# Direction of rotation of moment

1. Sep 23, 2015

### goldfish9776

1. The problem statement, all variables and given/known data
for the 400cos45 force , i think it should rotate anticlockwise, am i right? but the book gave it is rotated counterclockwise...

2. Relevant equations

3. The attempt at a solution

#### Attached Files:

File size:
49.4 KB
Views:
51
• ###### DSC_0076[1].JPG
File size:
45.6 KB
Views:
45
2. Sep 23, 2015

### SteamKing

Staff Emeritus
Uhh... "anticlockwise" and "counterclockwise" both mean the same thing, you know, "not clockwise".

RIP, Yogi Berra

3. Sep 23, 2015

### goldfish9776

Typo, imo it should be in counterclockwise, right? The book gave it in clockwise direction..

4. Sep 23, 2015

### SteamKing

Staff Emeritus
Nope, guess again.

The moment sign convention adopted by the book is a little unconventional. I prefer the right hand rule, which would make CCW moments positive.
The actual convention chosen doesn't matter, as long as you remain consistent throughout your calculations.

Last edited: Sep 23, 2015
5. Sep 23, 2015

### goldfish9776

I'm wondering is my direction of 400cos 45 force correct ? Should it point to the left ?

6. Sep 23, 2015

### goldfish9776

#### Attached Files:

• ###### hg.png
File size:
50.1 KB
Views:
54
7. Sep 23, 2015

### SteamKing

Staff Emeritus
Yes, it is correct.

8. Sep 23, 2015

### goldfish9776

then the moment should be anticlockwise , right ? by using right hand rule . I bent my finger from f to F , since it's r cross F

9. Sep 23, 2015

### SteamKing

Staff Emeritus
The moment you showed in Post #6 is indeed CCW.

10. Sep 23, 2015

### goldfish9776

Ok, then the book is wrong...

11. Sep 23, 2015

### SteamKing

Staff Emeritus
No, the moment shown in the book is properly CW.

The figure you drew in Post #6 does not match the layout of forces shown in the book, and the direction of that moment is CCW.

You can't change the position of the force relative to the axis of rotation without changing the direction of the moment which is produced.

12. Sep 23, 2015

### goldfish9776

I m confused now. Then can you show mew the r x f diagram in the book? I have no idea at all.

13. Sep 23, 2015

### SteamKing

Staff Emeritus
It's Fig. 3-27(a). Point A is located about half way up the y-axis, while the 400 N force is located about 0.8m from the origin.

You don't need a lot of diagrams and whatnot to figure this out. If point A is the point about which a force causes rotation, you should be able to visualize this from a picture if you can see where the force is applied relative to point A.

14. Sep 23, 2015

### goldfish9776

since the author assume clockwise as positive here , so shouldn't the positive 400cos45 (0.3) rotate anticlockwise? i have drawn a diagram for the moment in the previous post , showing it is rotated in anticlockwise direction... i cant understand why the moment is in clockwise direction for the author...