# Homework Help: Moment about a point using graph paper

1. Oct 7, 2015

### werson tan

1. The problem statement, all variables and given/known data
the ans given is 95Nm , but i gt 80.9Nm , which part i did wrongly ?

2. Relevant equations

3. The attempt at a solution
20(4) -39(5/surd 26(3)) +39(1/surd26)(1) -60(3/5)(1)+60(4/5)(3)
=80.9

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2. Oct 7, 2015

### BvU

What is a surd26 ?
Could you tidy up your notation in general as well. 20(4) probably means 20 N * 4 m = 80 Nm ?

 sorry, that 20(4) is a correct notation. Just had to get used to the sight.

Last edited: Oct 7, 2015
3. Oct 7, 2015

### mjsd

firstly, I don't think

20(4) -39(5/surd 26(3)) +39(1/surd26)(1) -60(3/5)(1)+60(4/5)(3)

would yield 80.9

The term 39(1/ Sqrt[26])(1) should not be there.... 60 (4/5)(3) the (3) should be a (2)

however with these adjustments, the answer is still not 95 Nm clockwise by my calculations (more like 25-ish), I believe there might be typo somewhere??

4. Oct 7, 2015

### BvU

Sigh... after wasting some time on this, I conclude that surd26 is $\sqrt { 26}$ and you use it to decompose the 39 N into x and y components. For the $\ \vec r\$ in $\ \vec \tau = \vec r \times \vec F\$ you then take $\ \vec r = (1,3)\$. Shouldn't that be $\ \vec r = (0,3) \$ ? It gives yet another answer, but I wouldn't trust a book answer in a book that lets 1 m be represented by a square ( )

PS I get what mjsd gets. The bat types faster ....

5. Oct 7, 2015

### werson tan

sorry , the book contain error , the 1m should be 12m on the 39N triangle . I have redo the question .
20(4) -39(5/13)(3) +39(12/13)(1) -60(3/5)(1)+60(4/5)(3)
=179N

6. Oct 7, 2015

### mjsd

As I said in my first post....
39(12/13)(1) should NOT be there as the (1) is actually (0)...can you see that?
and
60(4/5)(3) is actually 60(4/5)(2)
once you have got these

7. Oct 10, 2015

### werson tan

I know what u mean . In the above steps , I have resolved the 60N in this way( black and red) , whereas the green colour one is the resolution of force done by you .

For the 39N force , I have resolved the force in this way ( black and red)

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• ###### 60N.png
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8. Oct 12, 2015

### werson tan

yes , i know why it should be 39(12/13)(0), now my question is why cant I resolve the force in the way that i have posted earlier?
I have resolved the 60N and 39N in this way( black and red) , whereas the green colour one is the resolution of force done by you .

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