What is the rotation direction of a system with a force applied at point B?

[Faux Carnival]

[PLAIN]http://img137.imageshack.us/img137/8130/mompq.jpg

Hi, here is a system with a force acting on it. As you can see the force's components are shown. In systems like this, I can't decide in which direction the components rotate the system.

It is obvious that the 400N-component wants to rotate the system counter clockwise but what about the 300N-component?

Thanks!

 

[Doc Al]

It is obvious that the 400N-component wants to rotate the system counter clockwise but what about the 300N-component?

Assuming point 'A' is your axis, the 300N-component 'wants' to rotate the system clockwise. Figure out the torque from each component--the bigger torque wins.

Or you can just extend the line of the 500N force back toward the axis and see on which side it falls. You'll see that the overall force acts to rotate the system counter-clockwise.

 

[Faux Carnival]

Thanks Doc. But I still can't tell why the 300N-component rotates it clockwise. Because it's above point A, it seems to me that it would rotate counter clockwise.

 

[Doc Al]

Thanks Doc. But I still can't tell why the 300N-component rotates it clockwise. Because it's above point A, it seems to me that it would rotate counter clockwise.

Extend the line of the 300N component all the way to the left. It's above point A but it acts to the right, thus rotating clockwise.

 

[CharlesMareau]

Let's say the force is applied at point $$B$$, you can calculate the moment at point $$A$$ with the relation :
$$\overrightarrow{M_A}=\overrightarrow{AB} \wedge \overrightarrow{F}$$

Choosing the coordinate system such that $$x$$ and $$y$$ are respectively the horizontal and vertical axis, the sign of the $$z$$ component of the moment will give you the rotation direction (i.e. counter clockwise is the $$z$$ component is positive and clockwise if the $$z$$ component is negative).

This is another way of telling how it rotates...