## Search found 52 matches

- Sat Jul 16, 2011 12:56 am UTC
- Forum: Individual XKCD Comic Threads
- Topic: 0925: "Cell Phones"
- Replies:
**98** - Views:
**27757**

### Re: 0925: "Cell Phones"

Hey, remember when xkcd used to be statistically accurate ? This joke is really old (and, as we've seen, has been used in xkcd before), and Randall's art adds nothing to it. It seems to me that, lately, when Randall tries to be funny with science he does it by using it the wrong way and saying: 'Hey...

- Fri Apr 15, 2011 7:16 pm UTC
- Forum: Mathematics
- Topic: Assistance with an analytic solution to radical
- Replies:
**15** - Views:
**2284**

### Re: Assistance with an analytic solution to radical

Can't we just say, naívely, that r=\sqrt{1+\sqrt{2+...}}=\sqrt{1+\sqrt{2}r} ? Or, more rigorously, that r is the limit of the sequence: \{r_n\} | r_1=1, r_{n+1}=\sqrt{1+r_n\sqrt{2}} , It is not hard to prove that the sequence is bounded above by \frac{1+\sqrt{3}}{\sqrt{2}} , and that it is strictly ...

- Sat Jan 09, 2010 11:20 pm UTC
- Forum: Mathematics
- Topic: Bad at math
- Replies:
**15** - Views:
**2049**

### Re: Bad at math

Clarifying things a little: there is no such thing as a word problem once you get to math a little deeper. When there is, it is normally an application of such to another area such as physics in calculus. The point of them is simply to straighten out the facts to simplify the problem, removing as mu...

- Tue Nov 24, 2009 1:20 pm UTC
- Forum: Mathematics
- Topic: Unit circle
- Replies:
**15** - Views:
**89676**

### Re: Unit circle

Also important sometimes are the sine and cosine of \pi/10, \pi/5,3\pi/10 and 2\pi/5 For \pi/10 , make an isoceles triangle of angles \pi/5, 2\pi/5,2\pi/5. Now draw a line that bisects one of the 2\pi/5 angles, and use metric relations to arrive at sin ( \pi/10 )= \frac{5^{1/2}-1}{4} . Its cosine, a...

- Tue Oct 20, 2009 3:27 pm UTC
- Forum: Science
- Topic: Strange wording (astronomical laws)...
- Replies:
**6** - Views:
**827**

### Re: Strange wording (astronomical laws)...

His derivations of all of the three Kepler laws are, however, mathematically founded, not empirical.

- Mon Oct 19, 2009 2:05 pm UTC
- Forum: Mathematics
- Topic: proving a conjecture
- Replies:
**1** - Views:
**574**

### Re: proving a conjecture

Rephrase it in a way that people can read and understand (i.e.: What are you trying to prove about D or a?) and people should try harder to help. What exactly is the conjecture?

- Sun Oct 18, 2009 9:55 pm UTC
- Forum: Mathematics
- Topic: Prime problem
- Replies:
**6** - Views:
**1002**

### Re: Prime problem

Yes, sorry.

- Sun Oct 18, 2009 2:15 pm UTC
- Forum: Mathematics
- Topic: Prime problem
- Replies:
**6** - Views:
**1002**

### Re: Prime problem

Actually, what I meant is that for the proof be right, that must happen. I haven't been able to prove it always happens for q.

- Sun Oct 18, 2009 1:32 pm UTC
- Forum: Mathematics
- Topic: Prime problem
- Replies:
**6** - Views:
**1002**

### Prime problem

"Given a prime q=2p+1, where p is a prime, prove that there always is a non-zero multiple of q such that the sum of its digits in base 10 is less than or equal to 3." This isn't homework, and I have so far only been able to prove that there must be an i such that 10^i is congruent to -1 or...

- Tue Oct 13, 2009 1:30 am UTC
- Forum: Mathematics
- Topic: Compounding intrest proof! (Homework Headache)
- Replies:
**3** - Views:
**910**

### Re: Compounding intrest proof! (Homework Headache)

Assuming that a>0, of course, firstly, let f(n)=a(1+r/n)^n. Let's expand the binomials for n=k and n=k-1: f(n)/a=\sum_{p=0}^n {n \choose p} \frac{r^p}{n^p} , and f(n-1)/a=\sum_{p=0}^{n-1} {{n-1} \choose p} \frac{r^n}{(n-1)^p} Ergo, (f(n)-f(...

