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Trash Talk - Lets do physics :-)

Agurus - Tue Nov 07, 2006 8:20 pm
Post subject: Lets do physics :-)

Mmm I cant figure out this ridiculous problem, I hate doing variables only. Can someone take a crack at it for me, I would apreciate if you showed me the steps you did to get there

File b3low

Mine GO BOOM - Tue Nov 07, 2006 9:47 pm
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To help you along without giving answers right out, first list some equations you know that can be used in this example.

Blocks - Tue Nov 07, 2006 10:06 pm
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I'll give you some hints with which to get started.

Draw a free-body diagram. Call the horizontal direction x and the vertical direction y. You'll have two y forces from the weights, a force on the line of action of the wire (separate this out into x and y components), and a reaction at point A (defined by Asubx and Asuby).

Write out the equations for equilibrium, which would be the sum of the x forces equals zero, the sum of the y forces equals zero, and the sum of the moments about point A equals zero. You'll be able to write an inequality relating Asubx and Asuby under which equilibrium holds.

Maybe I gave away too much. Oh well, as long as you promise to understand what you're doing. There might be a simpler way to do it too.

Agurus - Tue Nov 07, 2006 10:11 pm
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Well here are a few I think we could use:

(SUM OF)F=MA

Fs(max) = Us*Fn

W=MG=-Fn

Net External Torque= I * angular acceleration

or

SUM OF Torques = SUM OF MASSES r^2*alpha

Xcm= (m1(change in X))+m2(change in x2)/ (m1+m2)

I think we are supposed to split it to X and Y components but then have no idea what to do with the torques.

Cyan~Fire - Tue Nov 07, 2006 10:17 pm
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Pay attention to blocks, you need to use the three equilibrium equations.

Agurus - Tue Nov 07, 2006 10:18 pm
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O_o didn't see his post, hmmm

CypherJF - Tue Nov 07, 2006 10:21 pm
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i dreaded physics... still not sure how i passed the classes? (C in the first semester and B in the following (which the material was a lot harder)). *shrug*

Mine GO BOOM - Tue Nov 07, 2006 10:29 pm
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CypherJF wrote:

i dreaded physics

I loved physics. Is it wrong that I knew the equations and method for how to solve that problem still, even though it has been about three years since I last bothered to touch torque/static friction?

Agurus - Tue Nov 07, 2006 10:35 pm
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So assist me mgb

Mine GO BOOM - Tue Nov 07, 2006 10:37 pm
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Blocks wrote:

Agurus wrote:

O_o didn't see his post, hmmm

Follow his directions and write out the equations.

Agurus - Tue Nov 07, 2006 10:41 pm
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Posts: 3001

Gj mgb ;P

Yeah ok I will try that

Mine GO BOOM - Tue Nov 07, 2006 11:46 pm
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Agurus wrote:

Posts: 3001

Gj mgb ;P

Well, it is my forums, and I do try to visit daily. Even so, I'm still not number one by a long shot. If you still want further help, make an MS Paint image of your diagrams and equations you currently have setup. If something is wrong, we'll help point it out for you.

Muskrat - Wed Nov 08, 2006 1:45 am
Post subject: Re: Lets do physics :-)

Agurus wrote:

I hate doing variables only.

If you actually want to retain an ability at physics or ever use this stuff, you should be doing variables only on every single problem until the last step. Starting with numbers and doing algebra(& mebe a little cal) will not give you a very good idea of what is really going on.

And yeah, free body diagrams help..... boy I'm glad I got my A and moved on from that class. But then again electricity & magnetism this spring is going to suck so much harder, taught by a man who simply loves calculus and has no qualms about banishing calculators with more than simple functions.

P.S. Virginia is down, Allen needs to stfu!

Cyan~Fire - Wed Nov 08, 2006 11:22 am
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MGB wrote:

Well, it is my forums, and I do try to visit daily. Even so, I'm still not number one by a long shot.

Agurus - Wed Nov 08, 2006 9:20 pm
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Pardon my handwriting, but here is my work any thing you guy see wrong?

Cyan~Fire - Thu Nov 09, 2006 12:39 am
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Working on a response, I'll edit in a few minutes...

Edit: OK, here's what I have to say. I'm tired right now and I need to get up early tomorrow (... for Physics) so I'll check it in the morning and I'll actually try to solve the problem.

Agurus - Thu Nov 09, 2006 8:34 am
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Fr= Force Reaction, same as Fn basically.

2w= weight of bar
w= weight of ball (go weights from problem)

Cyan~Fire - Thu Nov 09, 2006 12:27 pm
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OK, well you're right, it is the normal force. I guess you can call it whatever you want when you're doing your own work, but it would probably be good to call it the normal force when showing your work to others (such as us or your teacher).

So... everything else I said still stands. I realize that w is the weight of the ball (and the bar, reread the problem). So why did you multiply the coefficient of friction by the weight to get the frictional force? Why is the normal force even in (what I'm assuming is) your torque equation?

Finally, you didn't answer my question. What did you pick for your axis?

Quan Chi2 - Fri Nov 10, 2006 4:41 pm
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I wish I knew how to add.