1. Jun 29, 2009

### zrome

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

"In raising the pole from the position show, the tension T in the cable must supply a moment about O of 72 Knm. Determine T."

2. Relevant equations

M = Fd

3. The attempt at a solution

d = 12sin(30) = 6m
T = 72/d
==> 12m

I do not know what I am doing wrong here. I'm assuming there is a problem with "12sin(30)" but I checked atleast 3 times and I still get the same eq. Please assist in any manner possible. Thank You in advance.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

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2. Jun 29, 2009

### PhanthomJay

The moment of a force about a point O is the force times the perpendicular distance from the line of action of that force to the point O (M=F(d_perp)). Alternatively, the moment of a force about point O is the force times the distance from the point of the application of the force to point O, times the sine of the included angle between the two (M = rF sin theta = 30Tsin theta in this example). This becomes a geometrry/trig problem, where you must determine theta using the appropraite trig functions (as in using the law of sines and cosines, for example).

3. Jun 29, 2009

### zrome

Now I'm not too sure if I understand you correctly here, but what I make out of your response is that the angle between, I believe it to be 30 degrees; you take the sin of the angle and then multiply that by the force, in this case T.

Doing that I arrive at the conclusion that 30Tsin(30) = 72 and solving for T results a 4.8 knm

I see the answer is wrong and am still unable to make out how to correctly solve this problem. Unfortunately I have a quiz on this material tomorrow evening at 6 and I need your help in trying to understand how to do this problem, for which the author of the book decided not to give the EDIT: workto.

The final answer is 8.65 knm. Could you kindly walk me step by step through the problem so that I can clearly understand how it is done? (I agree my trig is horrible)

Last edited: Jun 29, 2009
4. Jun 29, 2009

### tiny-tim

Hi zrome!
Where do you get 30º from?

Hint: draw the vertical line from the top of the cable to the ground … so you have a right-angled triangle, and you want to know the sine of one of the angles.

5. Jun 29, 2009

### nvn

That's a good hint by tiny-tim, to get the height of the vertical line. And, you can compute the horizontal projection of the pole on the ground, right? After that... Hint: Use definition of tan to compute an angle.

6. Jun 30, 2009

### tiny-tim

Hi nvn!

Actually, we don't need to know the angle …

we only want its sine, whch we can get from the sides of the right-angled triangle (and Pythagoras)

7. Jul 1, 2009

### zrome

Ya computed the vertical line and was able to see the big triangle I was missing. Thank you for all the help.