# Learn something new everyday

Okay, I just learned how to check which post was the nth post here so just out of curiosity I punched in my birthday. 02/06/69
https://www.physicsforums.com/showpost.php?p=020669
Artman said:
There's a mentally deranged truckdriver driving along a remote country road. Whenever he sees a pedestrian near the road he enjoys swerving over and hitting them. He sees a preaher whose car broke down trying to flag down a ride, so he stops and picks him up.

The two drive along for awhile and then the truck driver sees a pedestrian. He thinks, how can I swerve over there without this preacher knowing what I'm doing? So he decides to pretend he is falling asleep. He does this and it works except he can't see if he got the guy or not. So he "wakes" up and says, "Oh my gosh, I must have dosed off, did I hit that guy walking back there?"

The preacher says "No my son. You missed him."

A little while later he sees another one and tries it again. "I was dosing again, did I hit that guy back there?"

"No my son. You missed him."

A little while ater he tries the same thing again. "Oh no, I must have dosed again. Did I hit the guy walking back there?"

"No my son. But you got close enough that time so I could get him with the door."
What's yours?

Math Is Hard
Staff Emeritus
Gold Member
Fascinating.
https://www.physicsforums.com/showpost.php?p=040167
Phobos said:
The Milky Way is about 100,000 ly in diameter, but I think the 3000 ly figure refers to the thickness at our location (28,000 ly from the center of the galaxy). The thickness of the galaxy is somewhat hard to pin down as it depends on what you use to define the upper/lower edges (stars? molecular clouds? dark matter?)
hmmmm..
Do you suppose it reveals something about my inner nature or destiny? Perhaps you have discovered the PF oracle?

I DO have an obsession with going to Joshua Tree to view the Milky Way before I die.

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Moonbear
Staff Emeritus
Gold Member
:rofl: This is turning out pretty funny. tribdog's post fits him too well! Here's mine, which is not nearly so fun.

krab: I only noticed yor post after I'd posted. I derived the equations as I was writing out my post, I thought that they would probably simplify, but I admit I was too lazy to do it.

The r exponent was a typo, I had to edit the post about four times to get rid of all the typing errors in the latex code, though the inline text worked just fine and it looks better than I thought it would.
Though, considering I'm completely inept with latex, maybe this is fitting afterall. :rofl:

Some technical information would be useful. Unlike you, I haven't learned how to do that yet.

loseyourname
Staff Emeritus
Gold Member
Just click on his link, and change the numbers at the end to reflect your birthdate rather than his.

Mine sucks. It's a one-line crackpot post by a guy that's been banned.

loseyourname
Staff Emeritus
Gold Member
Interestingly, it's not possible to go lower than the number 100, so no way to call up the first post. The earliest I can find is Kerrie asking if, after the last server changeover, we'd still have the same web address.

https://www.physicsforums.com/showpost.php?p=122386

phatmonky said:
There's an inferred personality trait in someone who sarcastically refers to a special forces member as "super brave" doing "ninja stuff", that allows me to say such a thing

You can say there was little risk, after the fact, but they were in baghdad, in hospital that was still guarded with armed soldiers. Everyone loves to play down these guys because of the rest of the story, but simply being in baghdad at this point was a dangerous thing.
The worst horoscope ever :-(.

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Sweet, It depends which way you do it
16/08/89 or 08/16/87

I don't understand either of mine. They both suck.

Hah, nice way to find people's ages.
Mine is a crackpot claiming to have a proof that god doesn't exist, but doesn't share it with others because apparently it's too complex.
Yeah, same old sh*t, I bet many others will get the same.

Now you will understand better that I am rejecting every uncertainty
approach. Uncertainty was created because there was a human
measuring problem which Heisenberg tackled. But when people started
to project his measuring solution for the explanation of the
cosmological events or to explain the fundaments of consciousness
systems ... it's too much. Did "uncertainty" became a CULT?

Sweet, It depends which way you do it
16/08/89 or 08/16/87

I don't understand either of mine. They both suck.
Wow you were born exactly two years before you were born

G01
Homework Helper
Gold Member
Mine kinda suck, one is someone discussing the photoelectric effect, the other is someone who said the bush would be drafting people into service right now.

Someone should turn this into a game. We could find random posts by inputing random numbers and then vote on which ones are the funniest. Kind of like the photo contest.

cristo
Staff Emeritus
Mine's a crank theory too, in the archived theory development forum. Fortunately, that forum was abolished long before I joined PF!

Homework Helper
https://www.physicsforums.com/showpost.php?p=041384

Or, in other words ...
You can do all the world transformation without matrices, it is only that matrices is a convinient way to save points, data, and transformations.
(i learned that while trying to learn Direct3D)
No kidding ... :tongue:

Evo
Mentor
Mine is just someone thanking someone for a link and doesn't even mention what it's for.

https://www.physicsforums.com/showpost.php?p=072274

HallsofIvy said:
a cannon fires a projectile (no air or wind) at 100 m/s at a 20 degrees angle north of west. it is fired down a "hill" (which is impossibly straight) the hill has a 20 degree angle north of east. find d (see picture.
You do understand, don't you, that we CAN'T "see picture" because you didn't post it! I assume that "d" is the distance from the point at which the projectile is fired to where it hits the ground. The real point of the problem is to find where it hit.

You say "at 20 degrees angle north of west". That sounds like you mean it is aimed 20 degrees to the north of due west but if that is true, we don't know the upward angle at which the projectile is fired.

