# Moment of force

## Main Question or Discussion Point

Moment of force= F*d*sin(alpha).
Now, Must d be the perpedicular distance from the force to the axis , or any distance?

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ZapperZ
Staff Emeritus
Moment of force= F*d*sin(alpha).
Now, Must d be the perpedicular distance from the force to the axis , or any distance?
You have given very little information here. For example, what is "alpha"? Is this the angle between F and d?

I think there's quite a bit of confusion here. You may want to look at this and see if you've misunderstood something important.

http://hyperphysics.phy-astr.gsu.edu/hbase/woang.html#waa

Please consider formulating as clear and complete of a question next time.

Zz.

No, Moment of force with respect to an axis,
Alpha is the angle betwen the force and d.

ZapperZ
Staff Emeritus
You mean that it doesn't matter if d is the perpendicular distance, because you included the angle?

ZapperZ
Staff Emeritus
You mean that it doesn't matter if d is the perpendicular distance, because you included the angle?
If they both have to be perpendicular, then the angle will always be 90 degrees, and it is a constant equal to one! So then why even bother writing "sin(alpha)"?

Zz.

Yeah I know, but thats not my point, my point is if d is any distance or it must me the perpendicular distance? You got me Mr.

ZapperZ
Staff Emeritus
Did you look at the figure in the link that I showed? I thought that is self-explanatory?

Zz.

So , it must be perpendicular.

cjl
Actually, it doesn't need to be perpendicular.

To be slightly more accurate, your equation already takes that into account. The moment is the force multiplied by the perpendicular distance. In your equation, d*sin(alpha) is the perpendicular distance.

ZapperZ
Staff Emeritus
So , it must be perpendicular.
I'm going to correct this and say no, it doesn't, which is what I said already. But obviously, it is not getting through to you, but I'm going to make sure others reading this do NOT get the same wrong information.

I have no idea why you are fixated with this "perpendicular".

Zz.

ZapperZ
Staff Emeritus
Actually, it doesn't need to be perpendicular.

To be slightly more accurate, your equation already takes that into account. The moment is the force multiplied by the perpendicular distance. In your equation, d*sin(alpha) is the perpendicular distance.
We need to be careful here because that is the perpendicular component of the distance vector. If you read what the OP wrote, he/she is simply not considering that, and somehow, refuses to accept that it can be ANY direction.

Zz.

rock.freak667
Homework Helper
Moment of force= F*d*sin(alpha).
Now, Must d be the perpedicular distance from the force to the axis , or any distance?
isn't the moment of a force about an axis given by

$$M=\hat{\lambda} \cdot (\vec{r} \times \vec{F})$$

where $\hat{\lambda}$ the unit vector in the direction of the axis?

Nope, we didn' took it this way.
M=F*d

Ohhhhh, I got it , thanks ZappperZ, and for all :d.
Sorry for confusion.