- Mon Sep 14, 2009 4:25 pm UTC
- Forum: Mathematics
- Topic: Help with Pappus' centroid theorem
- Replies:
**3** - Views:
**805**

### Re: Help with Pappus' centroid theorem

Fixed it. I'll try to make a picture:

B

|\....|

|.\...|

|..\..|

|...\.| A

|.../.|

|../..|

|./...|

|/....|

C

B

|\....|

|.\...|

|..\..|

|...\.| A

|.../.|

|../..|

|./...|

|/....|

C

- Sun Sep 13, 2009 11:36 pm UTC
- Forum: Mathematics
- Topic: Help with Pappus' centroid theorem
- Replies:
**3** - Views:
**805**

### Help with Pappus' centroid theorem

Let ABC be an isoceles triangle, such that m(AB)=(AC)=g and m(BC)=l, and that the height of the triangle with respect to A be h. We'll rotate ABC by an angle of 2\pi , in an axis that is parallel to BC and intersects the triangle in A. The surface area of the solid formed is clearly the sum of the a...

- Sun Mar 22, 2009 12:21 am UTC
- Forum: Science
- Topic: Comical mistakes in your homework.
- Replies:
**144** - Views:
**17268**

### Re: Comical mistakes in your homework.

Well, this once I found the constant of a spring to be about 12 900 N/m on an experimental test. Needless to say, I got the entire test wrong due to this.

- Fri Mar 20, 2009 2:52 pm UTC
- Forum: Mathematics
- Topic: I dont want to be eaten by a clown(math problem)
- Replies:
**12** - Views:
**2051**

### Re: I dont want to be eaten by a clown(math problem)

HINT: You do know that: cos (a+b) = cos a.cos b - sin a.sin b, cos (a-b) = cos a.cos b - sin a.sin b Add both, or subtract one from another and you get: cos (a+b) + cos (a-b) = 2.cos a.cos b cos (a+b) - cos (a-b) = -2.cos a.cos b Let a+b = x, a-b=y. Su...

- Sat Feb 28, 2009 8:00 pm UTC
- Forum: Mathematics
- Topic: An arithmetic progression problem
- Replies:
**3** - Views:
**700**

### Re: An arithmetic progression problem

Actually, the original phrasing of the question (I didn't have ot at the time of writing) is "Prove that (a_1,a_2,a_3,...,a_{n-1},a_n ) is an arithmetic progression, if, and only if \sum_{k=1}^{n-1} \frac{1}{a_ka_{k+1}} = \frac{n-1}{a_1a_n} " Does that change anything at all? If not, t...

- Sat Feb 28, 2009 6:59 pm UTC
- Forum: Mathematics
- Topic: An arithmetic progression problem
- Replies:
**3** - Views:
**700**

### An arithmetic progression problem

Well, I've got this problem for homework, and I have only been able to do part of it. So, I humbly ask for your help. The problem is: "Prove that if, and only if ( a_1,a_2,a_3,...,a_{n-1},a_n ) is an arithmetic progression, then \sum_{i=1}^{n-1} \frac{1}{a_ka_{k+1}} = \frac{n-1}{a_1a_n} ."...

- Wed Jan 07, 2009 11:11 pm UTC
- Forum: Science
- Topic: speed of sound vs the wind
- Replies:
**14** - Views:
**1842**

### Re: speed of sound vs the wind

Nevertheless, I believe it won't change things much, only the decrease in pressure would change depending on the variation on its specific mass (also, would it expand or contract at a wind speed of 10m/s?)

- Wed Jan 07, 2009 8:16 pm UTC
- Forum: Science
- Topic: speed of sound vs the wind
- Replies:
**14** - Views:
**1842**

### Re: speed of sound vs the wind

I believe that the pressure may have a bigger effect on that. The difference in pressure for an incompressible fluid is proportional to the square of the speed it has, and since the speed of the waves in a fluid changes with the pressure, it may be greatly affected, affecting maybe intensty.

- Sat Dec 13, 2008 12:07 pm UTC
- Forum: Science
- Topic: A Couple Seemingly Simple Physics Problems
- Replies:
**7** - Views:
**1606**

### Re: A Couple Seemingly Simple Physics Problems

For the second problem, remember that the the sum of the forces in the vertical direction must be zero, since it is stationary. Remember how the buoyant force equals the weight of the fluid dislodged by the cube. Solving for a general case: Solving for a general case: There is a body of volume V_1 ,...

- Sun Dec 07, 2008 11:55 pm UTC
- Forum: Mafia
- Topic: Vampire Werewolf - Dead
- Replies:
**47** - Views:
**6509**

### Re: Vampire Werewolf

Ooh, I wanna play!

