# Homework Help: 3D statics

1. Jun 20, 2011

### Femme_physics

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

http://img101.imageshack.us/img101/9546/3dstats.jpg [Broken]

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.

3. The attempt at a solution

http://img202.imageshack.us/img202/720/ohdear1.jpg [Broken]

http://img838.imageshack.us/img838/9387/ohdear2.jpg [Broken]

My Az is wrong, my Ay is correct

Last edited by a moderator: May 5, 2017
2. Jun 20, 2011

### I like Serena

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.

3. Jun 20, 2011

### Femme_physics

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.

4. Jun 20, 2011

### I like Serena

Now that wasn't so hard was it!

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

5. Jun 20, 2011

### Femme_physics

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 :)

6. Jun 22, 2011

### Femme_physics

Last edited by a moderator: May 5, 2017
7. Jun 22, 2011

### I like Serena

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

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

8. Jun 22, 2011

### Femme_physics

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!

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

http://img718.imageshack.us/img718/7568/doi0.jpg [Broken]

http://img27.imageshack.us/img27/355/doih.jpg [Broken]

http://img853.imageshack.us/img853/7369/doi2.jpg [Broken]

Last edited by a moderator: May 5, 2017
9. Jun 22, 2011

### I like Serena

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

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)?

10. Jun 23, 2011

### Femme_physics

Last edited by a moderator: May 5, 2017
11. Jun 23, 2011

### I like Serena

Last edited by a moderator: May 5, 2017
12. Jun 23, 2011

### Femme_physics

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 :)

13. Jun 23, 2011

### I like Serena

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

And you're welcome femme-physics :)