Function

O is the cross sectional area, not the surface area.

Well, yes.. Okay... Forgive me my linguistic ignorance.

Hi everyone

From my physics course in medicine:

[math]U=\int_{0}^{\Delta l}{Fdl}=\int_0^{\epsilon}{\sigma Old\varepsilon}=\cdots[/math]

With [math]F[/math] force applied on a beam, [math]l[/math] the length of the beam, [math]\Delta l[/math] the difference in length after appliance of the pulling force [math]F[/math] on the beam, [math]O[/math] the surface of the beam, [math]\sigma[/math] the stress [math]\frac{F}{O}[/math] on the beam and [math]\varepsilon[/math] the strain on the beam ([math]\frac{\Delta l}{l}[/math])

Can't seem to figure out why this is... Can someone help me?

Thanks.

F.

Hello everyone

Just asking myself this question over and over again:

Is every color we see the negative one from what it actually is?

Light, colours as we see it, is reflected light. If white light falls on an object, which in its turn sends out light in that colour that we accept is the colour of the object, then it must absorb the rest of the white light - the negative of the reflected colourful light.

E.g.

I have in front of me a computer mouse which appears black to me (let's assume it's pure black - not very realistic, but hey, we're reasoning theoretically here). Black (or 'no light') is being reflected from it. No light is reflected, so all light must be absorbed, making this mouse in fact white.

Is this actually the case, and are photo negatives the true image of our world as we don't know her?

Thanks. Thread open for discussion

-F.

P.S. Thread open for discussion, even discussions which will lead us to a deeper state of thinking (e.g. how do we know that we see colours all the same, that the colours that truly are, are not as they are seen by us (so actually another problem than the original problem stated here))

We've been focussing on bell-shaped metallic objects since this thread was started.

Update: do you know those metallic rings to make like 'rounds' of e.g. mashed potatoes or whatever? They show us the same phenomenon: if you hold your ear outside the ring, you hear a clear sound; if you hold your ear inside this ring (which is btw open on both sides), you hear almost nothing.

I guess we can expand the subject, no longer being obliged to stick to bells.

 

It is also the slope of the X-Y trajectory at t=0, that is it is

 

[math]{\left. {\frac{{dy}}{{dx}}} \right|_{t = 0}}[/math]

 

 

The point I wanted to establish was that this slope changes throughout the trajectory.

 

function was there anything else you wish to discuss about this subject?

 

You can derive further information from your formula if you wish.

I guess not, atm. It was the consequence of a strange equation I got by a wrong derivation... Thanks.

After new derivation, I get:

[math]t=\frac{x}{v_0\cdot\cos{\theta}}[/math]

Well... That's pretty useless.. (Units match up, though )

Errors fixed.

Oh God how did I oversee that... But what's [math]\dfrac{d\theta}{dt}[/math]? Is it [math]\omega[/math]?

And following this reasoning, [math]v_0[/math] should also be derived?

Edit: reread your reaction; theta is the initial angle; should it be derived?

Edit (bis): numerous mistakes in deriving. Shall review.

Hi guys

Everyone knows (or should know,) this pretty important formula:

[math]y=y_0+x\cdot\tan{\theta}-\frac{g\cdot x^2}{2\cdot v_0^2\cdot\cos^2{\theta}}[/math]

Now, I don't know if I'm physically allowed to do this, but I derived that, keeping in mind that every unit, independent of time, is constant:

[math]\frac{dy}{dt}=\tan{\theta}\cdot \frac{dx}{dt}-\frac{2\cdot g}{2\cdot v_0^2\cdot\cos^2{\theta}}\cdot\frac{dx}{dt}[/math]

[math]v_y=v_x\cdot\left(\tan{\theta}-\frac{g}{v_0^2\cdot\cos^2{\theta}}\right)[/math]

Now,

[math]v_{y,0}=v_0\cdot\sin{\theta}[/math]

and

[math]v_{x,0}=v_0\cdot\cos{\theta}[/math]

Am I thus allowed (?) to say that, using the formulas of kinematics:

[math]v_y=v_{y,0}-g\cdot t = v_0\cdot\sin{\theta}-g\cdot t[/math]

[math]v_x=v_{x,0}=v_0\cdot\cos{\theta}[/math]

Resulting in:

[math]v_0\cdot\sin{\theta}-g\cdot t = v_0\cdot\cos{\theta}\cdot\left(\tan{\theta}-\frac{g}{v_0^2\cdot\cos^2{\theta}}\right)[/math]

[math]v_0\cdot\sin{\theta}-g\cdot t = v_0\cdot\sin{\theta}-\frac{g}{v_0\cdot\cos{\theta}}[/math]

[math]t=\frac{1}{v_0 \cdot \cos{\theta}}[/math]

Now, if everything I did was 'legal' and correct, what [math]t[/math] is this? Is it the time the projectile needs to complete its trajectory? Or is just about everything I did wrong?

