[PHYSICS] Two questions whose solutions i dont understand

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[PHYSICS] Two questions whose solutions i dont understand

Postby Afif_D » Sat Jan 28, 2012 10:19 am UTC

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Here are two questions . I cant understand how they have been solved. Can anyone explain to me pls.
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Re: [PHYSICS] Two questions whose solutions i dont understan

Postby mfb » Sat Jan 28, 2012 3:54 pm UTC

The solutions have several steps, at which step do you have problems?

For 1, consider an infinitely small change in theta (or, mathematical, the derivative): The energy must not change, so the derivative is 0.
Moving the left fluid up (left side of the first hint) costs the same energy as moving the other fluid down (right side) gives.
The other lines are just steps to get an expression theta=... based on that.

For 2: The diagonal springs are treated as effective springs with a different constant k', based on the force they apply when the mass goes up/down.
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Re: [PHYSICS] Two questions whose solutions i dont understan

Postby Afif_D » Sat Jan 28, 2012 4:18 pm UTC

I think i am an idiot as i cant understand anything about this..

1.Could you detail me about how it is done with using mathematical symbols etc.

2.How does the (cos45)^2 term come in?

I dont understand..

Its better you show me all the steps. Sorry if its a pain for you. But i need it
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Re: [PHYSICS] Two questions whose solutions i dont understan

Postby Gigano » Sat Jan 28, 2012 8:27 pm UTC

Afif_D wrote:2.How does the (cos45)^2 term come in?


Caution: I am a biologist with limited knowledge of physics, so I may be totally wrong. I intuitively think the term comes into play by combining the force vector from the two springs that are at a 135 degree angle to the downward springs. Combining those two vectors ultimately simplifies the whole system to just two springs, each exerting a force in opposite directions.
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Re: [PHYSICS] Two questions whose solutions i dont understan

Postby Charlie! » Sat Jan 28, 2012 10:26 pm UTC

Gigano wrote:
Afif_D wrote:2.How does the (cos45)^2 term come in?


Caution: I am a biologist with limited knowledge of physics, so I may be totally wrong. I intuitively think the term comes into play by combining the force vector from the two springs that are at a 135 degree angle to the downward springs. Combining those two vectors ultimately simplifies the whole system to just two springs, each exerting a force in opposite directions.

So basically, when each spring is compressed, it exerts a force back. How much do the springs at an angle get compressed when you displace the particle straight up? What is the component of the force the springs exert along the vertical direction?
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Re: [PHYSICS] Two questions whose solutions i dont understan

Postby Afif_D » Sun Jan 29, 2012 4:29 pm UTC

Yea it is cos 45 . Why is it squared??
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Re: [PHYSICS] Two questions whose solutions i dont understan

Postby mfb » Sun Jan 29, 2012 8:54 pm UTC

One cos comes from the lower compression, and another from the angle of the spring force.
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Re: [PHYSICS] Two questions whose solutions i dont understan

Postby Afif_D » Mon Jan 30, 2012 5:33 pm UTC

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Y U NO work it out in details?
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Re: [PHYSICS] Two questions whose solutions i dont understan

Postby sigsfried » Mon Jan 30, 2012 6:48 pm UTC

Because it is your homework. What do you learn from any of us doing it for you?
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Re: [PHYSICS] Two questions whose solutions i dont understan

Postby yurell » Mon Jan 30, 2012 9:32 pm UTC

You know F = k x
What proportion of k is pushing the mass back down? What proportion of x is stretching / compressing the springs?
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Re: [PHYSICS] Two questions whose solutions i dont understan

Postby Sizik » Mon Jan 30, 2012 9:34 pm UTC

sigsfried wrote:Because it is your homework. What do you learn from any of us doing it for you?


