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).

 

[Mene]

In statics problems, it is important to consider the direction of the forces and their corresponding moments. When calculating moments, the direction of the force should be taken into account. In the first example, the force at point A is pointing downwards, so it should be represented as a negative value in the calculation. However, in the second example, the force is acting in the same direction as the axis, so it does not need to be represented as a negative value. It is important to carefully consider the direction of the forces and their effects on the calculation of moments in order to accurately solve the problem. This will ensure that the final answer is correct and reflects the actual physical situation.