1a. Bulvox (ha, take that Myrrh)

15. Tigerlion

2. Vieto

31. Megatriorchis

405. Moody7277

85.SuperTD

6. Mafia

75. crucialityfactor

117. Rodan

-11. Woofsie

56. Pepsiblue

3. Marslyr

√-1. Jar`O`Jam

∞. Vox Imperatoris

∆t. Istrom

3.14:CntRational

[imath]e^ {i\pi}[/imath]: b.i.o

[imath]\Phi[/imath]: Rgeminas

1a. Bulvox (ha, take that Myrrh)

15. Tigerlion

2. Vieto

31. Megatriorchis

405. Moody7277

85.SuperTD

6. Mafia

75. crucialityfactor

117. Rodan

-11. Woofsie

56. Pepsiblue

3. Marslyr

√-1. Jar`O`Jam

∞. Vox Imperatoris

∆t. Istrom

3.14:CntRational

[imath]e^ {i\pi}[/imath]: b.i.o

[imath]\Phi[/imath]: Rgeminas

- Sun Dec 07, 2008 11:34 pm UTC
- Forum: Forum Games
- Topic: Not when I'M watching [NOW IN ENDGAME, Plays until 31st Dec]
- Replies:
**6803** - Views:
**331024**

### Re: Not when I'M watching [NOW IN ENDGAME, Plays until 31st Dec]

I have 2 points now! Yipeee!

- Sun Dec 07, 2008 11:29 pm UTC
- Forum: Forum Games
- Topic: 10000 days of christmas
- Replies:
**14566** - Views:
**638409**

### Re: 10000 days of christmas

895 people who don't believe it got this far.

- Sun Dec 07, 2008 11:25 pm UTC
- Forum: Forum Games
- Topic: Not when I'M watching [NOW IN ENDGAME, Plays until 31st Dec]
- Replies:
**6803** - Views:
**331024**

### Re: Not when I'M watching [NOW IN ENDGAME, Plays until 31st Dec]

Just because you said so, I'm not doing it.

- Sun Dec 07, 2008 11:01 pm UTC
- Forum: Forum Games
- Topic: Courrupt-a-question
- Replies:
**465** - Views:
**33946**

### Re: Courrupt-a-question

The future holds the ultimate truth, only by means of [imath]e^{i\pi}+1[/imath]you will find your own essence.

How can I avoid boredom in vacation?

How can I avoid boredom in vacation?

- Sun Dec 07, 2008 10:57 pm UTC
- Forum: Forum Games
- Topic: Not when I'M watching [NOW IN ENDGAME, Plays until 31st Dec]
- Replies:
**6803** - Views:
**331024**

### Re: Not when I'M watching [NOW IN ENDGAME, Plays until 31st Dec]

I have just gotten a POINT! Yaaaaay!

- Sun Dec 07, 2008 10:54 pm UTC
- Forum: Forum Games
- Topic: It looks like... a nuclear bomb!
- Replies:
**424** - Views:
**33203**

### Re: It looks like... a nuclear bomb!

However, as you are being judged as a criminal, and eligible for immediate death, you'll be able to escape...

The ability to dye your butt-hair purple or orange at will.

The ability to dye your butt-hair purple or orange at will.

- Sun Dec 07, 2008 1:35 pm UTC
- Forum: Mathematics
- Topic: How many digits of Pi do you know?
- Replies:
**445** - Views:
**107041**

### Re: How many digits of Pi do you know?

Just 3,14159265358.9 I sort of feel stupid for that... : )

EDIT: Corrected a typo.

EDIT: Corrected a typo.

- Fri Dec 05, 2008 3:47 pm UTC
- Forum: Individual XKCD Comic Threads
- Topic: 0513: "Friends"
- Replies:
**914** - Views:
**223465**

### Re: "Friends" Discussion

If they go off with someone else then someone else had the right approach. Learn from someone else, he/she is a wise man/woman. Assertiveness = good, yes. However, in terms of specific approaches, what worked for that person may not work on the next one you're pursuing. Each case is so specific. Ho...

- Mon Nov 17, 2008 11:53 pm UTC
- Forum: Science
- Topic: how do you use kinetic energy and momentum to find speed?
- Replies:
**6** - Views:
**2859**

### Re: how do you use kinetic energy and momentum to find speed?

Classically speaking, [imath]K=\frac{mv^2}{2}[/imath] and [imath]\mathbf{P}=m\mathbf{v}[/imath]. For your problem, [imath]mv^2=246[/imath] and [imath]mv=36.9[/imath]. Divide one equation by the other: [imath]\frac{mv^2}{mv}=\frac{246}{36,9}[/imath]. [imath]v=6.67 m/s[/imath].

