Solving Tensions and Reaction Forces of Pole AB

[Femme_physics]

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

 

[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.

 

[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 F_zy... I don't know it. Argh, this problem is tough.

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

 

[I like Serena]

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!

As far as F.. you're right, I should have F_zy... 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.

 

[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 :)

 

[Femme_physics]

I forgot to scan this so I just went over the pencil graphite with a pen

http://img829.imageshack.us/img829/2628/cameasy1.jpg

http://img6.imageshack.us/img6/5398/cameasy2.jpg

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!

 

[I like Serena]

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 BC_xz, but BC also has an y component...

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

 

[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

http://img27.imageshack.us/img27/355/doih.jpg

http://img853.imageshack.us/img853/7369/doi2.jpg

 

[I like Serena]

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!

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)?

 

[Femme_physics]

Ah, then it's only a matter of rounding! :) And no, I didn't make a typo copying the figures, it's them who made a typo!


Now for the final kill!


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

Now I know that BDx is zero since it doesn't act on the X axis.


http://img30.imageshack.us/img30/9386/37090549.jpg

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

 

[I like Serena]

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?

 

[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 :)

 

[I like Serena]

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 :)