Moment of force about a point

[BigJon]

Homework Statement

The moment of force Fa about point O

Homework Equations

The Attempt at a Solution

I know how to setup with breaking the force into x,y component but how to tell if its rotating cw or ccw about the point. I think y is ccw but i can't tell on x. I think i would use right hand rule but need to know how to do it correctly.

 

[mishek]

Right hand rule would work here. How did you come to conclusion that Fy
"rotates" ccw? If You know the distance and direction of the force, it's easy to determine the direction of moment.

The distance has to be perpendicular to the force, and your fingers have to be directed in the direction of the force, i.e. the direction of rotation. Your palm shows moment direction.

 

[BigJon]

Well to setup the prob it would be like -200cos(60)(9in)+200sin(60)(30in), but why is the first term negative if x pulls straight out? My prof. didnt pull this from the book. Maybe I'm not understanding the principle behind moments etc.

 

[mishek]

Well to setup the prob it would be like -200cos(60)(9in)+200sin(60)(30in), but why is the first term negative if x pulls straight out? My prof. didnt pull this from the book. Maybe I'm not understanding the principle behind moments etc.

Please check the photo that i attached. If you try to rotate Fx around point P, your thumb would be faced down which is the negative z direction, and therefore that moment is also negative.

 

[Nickie]

The moment of force about a point is a measure of the tendency of a force to cause rotation around that point. To determine if the rotation is clockwise or counterclockwise, you can use the right-hand rule. Simply point your right thumb in the direction of the force and curl your fingers in the direction of rotation. If your fingers curl in a clockwise direction, the force is rotating clockwise about the point. If your fingers curl in a counterclockwise direction, the force is rotating counterclockwise about the point. You can also use the cross product to determine the direction of rotation, with the direction of the cross product being the direction of rotation.