# Solving Tensions and Reaction Forces of Pole AB

• Femme_physics
In summary, the conversation discusses a physics problem involving a pole supported by a pin joint and two wires, with a force acting on the pole at a specific angle. The conversation goes through different attempts at solving the problem, with the expert providing guidance and corrections along the way. Eventually, the problem is solved and the conversation ends with the learner expressing exhaustion and gratitude.
Femme_physics
Gold Member

## Homework Statement

http://img101.imageshack.us/img101/9546/3dstats.jpg

Pole AB is supported at point A by a pin joint, and held at point B by two wires, BC and BD, as described in the drawing. At point B acts on the pole force F which equals 500 [N]. The force F acts on a horizontal plane parallel to Axy, and is slated to an axis parallel to the y axis, and an angle psi that equals 30 degrees.

Calculate:

A) Tension on the wires.
B) That reaction forces at pin joint A

Measurements in meters.

## The Attempt at a Solution

http://img202.imageshack.us/img202/720/ohdear1.jpg

http://img838.imageshack.us/img838/9387/ohdear2.jpg

My Az is wrong, my Ay is correct

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Hi Fp!

What do your visual queues tell you about the size of alpha?

Furthermore, in this Y-Z-view, we will only look at y components and z components of forces.
What is the Fy component of F?
Hint: it is not equal to F.

I don't have visual queues in 3D! Heh. I wished we lived in 2D world calculations would've been easier...

*deep breath* Oooooooookay.

So it's actually

tan-1(2/6) = 18.435

As far as F.. you're right, I should have Fzy... I don't know it. Argh, this problem is tough.

Should I try from a different view? This view isn't helping me much.

Femme_physics said:
I don't have visual queues in 3D! Heh. I wished we lived in 2D world calculations would've been easier...

*deep breath* Oooooooookay.

So it's actually

tan-1(2/6) = 18.435

Now that wasn't so hard was it!

Femme_physics said:
As far as F.. you're right, I should have Fzy... I don't know it. Argh, this problem is tough.

Should I try from a different view? This view isn't helping me much.

Try a top view, just to find Fy.
Use that in your current view and you should be good.

Ah, you're right :) As always.

So I got Ay, and Az, but from some reason I got the wrong BC. I'll scan my attempt when I get back home-- I actually got some work to do today (summer break)^^

Thanks ILS you're amazing! Cya later :)

Femme_physics said:
My ay and az are correct, then it came to BC and I got it wrong. But how come? I used the correct F this time. Fx!

You calculated BCxz, but BC also has an y component...

Isn't it fun to wrap your mind around a 3rd dimension!

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Femme_physics said:
Argh!

Yes and no! I thought I already got the intuition but looks like I don't still. It'll take some time! But, I'll get there :)

I wish all the problems we solved so far had been in 3D, the transfer to it is just not that smooth!

I'd be very impressed if you could have done 3D problems straight away!

Femme_physics said:
And yet, I still appear to get the wrong BC. The answer in the manual says 476.70 [N]

I get 467.70 N.
If the solution manual says 476.70 N, I think they made a typo (or did you?).

The fact that you get a slightly different result will be due to rounding in the angles you calculated.
Actually you should have:
BCx = 250.000 N
BCy = 125.000 N
BCz = 375.000 N

Did you round the angles to 3 digits (instead of 6 digits)?

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Femme_physics said:
Now for the final kill!

http://img801.imageshack.us/img801/7223/68681533.jpg

Ah... my BD is off. Theirs is 974.01 [N]. But how is it possible? I've considered all the axes!

Ah, the final kill!

Perhaps you should plug in the numbers in your sum Fz formula again?

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Gooooooooooooooooooooooooooooooooooot it! :) At long lasts!

Phew, 3D problems are exhausting. I hope the other ones will go smoother. Many thanks master-of-all-things-physics-like :)

Femme_physics said:
Gooooooooooooooooooooooooooooooooooot it! :) At long lasts!

Phew, 3D problems are exhausting. I hope the other ones will go smoother. Many thanks master-of-all-things-physics-like :)

Long last? Only 12 posts!
You're definitely getting better at this!

And you're welcome femme-physics :)

## 1. How do you determine the tension in pole AB?

The tension in pole AB can be determined by using the equations of equilibrium. These equations state that the sum of all the forces acting on the pole must be equal to zero. By setting up and solving these equations, the tension in pole AB can be calculated.

## 2. What factors affect the tension in pole AB?

The tension in pole AB is affected by the weight of the pole, any external forces acting on the pole, and the angle at which the pole is being held. Additionally, the material and thickness of the pole may also impact the tension.

## 3. How does the angle of the pole affect the reaction forces?

The angle of the pole affects the reaction forces by changing the distribution of weight and tension on the pole. As the angle increases, the vertical component of the tension decreases, causing the reaction forces to increase. On the other hand, as the angle decreases, the vertical component of the tension increases, causing the reaction forces to decrease.

## 4. Can the tension in pole AB be negative?

No, the tension in pole AB cannot be negative. Tension is a vector quantity, meaning it has both magnitude and direction. A negative tension would indicate that the pole is being compressed, which is physically impossible. If the equations of equilibrium yield a negative tension, it means there is an error in the calculations.

## 5. How can solving for the tension in pole AB be useful in real-life applications?

Solving for the tension in pole AB can be useful in various real-life applications, such as construction and engineering. Knowing the tension in a pole can help determine the appropriate materials and design needed to support the weight and forces acting on the pole. This information can also be used to ensure the safety and stability of structures, such as bridges and buildings.

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