# How to calculate moments to find a force

1. Feb 17, 2012

### Sully1071

I'm looking at a problem and am having trouble finding the force using moments. To be honest i'm not sure am i going about it the correct way.

Attached is a sketch of the problem,

Is the cclockwise moment about A = .5m(1000N) = 500Nm
and the anti clockwise moment about A = .75m(F)

can these be equated to find F??

Thanks

#### Attached Files:

• ###### Sketch.jpg
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2. Feb 17, 2012

### zoobyshoe

3. Feb 17, 2012

### Sully1071

Sorry, attached is a bigger image in a word document.

#### Attached Files:

• ###### FBD.doc
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4. Feb 17, 2012

### tiny-tim

Welcome to PF!

Hi Sully1071! Welcome to PF!
Yes, that's exactly correct

(except that you've mixed up clockwise and anti-clcokwise! )

and yes, you now equate the clockwise to the anti-clockwise.

Why is that worrying you?

5. Feb 19, 2012

### Sully1071

Im just unfamiliar with unfamiliar with these calculations but have to do them as part of a project.

If i use the new free body diagram attached and the unit was to rotate about point a i assume my new moments would be calculated by:

Anticlockwise moments = 1000(.65)
Clockwise moments = F(.6)

#### Attached Files:

• ###### sketch 1 moments.doc
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6. Feb 19, 2012

### tiny-tim

Hi Sully1071!

Is this a new problem? …

sorry, I'm on an iMac, and it won't show images in .doc

(but I could see your original .jpg)

7. Feb 19, 2012

### Sully1071

jpeg is attached

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• ###### fbd11.jpg
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8. Feb 19, 2012

### tiny-tim

ah! nice and big!
yes that's right

generally, the moment of a force F is the dot product F.d,

which you can do as either F times the perpendicular distance (your method)

or as F times the whole distance times sine of the angle between