Miscellaneous Science Questions

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Re: Common Questions

Postby jmorgan3 » Sun Jan 11, 2009 8:51 pm UTC

Maybe it would help to see why energy is conserved. Every force in the macroscopic universe, whether tension, or elastic force, or anything else, is actually either an electric force or a gravitational force (I am excluding the microscopic forces, the strong and weak nuclear forces, because I am not entirely sure they behave the same way. Any physicists want to chime in?). The "normal force" of the earth pushing up on you is actually the electric force of the earth's electrons pushing against your electrons. Magnetism is merely a relativistic correction for electricity. Gravity is, well, gravity.

Gravity and electrical force are both conservative forces. Mathematically, that means that a vector field of gravitational or electric force is conservative. You can prove this rather easily if you know vector calculus. What this means is that the integral of the force vector dotted with the differential of the displacement vector of an object (in other words, force times displacement, or work) is the same between any two points, regardless of path. This means we can define a scalar-valued "potential function" (say, p(X), where X is a position vector) for the field such that the work done by the object moving from point A to point B is p(B)-p(A), regardless of path.

Now, let's look at the effect that force has on an object. If an object of mass m is moving at v1, then is subject to a force over a displacement, it can be shown mathematically that it will end up moving at some velocity v2 such that [math]\frac{1}{2} mv_2^2 =\frac{1}{2} mv_1^2 - \int_{position 1}^{position 2} force \cdot d(displacement)[/math] You should recognize the integral in that equation as the one I describe in my second paragraph. Per what I wrote in that paragraph, we can rewrite that equation as [math]\frac{1}{2} mv_2^2 =\frac{1}{2} mv_1^2 + p(position 1)-p(position 2)[/math] We rearrange that to get [math]\frac{1}{2} mv_2^2 + p(position 2)=\frac{1}{2} mv_1^2 + p(position 1)[/math] If we set the left hand equal to a constant (basically making position 1 a reference position), we get [imath]\frac{1}{2} mv_2^2 + p(position 2)= c[/imath] for any value of position 2! In other words, the sum of (.5mv2) and the value of the potential function of an object is the same regardless of position. Because it's convenient to give the terms in this equation names, we call .5mv2 kinetic energy and we call the value of the potential function potential energy. We say that total energy, the sum of potential energy and kinetic energy, is conserved.

Up until this point, I was talking about only one force field. Will these same principles apply to the real world where there are innumerable electric and gravitational fields emanating from every subatomic particle? Yes, because it so happens that the sum of two conservative vector fields is conservative, and its potential function is the sum of the two fields' potential functions. Therefore, the net force on an object is always conservative, and energy is always conserved.

At this point, you may want to offer friction or some other "non-conservative" force as a counter-example to my above wall of text. In actuality, friction only appears non-conservative at the (useful) macroscopic level. Force is transferred in friction using electric forces, so the energy is still conserved and converted to heat. Heat is merely the kinetic and potential energy of molecules.
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Re: Common Questions

Postby doogly » Sun Jan 11, 2009 9:01 pm UTC

jmorgan3 wrote:(I am excluding the microscopic forces, the strong and weak nuclear forces, because I am not entirely sure they behave the same way. Any physicists want to chime in?).


You still have conservation of energy, but you need to use field theory, so it is rare to do something like write down a force equation. You need to treat your interaction fields and your matter fields together, so it is tricky, but the conservation rules definitely work. That is a prime time to break out Noether. With her result handy, I could write down a Lagrangian even weirder than the standard model's (which is plenty weird), and if I don't include any time dependent terms, I still get energy conservation.
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Re: Common Questions

Postby j0sh » Mon Jan 12, 2009 8:53 pm UTC

this is a biology/chemistry question
I hate to sound ignorant, but hey lets face it the only way to get rid of our ignorance is by clearing up the matters which we are ignorant about, but recently a friend told me the human body can survive without a stomach.
this friend has a bit of a history of making up random facts Eg: milk feels pain (and whatnot) :?
but this sounds plausible enough if the other digestion organs worked more or something, i really dont know.

Can anyone help me out?

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Re: Common Questions

Postby thoughtfully » Mon Jan 12, 2009 9:10 pm UTC

The small intestine is where the nutrients are absorbed. The colon mainly removes water, and some salts. The stomach just makes the small intestine's job easier. With no stomach, you'll be less efficient, and might require supplements of some nutrients.
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Re: Common Questions

Postby Charlie! » Tue Jan 13, 2009 4:16 am UTC

thoughtfully wrote:The small intestine is where the nutrients are absorbed. The colon mainly removes water, and some salts. The stomach just makes the small intestine's job easier. With no stomach, you'll be less efficient, and might require supplements of some nutrients.

like... protein...

