Y!A: Newton's Second Law

this was a very nice question and i actually enjoyed answering it very much! it is a conceptual and experimental derivation (or confirmation) of Newton's Second Law. i think the asker really enjoyed my response and gained a good base for understanding the law.

http://answers.yahoo.com/question/index;_ylt=AgIrWQqqcF9zvC.Rpd2OTHDty6IX;_ylv=3?qid=20100808215327AAU7UHf&show=7#profile-info-YrShfcdFaa

Q:

If work is: force x distance... as well as: mass x acceleration, how can I relate these two ideas and think of?

force? Okay - let's break it down - we multiply mass by acceleration to get a force, which makes sense. I can think of it as an acceleration per mass, which takes force (and hence energy supplied). But if I have W = force x distance or W = Fd, I can equivalently say: F = W/d. So force equals work done per unit of distance. Which makes sense to me if I think of the fact that the two equations I have pointed out here require energy, that is again: mg and W/d.

Am I over analyzing this too much, or how can I really understand force? What throws me off is that I have two different equations for force - one is a product of two variables and the other is a quotient of two variables. Why couldn't I say F = m/g?? Or also say: F = Wd????

Additional Details

or instead of F = m/a say:

F = a/m

A:

a good way to think about these things and really get a good conceptual idea for what these equations mean is to create examples for yourself. sure, they wont get you the exact form of the equation, but they'll give you a rough idea of what needs needs to go where (like why mass is *multiplied* by acceleration to get force).

so first, think of newton's equation F=ma. what this says is that in order to move an object of mass 'm' with an acceleration 'a', you need a force 'F' equal to their product. so how does this make sense? well it'll be easy once you think of a few examples that you run into in everyday life!

imagine you are trying to push (or pull) a box along the ground. in your experience, does it matter if the box is REALLY heavy? of course! if the box weighs 2000 pounds, there's no way you're going to be able to move (or accelerate) it! you would need a much larger force to push it along! so then in our formulation of newton's law, we know that if you *increase* the mass of the object, you should also have to *increase* the force it takes to move it, right? well that makes sense, and it seems kind of obvious now! because it is certainly easier to move a feather than it is to move car - and the car even has wheels! so where should we have to put the 'm' if we want 'F' to increase as 'm' increases? well, certainly not in the denominator! if we did that, the force would *decrease* as mass increased! now that would be a sight! so we know 'm' has to be somewhere in the numerator!

now onto acceleration. again, let's use something we're familiar with! let's look at that box again. let's say you want to get it going up to 1/12 of a foot per second (an inch per second) in 1 minute. so you have all of 60 seconds to accelerate this box to a final speed of an inch per second. do you really think it takes a huge amount of force to move something that slow (about 0.0014 ft/s² or 23000 times slower than the acceleration of gravity!)? no, your baby sister could easily do that! but what if someone told you that they need to get this box going 100 miles per hour in the span of 1 second. could you do it? is that even possible?! (assuming the box isn't moving when you start, that would mean the acceleration would be 147 ft/s², or about 4.6 times greater than the acceleration of gravity!). well yes it can be done, but could *you* do it? it's nothing personal, but i don't really think you - or anyone else for that matter! - is that strong. it would require some very massive force to do it! (keep in mind the best cars can go from 0-60 mph in about 3 or 4 seconds, which is pitiful compared to what i'm asking you to do! and they've huge 500hp motors to do it!). so the conclusion is that the bigger acceleration you want to give to the box, the bigger the force you have to impart on it will need to be! and we know - from the previous example - that this requires 'a' to be in the numerator! therefore, F=ma, and there's no other way it can go!

so there you go! this was a rather simple method, but the power of this analysis is incredible! see if you can apply the same thinking to your work/energy problem and get the correct answer!

remember, all that physics is trying to do is describe the world around you. so all you have to do in order to understand it is play around with it! it's right at your fingertips :]

so try this same method with the other problem and i think you'll be pleasantly surprised at how physics describes the world. and if you have any other questions, i'd love to help you out some more! :]

happy hunting!

Y!A: Power series

This was a question i very much enjoyed answering because many people have trouble with - and actually absolutely hate! - power series and convergence and whatnot. but it is exactly the opposite! i LOVED the subject! (don't even get me started on Taylor Series!) and so i feel that this is an area in which i can do a lot of good and help clarify some subjects that some people have trouble with and dislike for no good reason at all!

and i'll also say that one of things that i think makes this subject hard for some people is that it requires the ability to really understand the mathematics and be able to draw conclusions and meaning from the equations. this is something A LOT of people lack, and it is not even something that is emphasized in schools! and this is a terrible circumstance, which i battle frequently, and i hope that i am doing my part to give people this familiarity with mathematics which is really useful and, above all, FUN!

it is also good to see that the asker was very pleased by my answer, and i really hope i was able to make a difference in the way he/she thinks about these subjects.

http://answers.yahoo.com/question/index;_ylt=AoITVMn2GywDSI2pg_R9oV3ty6IX;_ylv=3?qid=20100806112614AAmbpOX&show=7#profile-info-ewbG5Mazaa

Q:

Calculus: Power Series?