- Fri Nov 07, 2008 12:40 am UTC
- Forum: Science
- Topic: Problems with the Doppler effect
- Replies:
**4** - Views:
**913**

### Re: Problems with the Doppler effect

So the cases are not equivalent then? But, if I had light propagating in a vacuum, the results would have to be the same?

Nevertheless, thank you a lot, I am very grateful!

Nevertheless, thank you a lot, I am very grateful!

- Thu Nov 06, 2008 9:56 pm UTC
- Forum: Science
- Topic: Problems with the Doppler effect
- Replies:
**4** - Views:
**913**

### Re: Problems with the Doppler effect

The same 300 m/s. Should I have made it 270 or 330?

- Thu Nov 06, 2008 7:36 pm UTC
- Forum: Science
- Topic: Problems with the Doppler effect
- Replies:
**4** - Views:
**913**

### Problems with the Doppler effect

The problem I have is: --- Consider two motionless observers are receiving a frequency emitted by an object travelling between them, with a speed of 30m/s (see diagram) (observer) (emitter) (observer) Considering that the frequency emitted by the object is of 1000 Hz, and assuming the speed of the w...

- Thu Nov 06, 2008 6:38 pm UTC
- Forum: Fictional Science
- Topic: What is the mass of a feather?
- Replies:
**229** - Views:
**320439**

### Re: What is the mass of a feather?

First, you should measure the gravitational acceleration on the surface of Earth. In order not to mess with relativity, let's just say g=\frac{GM}{R^2} . Just put your scales upside-down, measure the mass of the Earth, get a ruler, measure the maximum circle of the planet. Assuming the Earth is a sp...

- Sun Nov 02, 2008 11:30 pm UTC
- Forum: Science
- Topic: Physics Help
- Replies:
**16** - Views:
**1589**

### Re: Physics Help

Only for the body that pulls. For the body that is pulled, it is exactly the opposite.

- Sun Nov 02, 2008 10:42 pm UTC
- Forum: Science
- Topic: Physics Help
- Replies:
**16** - Views:
**1589**

### Re: Physics Help

Not actually backwards, depending on you reference frame. Remember a string can never push things, just pull. EDIT: Yes, you can, but you are doing it the wrong way. Remember that the block is moving to the left, so things to the left are positive, and things to the right are negative. As the 2N for...

- Sun Nov 02, 2008 10:29 pm UTC
- Forum: Science
- Topic: Impulses and collisions
- Replies:
**22** - Views:
**2333**

### Re: Impulses and collisions

Shouldn't any frictionless collisions with a ball be perfectly frontal, and thus, transfer no angular momentum (by the way, I don't understand much of this, so I may be totally off).

- Sun Nov 02, 2008 10:21 pm UTC
- Forum: Science
- Topic: Physics Help
- Replies:
**16** - Views:
**1589**

### Re: Physics Help

Ok, ok, I'll show the complete solution. #9: f=m.a for the system of 3 blocks (you can do that, because the strings are supposedly inextensible, so the 3 objects will have the same acceleration): 11 - 2 = (3 + 5 + 1).a a = 1 m/s² Given acceleration, you can now work on the rest. f=m....

- Sun Nov 02, 2008 8:09 pm UTC
- Forum: Science
- Topic: Physics Help
- Replies:
**16** - Views:
**1589**

### Re: Physics Help

For #9: Because 11 is bigger than 2, the system moves as one big body to the left. According to Newton's 2nd law: For the system formed by the 3 blocks: 11 -2 = (m1+m2+m3).a Find a, then use the laws of motion for all other blocks to find Tl. For #11: Let f be friction; Using Newton's Laws for Block...

- Sun Nov 02, 2008 4:40 pm UTC
- Forum: Science
- Topic: Physics Help
- Replies:
**16** - Views:
**1589**

### Re: Physics Help

I don't know if I expressed myself well, I meant solve the linear system of equations, guess this word does not exist in English (I'm Brazilian).

- Sun Nov 02, 2008 3:56 pm UTC
- Forum: Science
- Topic: Physics Help
- Replies:
**16** - Views:
**1589**

### Re: Physics Help

When you exert a force in an ideal wire (i.o.w., massless, inextensible, non-absorbing and isotropic), it is transmitted to every point in the wire, and to whatever is on the other "tip" of the wire. An example: if I hold a wire with a rock tied to it, I am not touching the rock, only the ...