The Riddles Thread

[classical_hero]

What is the difference between a pickpocket and a peeping tom?

J

Nothing, they're both criminals.

 

[Perfection]

No gravity anywhere inside the Sphere? It seems you don't really know what you are talking about...
There is gravity inside Earth. This is very easy to prove.

Go in a deep mine. Jump. Are you floating in the air, or getting back down to the ground?

It cancels when you're in a hollow sphere. Earth is not hollow. In the mine scenario (given ideal spherical earth) we divide the Earth into two parts, the part above and the part below. The part above gets treated like a hollow sphere and gets canceled while part below follows inverse square law and describes the gravity one experiences.

 

[hobbsyoyo]

Unfortunately it would pop

Low pressure mylar balloons won't pop. The spacecraft I am designing for a NASA mission depends on it; also, several companies are developing inflatable antennas.

It cancels when you're in a hollow sphere. Earth is not hollow. In the mine scenario (given ideal spherical earth) we divide the Earth into two parts, the part above and the part below. The part above gets treated like a hollow sphere and gets canceled while part below follows inverse square law and describes the gravity one experiences.

While you did state this in your previous post on the matter, I'd just like to point out how often this gets overlooked.

I had a dynamics professor state there is no gravity in the Earth because he had read about the hollow sphere thing. I actually used the deep-mines retort against him as well.

 

[Perfection]

You and your fancy balloons.

 

[hobbsyoyo]

Not even that fancy really. Just ordinary, run of the mill weather balloons essentially.

 

[Perfection]

Admit it, your "research" is really just an elaborate scheme to put bouncy castles in low gravity environments.

 

[El_Machinae]

 

[hobbsyoyo]

I like this silly one --
In a one-story pink house, there was a pink person, a pink cat, a pink fish, a pink computer, a pink chair, a pink table, a pink telephone, a pink shower– everything was pink!
What color were the stairs?

 

[madviking]

there are no stairs

 

[hobbsyoyo]

Correct

 

[Truthy]

No gravity anywhere inside the Sphere? It seems you don't really know what you are talking about...
There is gravity inside Earth. This is very easy to prove.

Go in a deep mine. Jump. Are you floating in the air, or getting back down to the ground?

No, he does. He's talking about if the earth were a hollow sphere (i.e., a shell)

Edit: left the thread open for a while

 

[madviking]

it is a dark and stormy night. you are driving through a driving rainstorm when you stop at a rural motel to rest for the night. when you stop by the front desk, the owner of the motel is sitting under an awning nearby. you go up to the man and he says: "i got some good news and some bad news. bad news is that the storm has knocked out the power to the motel and it won't be fixed until morning. good news is that there are free rooms available."

not wanting to drive further in the storm, you decide to accept the man's offer and take the room. the owner hands you a flashlight and you go to your room. after you unpack, you both go to sleep.

in the middle of the night, you feel an arm grab you. and another. you realize that the owner and another man are restraining you. after the struggle, the owner yells at you, "sorry son, but today is not your lucky day. we're going to have to kill you. but we'll let you choose how you'll go. we have an electric chair and my buddy's shotgun. should you survive, we'll let you go, but we don't anticipate you surviving either method!"

except you can survive, very easily in fact. how?

 

[Perfection]

Shotgun. Juice or no juice, there's no way am I going to let those weirdos strap me into a chair.

 

[Kaitzilla]

The Earth isn't a perfect sphere so the gravity wouldn't be zero at the center. Even if it were a perfect sphere, you would essentially have to be a point mass at the very center to experience zero gravity. Anything else than a point mass would have differential forces acting on it, i.e. not zero gravity.

Ah that's true
Gravity would be close to zero then for a normal person.

The inverse square law applies to point sources. An imaginary small dot of infinite density.

Oh ya, inverse square law only applies to point sources of infinite density right.

Planets aren't that way, the part of a planet closer to you pulls harder per unit weight then parts farther away. However by quirk of mathematics any spherically symmetrical object will follow the inverse square law outside it's surface.

But what about inside the Earth?

There's another quirk that helps.

Imagine you're in the center of a planet sized hollow sphere. Gravity from all the parts of the surface pulls on you equally so you experience no net force. What happens if you move closer the surface? You'll experience stronger gravity from the area of the surface ahead of you but a larger percentage of the planet's mass is behind you. So which part wins? The answer is, it still cancels! You will experience no gravity no matter where you are within the sphere.

So for an arbitrary point inside a spherically symmetrical object the gravity of anything above it cancels and the gravity of anything below it follows the inverse square law. So as you descend you have a tradeoff of canceled gravity from the stuff above you versus increasing gravity from the stuff below.

That's really cool

On Earth the depth where that tradeoff is maximized is at the discontuity where there happens to be very little mass above you and a lot of mass Below. That discontuity is the boundary between the lithosphere and the troposphere. Which is better known as the surface.

No, that can't be right.
If I dig down a little bit, 99% of the Earth (that I'm now closer to) will pull down on me harder than the 1% above me pulls me up.

**Edit**
Ah here it is.
About 2500km down, gravity is around 10.8 meters per second squared. (1.1 g)
http://en.wikipedia.org/wiki/Gravity_of_Earth#/media/File:EarthGravityPREM.svg

 

[Perfection]

I remember playing around with that in high school, we used constant density as our toy model (where the surface has the highest gravity). I in a fit of arrogance presumed it held for whatever density profile Earth actually had.

Guess not lol.

Good catch

 

[Steph]

No, he does. He's talking about if the earth were a hollow sphere (i.e., a shell)
Edit: left the thread open for a while

Which is irrelevent since the Earth is not hollow.

 

[Kaitzilla]

it is a dark and stormy night. you are driving through a driving rainstorm when you stop at a rural motel to rest for the night. when you stop by the front desk, the owner of the motel is sitting under an awning nearby. you go up to the man and he says: "i got some good news and some bad news. bad news is that the storm has knocked out the power to the motel and it won't be fixed until morning. good news is that there are free rooms available."

not wanting to drive further in the storm, you decide to accept the man's offer and take the room. the owner hands you a flashlight and you go to your room. after you unpack, you both go to sleep.

in the middle of the night, you feel an arm grab you. and another. you realize that the owner and another man are restraining you. after the struggle, the owner yells at you, "sorry son, but today is not your lucky day. we're going to have to kill you. but we'll let you choose how you'll go. we have an electric chair and my buddy's shotgun. should you survive, we'll let you go, but we don't anticipate you surviving either method!"

except you can survive, very easily in fact. how?

I can think of lots of answers, but the "very easily survive" has me stumped.

No way "Electric Chair" is the right answer.

 

[Timsup2nothin]

It's still the middle of the night and the power was knocked out until morning. Unless they have a lightning rod to power the chair it seems safe enough.

 

[madviking]

tim's got it

 

[Timsup2nothin]

Group 1: A M
Group 2: B C D E K
Group 3: F G J L N
Group 4: H I O

What group does P belong in? How about Q? Why?