# B How to calculate the perpendicular tension?

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1. Dec 30, 2016

### jorgeha

Hello to everyone, merry christmas and happy new year! I was trying some exercises on the Feynman Lectures, on the topic of statics, and I came across with a problem which, simplified until where I am stuck, asks for the tension of a cable perpendicular to a bar with a weight on the top of it, as in the picture below. I don't know which trigonometrical function should I use, as the force going downwards y-axis is perpendicular to the tension on the x-axis, the tension seems to be 0! (That is obviously incorrect) Should I first calculate the force going down the bar, and then multiply it by the angle θ?
https://postimg.org/image/5401ebhg3/

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2. Dec 30, 2016

### Staff: Mentor

Have you heard of balance of moments?

3. Dec 30, 2016

### jorgeha

Sadly, I haven't. I am studying with the Lectures and it isn't a very complete book (I mean, it isn't like a textbook). With the balance of moments, which other concepts do you think I should learn? Can you show me how to do it anyways? Thanks

4. Dec 30, 2016

### sophiecentaur

Rather than trying to understand some cobbled-together explanation from a PF member, it would be good if you look up "The principle of Moments" in an on-line text and look at some worked examples. It's a very basic bit of book work and you will find it in many places. The very basic approach is to study the see saw, with all the forces acting up or down, at right angles to the beam. The next step is to have forces that are acting in a general direction (which is like your problem)
This link is just one of many that have the answer to how to work out your prob.

5. Dec 30, 2016

### Staff: Mentor

It is possible to do this problem without using a moment balance. You do force balances on the point where the rod meets the rope. The trick is to assume that the rod can exert a force only in the direction along its length.

6. Dec 30, 2016

### jorgeha

Thank you very much for your answer. Do you know any other important concept I should learn? I will study that principle tomorrow :). I am studying with the Lectures, and they aren't like a textbook, I don't know which particular concepts I must know... do you know any other which will be useful for me? Thank you!

7. Dec 30, 2016

### jorgeha

Thanks, that was my doubt!

8. Jan 1, 2017

### sophiecentaur

That's true in this case but it only works when all forces are applied to the same point at the end of a beam. Moments are very relevant in most cases and it is good to be able to tackle all problems in the same way - at least until you are very familiar with the whole business. Also, forces acting along the beam can be ignored when you use Moments.

9. Jan 1, 2017

### Staff: Mentor

The forces along the beam have zero moment arm, so their moments are zero.