Thanks!

Function

 

The force is going to be proportional to how quickly you make yourself accelerate away from the wall.

Isn't accelerating away from something somewhat.. "nonintuitive" in space? What is there to move in?

I think I fail to explain you that we are - in our example - in space, pressing against a wall, while floating around in the vacuum. I am convinced that a body (organism) with mass m can apply a maximal force against this wall, so can someone either approve this or convince me of my physical failure?

At first glance I think all of these records involve people exerting forces greater (sometimes a lot greater) than their own weight.

http://en.wikipedia.org/wiki/List_of_Olympic_records_in_weightlifting

Which includes a supporting surface and a gravitational force etc., which is why I put the example in outer space.

Pressure is different from force. Stiletto heels can do serious damage to a wooden floor because the weight (force) is concentrated in a small area.

Let's then from now on focus on one of those two, since they are related. First one to say "force" or "pressure" decides.

Hi everyone

Didn't really know where to put this; physics or biology...

Let's imagine an organism (human) with a mass of 80 kg.

My primary question: can an organism with mass m deliver a pressure greater than its mass?

I'm sorry, but I really can't explain my question better and since pressure is dependent on force and area, I don't really know if this question is even correct.

So let's define a 'pressure' expressed in kg. Can the man deliver a pressure greater than 80 kg?

At first sight: yes. Imagine him standing underneath a door opening and pushing with his arms again the door post, thus the force on the ground (his supporting surface) is (much) greater than his mass.

But now imagine him in space, near a big wall; Let's assume the wall is immobile. Can he push against the wall with a 'pressure' greater than 80 kg?

I'm sorry that I can't really explain the problem here. I hate that pressure is defined as the force per area, for since the area gets infinitesimale small, the pressure gets infinitely large, making this problem somewhat vague.

I could also try to explain this with force, so the force on the ground would be much larger than m*g when pressing against the door post, but since there is no g in space (let's just assume this), he can't deliver a force? Very confusing.

Thanks.

F.

I seriously doubt the cup sitting on the paper forms an air-tight seal.

Then the oxygen entering the cup should've been much less than the gas the mosquito exhales per time unit: it suffocated.

I found what the structures I was questioning in my original post are. They're "halteres" and used for flying.. I guess... added a Wiki link fyi

(Edited)

Found what the structures are, they're called "halteres"

http://en.wikipedia.org/wiki/Halteres

Hi everyone

I'd like to say "no animals have been harmed" in this 'experiment', which wasn't really an experiment, just a solution to a very annoying problem, but since mosquito's aren't animals, but monsters, I can't promise anything.

So I had been harrassed by this most annoying mosquito last night, and after half an hour looking for it, I found it sitting in some shade (yes, I had all my lights turned on; that's when their cloaking device activates --' )

And thus I found it, and since I had been successful on capturing insects with cups and paper, I decided to do this with this monster too.

First attempt didn't work, but in the second attempt, it flew right into the cup, I put it against my window, put a paper on it and transported the monster very carefully (so there wouldn't be an opening between the paper and the cup) to my desk.

My desk... where it stood for over 9 hours.

Don't see me as a sadist. See me as a saviour. For the religious ones here: see me as an eraser, which has the task to erase the Lord's mistakes.

After all, it's the mosquito who's the sadist.

I know there're quicker ways to deal with it, but I handle by this most famous Latin saying "Quid pro quo", in casu: those who make you suffer.. well.. let's not complete that sentence.

Now here's my point: this beast is still moving. I found it this morning still flying in the cup. WHAT?

9 hours of suffocation aren't enough to get rid of it?

It most certainly wasn't an after-death-spasm. When I shook the cup with the paper, however, I found it sitting on the paper, only moving its head and legs. After half an hour, that's when the spasms came in.

Don't know what it's doing right now... Won't be flying anyway...

Now, how can this beast be still alive? Or still show spasms?

The cup has a volume of around 396,14 cc and I'm sure that a mosquito needs air to survive, so perhaps the cup held just about enough air for the mosquito to breath for around 9 hours?

Once again, I'm not a sadist just a savior. For the sake of science.

No comments on my potential inhumanity will be treaten (by me). Only comments on the final question.

Thanks.

F.

Edit: another question:

Couldn't resist putting it under my microscope.

about at the end of its 'tail' and just before its abdomen, with wings, it has both left and right a very interesting structure. It looks like a very small stone attached with.. well.. it looks like grey muscles, but don't shoot me for being wrong. Does someone know what these structures are? The things attaching these two very small 'stones' have about the same structure as the thing that attaches the wings to the abdomen; but much smaller.

Congratulations, Function! Now the hard work starts.

Thanks!

Only 12 years?

 

You will still be a young man when you finish then.

