## Search found 13 matches

Wed Dec 07, 2011 2:21 am UTC
Forum: Science
Topic: KA's Q&A. This week: What exactly is a "dimension?"
Replies: 99
Views: 15942

### Re: The physicality of time vis-a-vis "before the big bang."

Stephen Hawking has compared the question of what was before the big bang to asking what's north of the north pole; i.e. there's no such place, yet there is no barrier / boundary. So I guess if you took your time machine back to the big bang, you'd find that all "directions" point future-w...
Wed Oct 12, 2011 1:52 am UTC
Forum: Science
Topic: 1-D Kinematics Problem
Replies: 19
Views: 2825

### Re: 1-D Kinematics Problem

Assuming the ramp has constant slope, and both balls have the same moment of inertia (or, in the case of no friction, are really sliding rather than rolling), both balls should have the same acceleration - not opposite, since they're both accelerating in the same direction (towards the bottom of the...
Tue Sep 13, 2011 10:55 pm UTC
Forum: Science
Topic: CMB photon question
Replies: 17
Views: 2750

### Re: CMB photon question

The thing is, these photons are part of the universe. When they interact with matter, it isn't energy "trickling into the universe", just energy changing from one form into another. So, to answer your question, yes, they are accounted for. The radiation contributes both to the energy densi...
Mon Aug 09, 2010 11:01 pm UTC
Forum: Logic Puzzles
Topic: Eight problems for intuition in basic classical mechanics
Replies: 59
Views: 11609

### Re: Eight problems for intuition in basic classical mechanic

what I mean is the velocity of the target increases by the same amount as the velocity of the rubber ball changes when it bounces off the target? Or am I not understanding this correctly The momentum of the target increases by the same amount as the momentum of the rubber bullet changes when it bou...
Mon Aug 09, 2010 5:32 pm UTC
Forum: Logic Puzzles
Topic: Eight problems for intuition in basic classical mechanics
Replies: 59
Views: 11609

### Re: Eight problems for intuition in basic classical mechanic

Re: Adacore on #1:
Spoiler:
This is essentially brachistochrone problem, the solution of which is an upside-down cycloid. It looks somewhat like a parabola at a glance, but it's not the same curve.
Mon Mar 01, 2010 3:06 pm UTC
Forum: Forum Games
Topic: The Intersecting Geekeries Test
Replies: 9
Views: 1949

### Re: The Intersecting Geekeries Test

I was excited that I got 25/25 until I saw that the extra credit question was worth 5 points. I'd be nice to see all the correct answers.
Sun Sep 20, 2009 3:59 pm UTC
Forum: Science
Topic: Need help with Physics-Kinematics in 1 dimension.
Replies: 11
Views: 1192

### Re: Need help with Physics-Kinematics in 1 dimension.

Small correction: [imath]\Delta x = v_0 t + \frac{1}{2} a t^2[/imath], so in this case [imath]\Delta x = 4.9 {t_r}^2[/imath]
Sun Sep 20, 2009 3:31 am UTC
Forum: Logic Puzzles
Topic: Infinite Balls and Jugs
Replies: 12
Views: 12716

### Re: Infinite Balls and Jugs

A slight variation to case A: Each step, you add nine balls (not ten), and instead of removing the lowest numbered ball, you get out a pen and add a '0' to the end of its number. So you add 1-9, and the 1 becomes 10; second step add 11-19, and the 2 becomes 20. So, at every step, the state of the ju...
Tue Nov 25, 2008 3:22 pm UTC
Forum: Logic Puzzles
Topic: My write-up of the "Blue Eyes" solution (SPOILER A
Replies: 1368
Views: 424629

### Re: My write-up of the "Blue Eyes" solution (SPOILER A

Surely the guru will leave the island if she has blue eyes. Examining the two situations (Guru = Blue and Guru != blue) != Blue: We have the exact situation described in the original problem =Blue: Every blue eyed person can see 100 blues, rather than 99. Therefore, when nobody leaves after day 100...
Wed Sep 17, 2008 3:36 pm UTC
Forum: Mathematics
Topic: i^i
Replies: 4
Views: 1327

### i^i

A while back, I was studying for quals and I came across the problem: Express i^i using only real numbers. The given solution was to take i = e^{i\pi/2} , so i^i = (e^{i\pi/2})^i = e^{-\pi/2} But wait a minute, e^{i5\pi/2} = i as well. Couldn't we just as easily use that to get i^i = e^{-5\p...
Sun Oct 14, 2007 1:57 am UTC
Forum: Science
Topic: Speed of Light
Replies: 68
Views: 6803

### Re: Speed of Light

I would just like to add that there is no such thing as a "stationary observer" in any absolute sense. There are only reference frames. Remember, the Earth is revolving around the sun, the sun is circling the galaxy, the galaxy is moving in relation to others... Everything is in motion rel...
Tue Sep 25, 2007 2:21 pm UTC
Forum: Logic Puzzles
Topic: Only Natural
Replies: 12
Views: 3492
This one seemed pretty easy. I didn't look at any of the others. Just to check, did everyone else get 23/81 for the second part of question 3? Actually, I got 1 - (8/9)sqrt(2/3). I got sqrt(2/3) for the price in case 2 (with the probability distribution for the total value to the customer being P(x...
Mon Sep 24, 2007 8:24 pm UTC
Forum: General
Topic: Who the dickens are you?
Replies: 10922
Views: 2418782
'Ello. I'm a 22 year old guy in my first year of grad school studying physics. I discovered xkcd about half a year ago, and it instantly became my favorite webcomic. I've been lurking here for a while, and will probably continue to do so mostly, but I figured I'd go ahead and introduce in case I eve...

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