Although I suppose you could get by stomachless if you simply ate special food.
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Re: Common Questions

Postby Carnildo » Tue Jan 13, 2009 8:15 am UTC

A good starting point would be the Wikipedia article on gastric bypass.

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Re: Common Questions

Postby noobius » Tue Jan 13, 2009 5:30 pm UTC

gmalivuk wrote:Well on one level (which is probably unsatisfying), the justification is because that's just what the units themselves are telling you. Units of energy are, in MKS, kg m2/s2, which can be seen as (kg m/s2)m, or force*distance. There doesn't need to be any further justification, because the units and terms are defined that way.


Well, fine, but shouldn't there be a logical sequence of steps that gets you to that point? I've always thought, although this may just have been wishful thinking, that as things happen in nature change occurs according to a series of natural laws and that nature didn't just take all the variables, fiddle with them to get them in the right form, and spit the answer out. Ie, I thought of natural laws as a bunch of machines that all did their jobs in order and that algebraic manipulation may not be a good representation of the mechanisms because order matters in a meaningful explaination of what happened.

Anyways, what originally got me thinking about energy was how for a given energy the average speed of a bunch of particles was inversly proportional to the root of mass. And my understanding is that this quantity 'energy' was eventually more and more evenly dispersed to all the particles. Thinking about it, it seemed like momentum was what should be equalized between two particles when they shot away from each other because they're shoving with equal force, so I equated energy and momentum. Can someone tell me what was wrong with this train of thought?

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Re: Common Questions

Postby gmalivuk » Wed Jan 14, 2009 4:22 pm UTC

It's physically motivated by the fact that energy is a conserved quantity. So is momentum, which is why they both have specific names and formulas. I suppose we could have just as easily called mv energy, and 1/2 mv2 momentum, but then the question would be the same but with different words...

(The natural laws you're talking about are the ones that result in 1/2 1/2 mv2 to be a conserved quantity. The units come from that, and then turn out to be the same as the units of force*distance.)
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Re: Common Questions

Postby jmorgan3 » Thu Jan 15, 2009 7:41 am UTC

gmalivuk wrote:(The natural laws you're talking about are the ones that result in 1/2 1/2 mv2 to be a conserved quantity. The units come from that, and then turn out to be the same as the units of force*distance.)

Just being the same units doesn't fully justify force*distance equaling a change in energy. Work is the dot product of force and distance, which has units of Joules. The magnitude of the cross product of force and distance also has units of Joules, but it is certainly not Work.
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Re: Common Questions

Postby gmalivuk » Sat Jan 17, 2009 7:25 pm UTC

jmorgan3 wrote:
gmalivuk wrote:(The natural laws you're talking about are the ones that result in 1/2 mv2 to be a conserved quantity. The units come from that, and then turn out to be the same as the units of force*distance.)

Just being the same units doesn't fully justify force*distance equaling a change in energy. Work is the dot product of force and distance, which has units of Joules. The magnitude of the cross product of force and distance also has units of Joules, but it is certainly not Work.

Well no, *just* being the same units doesn't fully justify it, sure. But that was already addressed by doogly above, with why it's not mv^4/c2.

The thing is, force (dot) displacement = mass*(acceleration (dot) displacement) = the change in 1/2 mv2, which is the conserved quantity we're interested in.
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Re: Common Questions

Postby jmorgan3 » Sat Jan 17, 2009 7:55 pm UTC

Sorry, I think I read the "turn out to be" in your post as indicating that the unit equivalence was a coincidence. doogly's link does do a very good job of explaining why [imath]F \cdot d =\Delta .5mv^2[/imath]
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Re: Common Questions

Postby Charlie! » Tue Jan 20, 2009 6:51 pm UTC

What about generation numbers? Or the effects of stronger selection on diversity?
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Re: Common Questions

Postby betsapp91 » Fri Jan 23, 2009 3:30 am UTC

so what's on the outside of the universe? is there any way to know?

also, if anyone knows, what was the farthest distance traveled from earth yet?

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Re: Common Questions

Postby oxoiron » Fri Jan 23, 2009 4:04 am UTC

betsapp91 wrote:also, if anyone knows, what was the farthest distance traveled from earth yet?
A little past the moon, but I don't know precisely how far. I'm sure Wikipedia can tell you.
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Re: Common Questions

Postby Sir_Elderberry » Fri Jan 23, 2009 4:13 am UTC

betsapp91 wrote:so what's on the outside of the universe? is there any way to know?


