How Is the Net Moment Calculated in a Circular Disc?

[xunxine]

I understand that to find moment of force, we look at direction of force and perpendicular distance. In the diagram, O is the centre and pivot of the circular disc.

Would the anticlockwise moment at A be (30N x 4m)? I'm not sure if this is true, cos the distance 4m is only AB. It does not include O which is the pivot.
Should we resort to resolving the 30N force to 2 components at 5m (OA)?
(Clockwise moment is straightforward here, isn't it?)

 

[Doc Al]

Would the anticlockwise moment at A be (30N x 4m)?

Yes.

I'm not sure if this is true, cos the distance 4m is only AB. It does not include O which is the pivot.

What counts is the perpendicular distance from the line of the force to the pivot, which equals AB.

Should we resort to resolving the 30N force to 2 components at 5m (OA)?

That's a perfectly fine thing to do. Try it and you'll see that you get the same answer for the moment, since OA*F*sinθ = AB*F.

(Clockwise moment is straightforward here, isn't it?)

Sure.

 

[charng]

I can confirm that your understanding of finding the moment of force is correct. The moment of force is the product of the force and the perpendicular distance from the pivot point. In the case of a circular disc, the pivot point is the center of the disc.

Regarding your question about the anticlockwise moment at point A, you are correct that the distance should include the pivot point O. This is because the force is acting at point A, which is 4m from the pivot point, but the moment is calculated with respect to the pivot point.

To accurately calculate the moment at point A, you can either resolve the 30N force into two components at 5m from the pivot point, or you can use the perpendicular distance from the pivot point to point A, which is 5.66m. Both methods will give you the same result.

As for the clockwise moment, it is indeed straightforward in this case as it is simply the product of the force and the perpendicular distance from the pivot point, which is 4m.

I hope this helps clarify your understanding of the net moment in a circular disc. Remember to always consider the pivot point when calculating moments of force.