I thought that determining weather series converged or diverged was easy, but I seem to be having some problems figuring out how to do that with power series, so can you help me with this problem. Thanks :)

Find the interval of convergence for the given power series:

∑ (n=1 to ∞) [((x-9)^n) / (n(-7)^n)]

Additional Details [the asker posted this after someone else had posted a response with just the answer and nothing else. really pathetic in my opinion...]

I already knew that the answer was (2,16] so you haven't helped me at all. I want to see the steps taken to get to the answer because that is the part that I need help with.

A:

the key you need to solving this problem is to use the ratio test. most of the time, the ratio test will get you the answer you need to convergence problems.

if you recall, the ratio test is the limit (as n approaches ∞) of the absolute value of the ratio of the n+1th term to the nth term of the sequence. or in a different language:

lim(n-->∞) | (a_n+1) / (a_n) |

in this case, the nth term of your sequence is a_n = (x-9)ⁿ / n*(-7)ⁿ.

it follows that the next term (the n+1th term) is a_n+1 = (x-9)ⁿ⁺¹ / (n+1)*(-7)ⁿ⁺¹.

it also happens that our sequence is a function of another variable (x), but this is of no consequence when we consider a limit whose parameter is n (and n and x are in no way related).

so now we need to take the ratio of these as n goes to infinity. we write:

lim(n-->∞) | [ (x-9)ⁿ⁺¹ / (n+1)*(-7)ⁿ⁺¹ ] / [ (x-9)ⁿ / n*(-7)ⁿ ] |

this looks ugly, so we simplify the fractions:

lim(n-->∞) | [ n*(x-9)ⁿ⁺¹ (-7)ⁿ ] / [ (n+1) (x-9)ⁿ (-7)ⁿ⁺¹ ] |

as you can see, some nice things cancel here (since Aⁿ⁺¹ = Aⁿ A¹)

[at this point i highly suggest you write this out as you read it because it become much easier to read and see for yourself what i mean]

now that we've cancelled, our limit looks as follows:

lim(n-->∞) | n*(x-9) / (n+1)*(-7) |

now, if we wish (and we do), we can take things which do not depend on n out of the limit.

notice that x-9 does not depend on n (it only depends on x) and -7 doesn't really depend on anything (especially n!) so we can take these both out of the limit. but the absolute value signs stay on! (don't forget those!)

so, our limit now looks like this:

|(x-9)/(-7)| lim(n-->∞) | n / (n+1) |

the next step is to evaluate this rather simple limit. note also that you can just take those absolute value signs right off of the n/(n+1). this is because everything inside them is guaranteed to be positive, so the signs are superfluous. (but don't take them off of the (x-9)/(-7)! those aren't guaranteed to be positive!)

so, we know that n/(n+1) approaches 1 as n approaches infinity, and so the value of the limit is 1.

our expression now looks simply like:

|(x-9)/(-7)|

now, i've been neglecting it, but the point of the ratio test is to see of the value of the ratio is less than 1 because this is where the series converges (and don't forget that it is inconclusive when it equals 1!)

so we really have to solve this problem:

|(x-9)/(-7)| < 1

also note that that -7 on the bottom can be written as just a 7 and taken out of the absolute value signs. (because |-7| = 7)

so now let's solve

|x-9| / 7 < 1

|x-9| < 7

as you know, you can write this 2 ways and get rid of the absolute value signs as:

x-9 < 7, -(x-9) < 7

solving these (finally), we get:

x < 16, x > 2

(don't forget that when you multiply by a negative in an inequality, you flip the sign)

so we're done right? almost. remember that the ratio test inconclusive when it equals 1. so we have to manually check those possibilities. we do this by checking convergence of the original series at a particular value of x. recall the original series:

∑ [ (x-9)ⁿ / n*(-7)ⁿ ]

the 2 we have to check now are x = 16 and x = 2. notice that at x = 2, the (-7)ⁿs cancel on the top and bottom and you are left with 1/n. as you should know, ∑ 1/n diverges. so, x = 2 does not yield a series which converges.

at x = 16, you get a different series. i'll actually leave it to you to check this one for convergence (hint: use the ratio test again). but if you have problems with it, just let me know, and i'll walk you through it again.

i hope this has helped! and i'm always happy to answer more questions if you have any :]

Y!A: Heisenberg's principle

this is a question i answered about basic knowledge of Heisenberg's principle. but i also tried to give the asker a good background on the subject and even a nice illustration of the meaning of the results at the end.

http://answers.yahoo.com/question/index;_ylt=AgUt5CPVFqxv56o4K4ZEV__ty6IX;_ylv=3?qid=20100805131936AA3iGQD&show=7#profile-info-KN1zv8xbaa

Q:

At a baseball game, a radar gun measures the speed of a 140 g baseball to be 137.32 ± 0.05 km/h.

