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Discussion Starter · #1 ·
I, for the life of me, can not get the right answer on a problem I have for physics. Doing it in class tomorrow isn't an option otherwise I would probably do that. Anyone good at physics?

Here is the problem:
Three masses, (m1 = 3.0kg, m2 = 1.0kg, m3 = 2.0kg), all in the x-y plane, are connected to each other by rigid rods of negligible mass. They are also connected to a bearing on the axis of rotation, the z-axis, by a similar rigid rods of length r1 = 0.40m, r2 = 0.60m, and r3 = 0.20m, respectively to the masses. The rod r1 makes an angle of 30 degrees with the negative x-axis, r2 makes an angle of 60 degrees with the positive x-axis, and r3 lies along the positive x-axis. The assembly of masses is held stationary with r3 horizontal and then released. (Gravity is acting on the system to cause it to rotate)

a) Find the angular acceleration of the assembly just after release
b) Find the linear tangential accelerations of each mass.

Here's a pic for clarification:


I planned on using the fact that Torque = Rotational Inertia * rotation acceleration. I calculated the total rotational inertia to be .92 (by using I=m1*r1^2 + m2*r2^2 + m3*r3^2). And I tried calculating total torque by T1+T2+T3, assuming T=r*F*sin(Theta), and F would be mass*cos(theta)*9.81. That didn't get me the answer I was looking for.

The answer to part a is 3.62 rad/s^2 but I did'nt get there.
Part b is easy as soon as I can finish part a.

Thanks for any help!
 

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Discussion Starter · #3 ·
I'm a bit confused....is the z-axis going in and out of the screen?
Indeed it is, and the system is rotating around that point.

If you had a clock with 3 arms that were fixed to each other, thats how it would rotate. (Except it might rotate counter-clockwise, I can't fricken figure out what the acceleration is so I wouldn't know)
 

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Initial acceleration should be the difference in force from the left to the right. Or, cancel out the torques (F*d). That's horizontal distance to the axis of rotation. On the left, that's - if my trig is still good - R1*cos(theta1)*g*m1 for the counterclockwise torque. Clockwise torque should be R3*g*m3 + (R2*cos(theta2)*g*m2). Subtract one from the other to get your effective torque.
 

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Well, first your inertia calculations are off. You have to use the perpendicular distance from the axis of rotation. So the equation would be I=m1*(r1*Cos(30))^2+r3^2*(m2+m3) which should be like 0.4something...

I don't really have a pen and paper next to me..
 

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Discussion Starter · #6 ·
Well, first your inertia calculations are off. You have to use the perpendicular distance from the axis of rotation. So the equation would be I=m1*(r1*Cos(30))^2+r3^2*(m2+m3) which should be like 0.4something...

I don't really have a pen and paper next to me..
Ah Ha! I think that is what I have screwed up. I just used r1 r2 r3 as my distance, I didn't know (or forgot), that it had to be perpendicular distance.

Let me run some numbers to see what I get.
 

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Discussion Starter · #9 ·
Lol nice.

Well after much deliberation, (and by deliberation I mean A LOT of wasted time), I got to the final answer. YAY! Lol. Thanks guys. Its great that I can go to a motorcycle forum and ask for physics help. And more than likely I'm going to post another one up here in a bit haha.
 

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Its great that I can go to a motorcycle forum and ask for physics help
It's only because we ride girls' bike that can't really wheelie that we have to resort to mental tricks like physics and the calculus to compensate.
 

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Its great that I can go to a motorcycle forum and ask for physics help.
I have asked plenty of calc2 questions and maybe even a physics question. efnar is a math genius ;D.
 

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Discussion Starter · #15 ·
Lol I'm going for my ME degree but lately I've been looking at EE too. Afterwards I'm planning on being a fighter pilot for either the Air force or Navy.
 

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Lol I'm going for my ME degree but lately I've been looking at EE too. Afterwards I'm planning on being a fighter pilot for either the Air force or Navy.
I got my BSME with the same goal of flying for the USAF. They offered me a nav position and put me on the alternate list for pilot. I should've taken it, but turned down the nav slot as well as pulled my name off the pilot alternate list to be an engineer. Stupid me, I got slapped with a RIF almost two years ago.

Moral of the story: Engineers are the whores of this society. Everyone harps on how important it is to have engineers and the shortage of us. Then, they pay you for a job, phuck you in the ass and don't even cuddle with you a little. Now I'm stuck working in an administration seat, paid like sh!t, just had my pay cut, can't find any REAL engineering job and will do anything to get the phuck out. Take any avaitor slot you can get.
 

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Moral of the story: Engineers are the whores of this society. Everyone harps on how important it is to have engineers and the shortage of us. Then, they pay you for a job, phuck you in the ass and don't even cuddle with you a little.
I feel like as a mod I should censor that, but as an engineer, I understand the truth behind it :'(
 

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I feel like as a mod I should censor that, but as an engineer, I understand the truth behind it :'(
Censor? OK, I'll try to put it a better way ;)

President Obama:
On K through 12 across the board we've got improved math and science instruction. And that means focusing on recruiting more math and science teachers, emphasizing math and science instruction, finding innovative ways to make it interesting for students. And I have to say this is an area where the president has the power to use the bully pulpit and to make math and science interesting and vibrant again. One of the things that I'm always struck by when I talk to engineers and scientists who are in their 50s and 60s is how many say they were inspired by JFK and the space program for going into science and math. And one area where I think we could actually do that is to really make a huge effort around energy independence. And if a president is talking about the importance of us engaging in research and development, doubling the amount of research dollars that are being put into basic science and basic research, all that can help lift up the importance of these areas of study for young people who basically take their cues from the larger culture….
If that doesn't make you feel improtant... ::)

And to finish the quote, "When we're through, we're going to put you on meth and kick you to the curb... after we phuck your ass."


Wait, I can't seem to get through it without the ass rape.
 

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My mind blanks out when I see mathematical equations like that. It induces narcolepsy in me. You might think I'm just trying to be funny, but it's true. I slept though all my math classes and english/writing classes all though out high school. I have no explanation. I excelled in drafting, wood shop, graphic design, lapidary, auto shop, science(up till chemistry... the formulas and memorization of the periodic table... couldn't do it.) I would be a 4.0 student if it weren't for math, history, PE(I ditched all soph and junior year), foreign language, English/writing classes. you dont get to use any of that crap anyways...


Calc and Trig.... I don't know how the fcuk you guys can process that stuff. My brain refuses to learn any of it.
 
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