 

Depends on what you're doing:

3 years to get Bachelor of Science in Medicine

3 years to get Master of Medicine in Medicine

(Sounds stupid, but in Dutch: Master of Medicine in de geneeskunde)

In those last 3 years, you decide whether you're going to be a family doctor, a specialized doctor (hospital doctor), youth doctor, company doctor, ...

After those 6 years, you get to call yourself a doctor... or physician... or whatever term you use for it (but not "doctor" as in: hey I got a PhD)

Then you specialize yourself

If you 'really' specialize yourself, you can pick between a large scala of specializations. Those specializations are rather 4, 5 or 6 years of paid internship and some study, but very few theory.

If you pick family medicine, you get to specialize 2 years

And so on

My big dream: neurosurgery

That's 6 years, so a total of 12 years

Hope it's clear

Extra info:

In one way, the entrance exam is good: study success is varying from 85-93% for med students, highest of all studies.

On the other hand, we have too few doctors. Our government has tried to get rid of that exam, but the universities don't want that

(Can you believe there are so few family doctors that starting ones now get a launching aid (or up aid, whatever you call it) of € 20,000!?)

Sorry, poor lexicon. Thanks for teaching me new vocabulary.

I've got only one rule: don't apologize for ignorance

Congratulations. Is Flemish University in Belgium?

Thanks!

I meant "any Flemish university"

Flemish University doesn't exist

I meant: a university in Flanders, which happens to be in Belgium

I'll go to Ghent University, a university in Flanders.. in Belgium

Hi guys

Those who know me a bit, also know that tuesday (previous week), I had to take my entrance exam in order to be allowed to follow medicine in a Flemish university.

Today, I got the result.

Knowledge and Insight in Sciences (KIS) (10 questions of each science) (a correction for guessing is implied; also, some of the questions were deleted because they wouldn't have been 100% correct; otherwise my result would've been a bit better):

-Mathematics: 2,04/5

-Chemistry: 2,41/5

-Biology: 2,71/5

-Physics: 4,26/5

-----------------------

TOTAL: 11,4/20

Acquiring and Processing Information (API) (exists out of 30 questions on communication between physician/patient, about empathy etc., 30 questions about 6 texts of 1 page where you can keep your texts with you (part 1) and 30 questions about the texts, without the texts (part 2)):

-Communication: 4,57/6

-Reading part 1: 5,44/7

-Reading part 2: 4,23/7

------------------------

TOTAL: 14,3/20

------------------------

GRAND TOTAL: 25,7/40

------------------------

The prequisites to pass are: 10/20 for KIS, 10/20 for API and 22/40 in total.

Conclusion: I PASSED!

Guess I'll be spending the next 12 years of my life on medicine

Now, this message is just to thank everyone once again for their support and help (and patience ) with all my questions.

Ta-ta.

Function

Correct. What is really interesting is that give someone "pick a number between 1 and 10" and there is a much larger than 10% chance they will say 7.

 

But, this is much more a question of psychology and how the human mind works more than a mathematical or statistics problem.

Indeed. Suggestion plays a big role here: say a lot of sentences (in a subtle way, of course), with the sound "eight" in it, and the person will most likely say 8: wait, late, gate, ...

That's why I never let this game decide my fate people may have tricked my mind.

 

Can't agree with this, they are not the same, even in scientific English.

 

Function, please look at my edit to post#4.

 

To further continue, random ( in the statistical sense) implies that the probabilities are the same for all outcomes. It the probabilities are not the same it implies that there is some preference or selection or other driving agent involved.

Ah, now that's what this example is all about. Selection and preference. Thanks. +1

It's like: "pick a random number from 1 to 10". There's an infinitesimale chance of picking the number 2, and the same chance for pi or e. Yet, the probability, in my opinion, of picking 2, is much larger than picking pi or e.

The problem was that on a Dutch forum, everyone said that "kans" (chance) and "waarschijnlijkheid" (probability) are both the same. No matter what.

Damn you go to a good school if you learn this at high school.

Well, just Belgian high school

By 'psychological chance' do you mean your personal degree of belief that it will land on a specific point?

 

If so we are getting onto Bayesian probabilities (not my forte), which it sounds like you know about. But Bayesian probability is just as rigorous as traditional probability, they come from the same axioms, so the answer is the same. I've not read around the subject but my lecturer made a distinction between 'subjective' Bayesian stats and 'empirical' Bayesian stats. The former involves probabilities plucked from thin air, while the latter tries to justify why a particular probability was picked. So I would say it is still mathematics, but the end of mathematics that tries to meet with the real world. Physicists probably do this best - maybe one would like to comment.

I mean the chance that the bee will be 'convinced' of landing on the honeyed surface

Well yes, we dealed with this stuff in high school, but I really do mean, that the "psychological chance" of the bee landing in a honeyed place, is larger than that of it landing on a non-honeyed place, even if the honeyed place is much smaller than the not-honeyed place?

Because it does matter for this bee to land on a specific type of surface (food or nothing), but then I'm afraid we aren't in mathematics any longer?