Well, "the universe" is usually defined as "everything that exists". The question doesn't make sense in that context, there isn't any "where" to be if you're not in the universe.

But some have theorized that our "universe" is really just one universe floating about in a sea of other ones. This is for the moment untestable if a pretty cool idea. Here, the word universe means something a little different, but then "atom" originally mean "completely indivisible particle", so that's science for you.
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Re: Common Questions

Postby Mr_Rose » Fri Jan 23, 2009 11:57 am UTC

oxoiron wrote:
betsapp91 wrote:also, if anyone knows, what was the farthest distance traveled from earth yet?
A little past the moon, but I don't know precisely how far. I'm sure Wikipedia can tell you.

That's for humans though; the furthest man-made object is Voyager 1, which is on the very edge of the solar system (106.4AU) and about to hit the interface between the solar wind and the interstellar galactic particle flux. No-one knows precisely what happens next, but I expect it will be like being caught in a semi-tangible electric whirlpool and we will likely lose the probe, which would be a great shame.
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Re: Common Questions

Postby oxoiron » Fri Jan 23, 2009 3:21 pm UTC

Mr_Rose wrote:That's for humans though
You are correct, Sir! I foolishly assumed he meant 'people' when he was talking about travel.
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Re: Common Questions

Postby hideki101 » Thu Jan 29, 2009 12:05 am UTC

Question:( for labwork)

I have a voltage of 546V across two plates spaced ~4mm apart. I know the equation to find the field here is E=(deltaV)/(distance between the plates).

The question is for the V: do I just use the V given, or do I have to double it (DeltaV between +/- charged plates)?
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Re: Common Questions

Postby Sir_Elderberry » Thu Jan 29, 2009 12:39 am UTC

hideki101 wrote:Question:( for labwork)

I have a voltage of 546V across two plates spaced ~4mm apart. I know the equation to find the field here is E=(deltaV)/(distance between the plates).

The question is for the V: do I just use the V given, or do I have to double it (DeltaV between +/- charged plates)?


Er, the voltage given, generally. Since voltage is relative, when we say that there is a voltage of V "across" something or "between" something, usually we mean that the change in potential between the two is that voltage.
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Re: Common Questions

Postby Sir_Elderberry » Thu Jan 29, 2009 2:41 pm UTC

Yeah, but you can pick any point you wants and say "this? this right here? This is 0 volts."
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Re: Common Questions

Postby Sir_Elderberry » Thu Jan 29, 2009 9:21 pm UTC

Earth has a voltage--I think the atmosphere has a slight positive voltage compared to the ground, actually, but I may have that backwards. Everything has a voltage, but the point I was making was that where you put 0V is completely arbitrary. There is no physical difference between calling one wire +120V and the other one 0, and calling them 0 and -120V.
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Re: Common Questions

Postby hideki101 » Fri Jan 30, 2009 1:13 am UTC

I got it, thanks you two. Oh, and Sir Elderberry? Happy ruing day! :D
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Re: Common Questions

Postby Sir_Elderberry » Fri Jan 30, 2009 1:14 am UTC

hideki101 wrote:I got it, thanks you two. Oh, and Sir Elderberry? Happy ruing day! :D


I have the flu. Second Talon's arm is long, indeed, to sway even viruses.
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Re: Common Questions

Postby hideki101 » Fri Jan 30, 2009 1:24 am UTC

Sir_Elderberry wrote:
hideki101 wrote:I got it, thanks you two. Oh, and Sir Elderberry? Happy ruing day! :D


I have the flu. Second Talon's arm is long, indeed, to sway even viruses.

Ouch. Well, Hope you feel better soon.
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Re: Common Questions

Postby cowsofdoom23 » Mon Feb 02, 2009 12:00 am UTC

Someone please help. I'm in physics and I need some help with this homework question. It's really basic, but I need help. I don't know if I keep doing the math wrong or what, but please, SOMEONE HELP!

In introductory physics laboratories, a typical
Cavendish balance for measuring the gravita-
tional constant G uses lead spheres of masses
2.01 kg and 16.1 g whose centers are separated
by 3.25 cm.
Calculate the gravitational force between
these spheres, treating each as a point mass
located at the center of the sphere. The
value of the universal gravitational constant is
6.67259 × 10−11 N · m2/kg2. Answer in units
of N.

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Re: Common Questions

Postby cowsofdoom23 » Mon Feb 02, 2009 4:10 am UTC

F = G(m1m2)/ r2. Reads Force is equal to gravitational constant x (mass 1 x mass 2) divided by radius square, but you probably knew that. Thanks for helping, I need to pull up my grade.

