# Why are these angles the same?

1. Sep 24, 2014

### xatu

1. The problem statement, all variables and given/known data

In my statics book (13th ed. Hibbeler) I'm reviewing the section about using vector analysis to calculate the moment of a force about a specific axis. I understand the theory fine, but I don't completely understand the figure in the book (pic attached). Specifically, why are the two angles labeled theta equal? The first tangle is between the position vector and the x axis, and the second angle is between the moment and it's y component.

2. Relevant equations

Trig relations would obviously be of some help.

3. The attempt at a solution

Since $r$ is any vector from point O extending to the line of action of the applied force, doesn't that mean we can adjust $r$ and still maintain the same moment $M_O$? Thus $\theta$ between $r$ and the x axis is changing while $\theta$ between $M_O$ and $M_y$ remains the same?

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2. Sep 24, 2014

### nasu

What is Mo?

3. Sep 24, 2014

### xatu

$M_O$ is the moment of the force $F$ about the axis through point O.

4. Sep 24, 2014

### nasu

OK. What is the direction of Mo in respect to the plane determined by F and r? What is the angle between Mo and r?

5. Sep 25, 2014

### xatu

$M_O$ is, by definition, perpendicular to the plane containing $F$ and $r$. The angle between $M_O$ and $r$ is thus 90°.

6. Sep 25, 2014

### nasu

Great. so now you see why the two angles are equal?
The angle between the y axis and r should give 90 when added to theta, right?

7. Sep 25, 2014

### xatu

Yes, but I don't exactly understand how the angle between the y axis and $r$, and the angle $\theta$ being complementary to one another tells us anything about the angle between $M_O$ and $M_y$.

I get that the angles adjacent to $\theta$ are equivalent to $90-\theta$, but, to me, that doesn't explain why both $\theta$'s seem to be equal.

Can you please clarify?

8. Sep 25, 2014

### nasu

If we call "alpha" the angle between r and My, it is complementary to both angles labeled "theta".
Or both angles have the same complementary angle (alpha).

9. Sep 25, 2014

### xatu

Thanks a ton, I got it now! Interesting way of looking at things!