I'm going to take it that the cannon is aimed "north-west" (that is "45 degrees north of west", and that it is pointed 20 degrees above the horizontal. Also that the hill slopes downward in the same direction as the cannon is aimed (which makes the problem MUCH easier) with a 20 degree downward slope.

Here's how I would do the problem: We can ignore the direction. Since the cannon is fired in the same direction as the hill slopes, just do it as a standard two dimensional problem- take (0,0) as the position of the cannon and the x axis in the direction the cannon is fired. First ignore the hill. Use the standard (parabolic) formula for a projectile launched at angle [theta] with initial speed v0:
x= v0 cos[theta] t, y= -(g/2) t2+ v0 sin[theta] t.
You can easily solve solve for t in terms of x and replace it in the formula for y to get y as a function of x. In a simple, "standard" problem, the projectile would hit where y= 0 so you would solve y= 0 for x.

Here, the ground slopes. Knowing that the ground slopes downward in the x direction, find the equation of the straight line:
it is y= (tan[theta])x where [theta] is the angle the line makes with the x axis (in this problem it is -20 degrees).

The projectile "hits" the ground when it crosses that line.
Solve the two equations y= f(x) for the projectiles motion and
y= mx for the ground simultaneously to find the point at which the projectile hits the ground. Once you know the coordinates of that point, you can find d, the distance between (0,0) and that point.
or

https://www.physicsforums.com/showpost.php?p=220774
Someone now says that the penguins can fly because they will have developed to a level that their wings are larger than now, they have many feathers, and so, flying is possible...

Would you please tell me if that guy is correct or just crazie ?

Thank you,

I think I got the post that was meant for dduardo or penguino.

Moonbear
Staff Emeritus
Gold Member
Wow you were born exactly two years before you were born
:rofl: Glad someone caught that! :rofl:

Mine was hilarious. Best joke I've heard in quite some time and yet really gets to the heart of the human condition.

BobG
Homework Helper
Mine was hilarious. Best joke I've heard in quite some time and yet really gets to the heart of the human condition.
Well, tell us the joke, dammit!

Mine is pretty cryptic. I'm not sure I completely understand it:

Just my luck. Mine was probably a post by someone like Saint - someone who not only was banned, but had every post they ever made deleted from the system.

Or, in a different format:

We are covering this in class next week. So stay tuned. :D
Well, well!!?? That was years ago and we've stayed tuned all this time and there's still no follow-up!!! Did you get hit by a bus??!!

Let me try, yet again:

So Happy!

Thank You, Tom!

You know i looked at that section numerous times but after taking a days break and reading what you wrote it was like a lightbulb lit up in my head. I cant believe I didnt see it earlier always the simple ones that seem to give me the most trouble.
Meh, at least it's upbeat and positive.

I'll try, yet again:
Help! I'm posting this from my Blackberry in the middle of the Rose Bowl. Liu Ailing's PK in shots from the mark was saved, but Scurry came off her line early. My question is:

There's 90,185 fans in the stands, each holding a 1 meter square mirror. It's 3:10 PM. If they all simultaneously reflect the sunlight at my head, how long can I stand in one place without my brain boiling in my own blood?

Dang! Now that's a dilemma! I'd gladly make up a date for my birthday and a post to go with it for a horoscope like that!

Astronuc
Staff Emeritus
BobG said:
Mine is pretty cryptic. I'm not sure I completely understand it:
Make sure the url has '/showthread.php?p=######' and not '/showthread.php?t=######' after the www . physicsforums . com

loseyourname
Staff Emeritus
Gold Member
Hmm, when I go with the other date format, I get a much better one:

quasar987 said:
Another one of the physics teacher working at the college I attended (see my previous post) wrote a book on basic http://universite.deboeck.com/Resources/Titles/28011100266200/Images/28011100266200L.gif [Broken]. In this book it says that the expansion of the Universe is due to the expansion of space itself and that this expansion of space is entirely predicted by Einstein's theory of GR. Now, if this is so, where does the DARK ENERGY (suposedly a MYSTICAL force of repulsion between matter) fit it ?!
The term Dark Energy is given to that matter which is causing the universe to expand at an accelerating rate. This is what some call "anti-gravity" since this is clearly gravity acting in a repulsive manner.

Back in Einstein's day nobody knew of any kind of matter which could produce such an effect. since Einstein assumed that the universe was static he added a term to his field equations to allow for this repulsive effect. Einstein's equations changed from

$$G^{\alpha\beta} = -\frac{8\pi G}{c^4}T^{\alpha\beta}$$

to

$$G^{\alpha\beta} + \Lambda g^{\alpha\beta} = -\frac{8\pi G}{c^4}T^{\alpha\beta}$$

$\Lambda$ is called the cosmological constant. In modern terms the cosmological constant is also called "Dark Energy." This is the term which, for normal matter, allows for anti-gravity when $\Lambda$ > 0. However this was before the concept of negative pressure was around. It is now conceivable that pressure terms in the $T^{\alpha\beta}$ can be negative. This would also give rise to resulsive gravitational effects. For weak gravitational fields Einstein's equations, for $\Lambda = 0$, can be expressed as

$$\nabla^2 \Phi = 4\pi G(\rho + 3p/c^2)$$

Let $\rho_A = 4\pi G(\rho + 3p/c^2)$ = effective active gravitational mass. If the pressure is negative enough then $\rho_A$ < 0. Therefore when there is a large negative pressure the matter acts like a negative active gravitational mass (defined as that which generates a gravitational field).

Pete

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