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Re: Common Questions

Postby PhantomPhanatic » Tue Feb 03, 2009 5:00 am UTC

I was under the impression that LIGO has not yet detected gravity waves. What study does the OP stand by that determines gravity to travel at c?
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Re: Common Questions

Postby PhantomPhanatic » Tue Feb 03, 2009 5:23 am UTC

Meteorswarm wrote:
PhantomPhanatic wrote:I was under the impression that LIGO has not yet detected gravity waves. What study does the OP stand by that determines gravity to travel at c?


There was a thing, with Jupiter that measured the speed of gravity to (1±.3)c, I just can't remember exactly. I'll dig around tomorrow for you if you like.


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Re: Common Questions

Postby crisp » Wed Feb 04, 2009 8:20 pm UTC

I was wondering if anyone can explain the mechanics of the inflationary period of the universe when matter was expanding at a speed greater than c.

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Re: Common Questions

Postby SWGlassPit » Wed Feb 04, 2009 8:25 pm UTC

Someone can probably explain it better than I, but c is the limit of the velocity with which matter may travel through space. At the initial inflationary period, space itself was expanding faster than c, which gets around this limit idea.
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Re: Common Questions

Postby Sir_Elderberry » Thu Feb 05, 2009 12:11 am UTC

crisp wrote:I was wondering if anyone can explain the mechanics of the inflationary period of the universe when matter was expanding at a speed greater than c.


But of course! First of all:

SWGlassPit wrote:Someone can probably explain it better than I, but c is the limit of the velocity with which matter may travel through space. At the initial inflationary period, space itself was expanding faster than c, which gets around this limit idea.


Let's talk about why this is true. This is true, essentially, because the laws of electromagnetism dictate how fast an EM wave (ie, light) should travel, in terms of the properties of the vacuum, called permeability and permittivity. Now, we couple this with the fact that all viewpoints are equal--the laws of physics do not change if you are moving, no matter how fast or slow you're moving. In fact, you can define your velocity to be whatever you want if you pick the right reference point. Who's to say, after all, that the rest of the universe isn't going left, instead of you going right? There's no experiment you could do to distinguish the two. (You've heard of time dilation and similar effects, however, these do not violate this principle.) So, if I'm an observer, and I'm cruising along, let's say I decide to measure the speed of light.

Now, here's a tricky situation. If I measure it to be anything but c, that means empty space would have to have different properties to me because I was moving--which doesn't make sense, as it would imply some kind of universal reference point for movement. If this existed, all our earthly physics would be null and void in some sense, as we're constantly moving. Since we can't distinguish between two reference frames, we have to assume that all observers would measure the same speed of light--c, 3 x 10^8 m/s.

So, if you follow this to its logical conclusion, you find that kinetic energy doesn't quite work as we usually understand it--anything with mass has energy that asymptotically approaches infinity as velocity approaches c, anything without mass has to travel at c.

But space is different. It has no mass. It's not an object. Space does not have kinetic energy. It's not galaxies retreating from each other, it's space itself expanding. The usual analogy is quarters taped to a balloon or seeds ina piece of baking bread--space gets bigger. The quarters themselves stay put, roughly.

This is probably longer than it needed to be, but I enjoy explaining relativity.
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Re: Common Questions

Postby PhantomPhanatic » Fri Feb 06, 2009 6:12 pm UTC

Meteorswarm wrote:As to the speed of gravity, see the "speed of gravity" thread.

http://forums.xkcd.com/viewtopic.php?f=18&t=31800


Thanks. Didn't realize there was a whole thread on the discussion.
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Re: Common Questions

Postby TheCitadel » Wed Feb 11, 2009 6:07 pm UTC

Ok, this is not a common question. I don't know what it is, or in which thread it should be, but here goes: does anyone know what programs/software/computerthings are frequently used when you're studying biology? I've used Mathematica and MatLab but surely there are more specific tools for Biology, and since I have a long summer before me I would like to know some of them so I can begin to learn how to use them.
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Re: Common Questions

Postby Hatter » Wed Feb 11, 2009 6:54 pm UTC

Sir_Elderberry wrote:Not only does matter not travel faster than light, neither does information. No matter how hard you spin those entangled photons.

If I understand this as you intended it, I think your wrong. Imagine a solid plastic tube, filled with marbles, If I push a marble in one ed, one falls out the other almost instantly. Although all the individual marbles move very slowly the speed at which it takes for one to come out the other end is almost instant. Am I right?