He learns the thought processes you go through in order to solve the problems. It's not like he's going to pass it off as his own work.
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Re: [PHYSICS] Two questions whose solutions i dont understan

Postby starslayer » Tue Jan 31, 2012 4:24 am UTC

If it is homework/coursework, Afif_D, have you already talked with your professor/teacher about these problems? If not, why not? They should be your first recourse for any questions you have; that's what they're there for. If you have already asked them about these problems, what about their explanations was unsatisfactory? Note that just giving you the exact detailed solution hardly ever lets you learn much. Trying to work it out for yourself with some hints is usually much better for gaining actual understanding.
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Re: [PHYSICS] Two questions whose solutions i dont understan

Postby Afif_D » Tue Jan 31, 2012 4:43 pm UTC

starslayer wrote:If it is homework/coursework, Afif_D, have you already talked with your professor/teacher about these problems? If not, why not? They should be your first recourse for any questions you have; that's what they're there for. If you have already asked them about these problems, what about their explanations was unsatisfactory? Note that just giving you the exact detailed solution hardly ever lets you learn much. Trying to work it out for yourself with some hints is usually much better for gaining actual understanding.



Coz our school is closed. I have been trying to figure out how these two things come up
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Re: [PHYSICS] Two questions whose solutions i dont understan

Postby Afif_D » Tue Jan 31, 2012 4:51 pm UTC

Its not a homework.Not a goddamn homework . Trust me
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Re: [PHYSICS] Two questions whose solutions i dont understan

Postby sigsfried » Tue Jan 31, 2012 8:26 pm UTC

Ok. Let's work through the first one.

Can you write an expression for the total energy of the system?
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Re: [PHYSICS] Two questions whose solutions i dont understan

Postby yurell » Tue Jan 31, 2012 9:36 pm UTC

Afif_D wrote:Its not a homework.Not a goddamn homework . Trust me


Did my advice help you? I got caught on where the cos2 came from too, and I think the problem is that once you write F = kx y = 2k cos(45) y, you are treating the two springs as a single vertical one.
The thing is, when the particle moves y in the vertical direction, how much of that is compressing the springs?
cemper93 wrote:Dude, I just presented an elaborate multiple fraction in Comic Sans. Who are you to question me?


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Re: [PHYSICS] Two questions whose solutions i dont understan

Postby starslayer » Wed Feb 01, 2012 4:44 am UTC

Afif_D wrote:Coz our school is closed. I have been trying to figure out how these two things come up
They come up because physics-literate forums see this kind of thing all the time. Some people will just post their homework and ask for the answer, without mentioning that it's homework. Since you say this isn't homework, great. It still doesn't mean that presenting you with the solution in its entirety would be all that helpful to you for understanding the principles behind the solutions. Hints like the ones given are much better for that, especially since you already know what the answers are and can easily check your work.
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Re: [PHYSICS] Two questions whose solutions i dont understan

Postby Dopefish » Wed Feb 01, 2012 5:18 am UTC

To be fair, in this case the solution is basicly provided along with the problem with essentially all the work (although I can't seem to display it all), making it more likely to be review material rather then any form of homework, it's just the justification for each step that's missing.

The cos^2 bit is sneaky, but if you think of it as two seperate cos's (one nestled with the x, and one with the overall force) it works out I believe. I haven't looked at it too closely though since the full thing is cut off for me.
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Re: [PHYSICS] Two questions whose solutions i dont understan

Postby starslayer » Wed Feb 01, 2012 5:44 am UTC

Dopefish wrote:To be fair, in this case the solution is basicly provided along with the problem with essentially all the work (although I can't seem to display it all), making it more likely to be review material rather then any form of homework, it's just the justification for each step that's missing.

The cos^2 bit is sneaky, but if you think of it as two seperate cos's (one nestled with the x, and one with the overall force) it works out I believe. I haven't looked at it too closely though since the full thing is cut off for me.
Point taken, however, the most the important parts of the solution aren't. For example, the first problem is all about getting the first expression (I haven't figured it out yet, but I haven't thought about it all that hard), and the rest is gravy.

To view the whole problem, right click the image and select "view image."
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Re: [PHYSICS] Two questions whose solutions i dont understan

Postby Afif_D » Thu Feb 02, 2012 11:29 am UTC

So how to write the energy of system??
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