statics Problem - understanding the directions.

[kpx001]

Homework Statement

Ok so basiclly I know how to do the problems, but I'm confused when to make the force negative and when to ignore the negative.

forexample: http://yfrog.com/jastaticsprobp
solving part a:
Moment at A = -24*39 -(-14*39) = 2028
notice how i had to do -(-14*39) to compensate for the force going down

VS.

http://yfrog.com/jqstaticsprob2p

where solving for
Moment at xaxis= -230*.250- 230*.210 = -105.8
notice i did not do -(-230*.210)

My assumption?
either keep the direction of force if the problem is 2-D ?? i have no idea, can someone help make this clear? thanks

 

[tiny-tim]

Hi kpx001!

"up" and "down" doesn't matter, what matters is "clockwise" and "anti-clockwise" …

so in both your pictures, the torque (the moment) is clockwise for both forces, so they have the same sign (and your answer to the first is wrong).