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Re: Common Questions

Postby thoughtfully » Wed Feb 11, 2009 6:59 pm UTC

Hatter wrote:
Sir_Elderberry wrote:Not only does matter not travel faster than light, neither does information. No matter how hard you spin those entangled photons.

If I understand this as you intended it, I think your wrong. Imagine a solid plastic tube, filled with marbles, If I push a marble in one ed, one falls out the other almost instantly. Although all the individual marbles move very slowly the speed at which it takes for one to come out the other end is almost instant. Am I right?

No. The force applied to the marble is transmitted through the marble at the speed of sound, and similarly with the following marbles, if there is no gap between any.
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Re: Common Questions

Postby Sir_Elderberry » Thu Feb 12, 2009 1:16 am UTC

Hatter wrote:
Sir_Elderberry wrote:Not only does matter not travel faster than light, neither does information. No matter how hard you spin those entangled photons.

If I understand this as you intended it, I think your wrong. Imagine a solid plastic tube, filled with marbles, If I push a marble in one ed, one falls out the other almost instantly. Although all the individual marbles move very slowly the speed at which it takes for one to come out the other end is almost instant. Am I right?


No, for reasons explained above. Really, really long levers don't work either. Nothing is perfectly rigid.
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Re: Common Questions

Postby Carnildo » Thu Feb 12, 2009 5:28 am UTC

thoughtfully wrote:
Hatter wrote:
Sir_Elderberry wrote:Not only does matter not travel faster than light, neither does information. No matter how hard you spin those entangled photons.

If I understand this as you intended it, I think your wrong. Imagine a solid plastic tube, filled with marbles, If I push a marble in one ed, one falls out the other almost instantly. Although all the individual marbles move very slowly the speed at which it takes for one to come out the other end is almost instant. Am I right?

No. The force applied to the marble is transmitted through the marble at the speed of sound, and similarly with the following marbles, if there is no gap between any.

As a specific example, if you were to run your marble-filled tube from here to Mars at closest approach, and shoved a marble in the Earth end, one would pop out the Mars end about six months later.

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TheCitadel
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Re: Common Questions

Postby TheCitadel » Thu Feb 12, 2009 4:08 pm UTC

Meteorswarm wrote:
TheCitadel wrote:Ok, this is not a common question. I don't know what it is, or in which thread it should be, but here goes: does anyone know what programs/software/computerthings are frequently used when you're studying biology? I've used Mathematica and MatLab but surely there are more specific tools for Biology, and since I have a long summer before me I would like to know some of them so I can begin to learn how to use them.
I know it sounds like I haven't got a clue. It's true. :)


For statistical analysis, you will likely use something like SAS or R. I assume, rather presumptively, that you're entering college as a freshman soon. If this is the case, then don't worry about learning those kinds of tools. Programming languages are nice, but they're really not difficult to pick up. You will get more use out of reading books about the topics you're interested in.


Fully noted. And, yes, you're right, it is college, only in my country, college means you have subjects related ONLY to the career you've chosen (no literature, languages, etc- only subjects that have to do specifically with Biology), and this appears not to be the case in most of the other countries, or at least that's what my friends abroad say.
Anyway, thanks for the advice.
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Charlie!
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Re: Common Questions

Postby Charlie! » Wed Feb 25, 2009 8:22 am UTC

Carnildo wrote:
thoughtfully wrote:
Hatter wrote:
Sir_Elderberry wrote:Not only does matter not travel faster than light, neither does information. No matter how hard you spin those entangled photons.

If I understand this as you intended it, I think your wrong. Imagine a solid plastic tube, filled with marbles, If I push a marble in one ed, one falls out the other almost instantly. Although all the individual marbles move very slowly the speed at which it takes for one to come out the other end is almost instant. Am I right?

No. The force applied to the marble is transmitted through the marble at the speed of sound, and similarly with the following marbles, if there is no gap between any.

As a specific example, if you were to run your marble-filled tube from here to Mars at closest approach, and shoved a marble in the Earth end, one would pop out the Mars end about six months later.

Hrm. Assuming the glass transmits sound fairly slowly (for glass) it still sends it a good 6 times faster than in air. It works out to about 112 ish days for not-very-stiff glass (or only 37 ish days for stiff glass). :P

Oh, nitpicking, where would I be without you? (sorry)
Some people tell me I laugh too much. To them I say, "ha ha ha!"

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superglucose
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Re: Common Questions

Postby superglucose » Thu Feb 26, 2009 7:01 pm UTC

What, exactly, is a capacitor and how is it useful in a circuit? This is the second time a class has talked about capacitence and how they affect a circuit, but I still don't know why we even need them for anything. If anyone could explain this with moderately simple english I would be much obliged.
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