(a) What is the minimum uncertainty of the position of the baseball?

answer in m

(b) If the speed of a proton is measured to the same precision, what is the minimum uncertainty in its position?

answer in m

please help me out i'm stuck it would be greatly apprieciated if you help me out..thanks

A:

a ) here you need the relationship between uncertainty in position and uncertainty in speed. unfortunately, no such one exists, but there's an equally useful one which relates uncertainty in position to uncertainty in momentum. it is one of Heisenberg's equations:

∆x * ∆p ≥ ħ/2

(where ħ is "h-bar" or h/2π)

(and ∆ means "uncertainty in")

it is good then that you are given the mass of the ball, so that p can be written as mv, with those 2 values given in the problem.

now, all that is left to do is solve for ∆x (or the uncertainty in position):

∆x ≥ ħ / (2*∆mv)

note that since there is no uncertainty in the mass, we can take it out of the ∆ operator - not a necessary step, but still worth noting.

∆x ≥ ħ / (2m*∆v)

so now all we have to do is plug in our known values for m, ∆v, and ħ (in the correct mks units, of course)

m = 140 g = 0.140 kg

∆v = 0.05 km/hr = 50 m/hr = 0.0139 m/s

ħ = 1.0546 * 10^-34 J*s = 1.0546 * 10^-34 kg*m²/s

thus we have:

∆x ≥ (1.0546 * 10^-34 kg*m²/s) / (2 * 0.140 kg *0.0139 m/s)

cancelling units and evaluating, we get:

∆x ≥ 2.71 * 10^-32 meters <<<<<<<<<<<<<<< (answer part a)

you'll notice that this is so small, it is not even perceptible - or anywhere near that! that is why you don't notice this phenomenon while watching a game of baseball.

b) for this problem, we need the same uncertainty relationship. ∆v is again 0.05 km/hr (or 0.0139 m/s). and the mass of the proton can be looked up and is approximately 1.6726 * 10^-27 kg (much much smaller than the baseball)

starting again with the basic relationship

∆x * ∆p ≥ ħ/2, and rearranging:

∆x ≥ ħ / (2m*∆v)

plugging in values exactly the same as last time:

∆x ≥ (1.0546 * 10^-34 kg*m²/s) / (2 * 1.6726 * 10^-27 kg * 0.0139 m/s)

again, all we are left with is meters, and the value of the expression is

∆x ≥ 2.269 * 10^-6 meters <<<<<<<<<<< (answer part b)

although this is still relatively small, it is many many many orders of magnitudes larger than the ∆x of the baseball.

consider also that it is roughly 2.27 * 10^-6 meters (2 micrometers). it is interesting to see this value compared to the approximate size of of an atom, which has a diameter of roughly 100 PICOmeters (a picometer is 10^-12 meters. or 100 of them is 10^-10 meters).

so the uncertainty in the position of this moving proton is almost 4000 times larger than the diameter of an atom!! so forget about it if you wanted to pinpoint the location of this particular proton among other atoms.

i hope this was helpful! and i'm always happy to answer more questions :]

Y!A: time and relativity

an interesting question from Y!Answers about Einstein's relativity and the constant nature of time. this was a nice conceptual question to answer, which is refreshing from time to time.

i also very much enjoy helping people (especially those who have not had much exposure to the topics) about these complicated subjects in ways that are easy for anyone to understand. i find this is often difficult, but it is an extremely worthwhile endeavor. and nothing beats a strong conceptual understanding base from which to build your knowledge of a subject - it is immensely useful. so that is what i tried to give when answering this question...

http://answers.yahoo.com/question/index;_ylt=AvZ93LSZd2IftUoOVnMet4Dty6IX;_ylv=3?qid=20091017130251AAdpDQO&show=7#profile-info-sh6r0wETaa

Q:

How do we know that time is what is what we believe it to be?

Basically I would like to know whether time is what class it as i.e. that how do we know that a second or an hour lasts the duration that we say a second or an hour lasts. Also will the strength of gravity change the speed at which time travels (and if so is time therefore not a constant and as Einstein said only "Relative") I hope that makes sense I'm not really fully sure what I mean myself, i know that the theory of relativity says that space and time are relative but relative to what?

A:

a second or an hour lasts as long as it does because that is the way that we defined it. for example, a second is defined (in modern terms) as the time it takes light to travel 299,792,458 meters. other modern measurements include the time it takes cesium-133 to radiate a certain number of times.

in other words, the universe did not invent the duration of the second, we did. we are using universal constant to define it now, which will never vary so that we can accurately measure time in any circumstance!

regarding your gravity question, no the speed of light will never vary! but it is interesting that you brought this up! let's analyze this situation. if we have light traveling against a gravitational field, the light will need to expend or lose energy. however, the energy it loses will not come out of its speed, but rather, its wavelength. there is a property of waves which relate their wavelengths (or frequencies) to their energy. so, light will expend energy from its wavelength rather than its speed to travel in a gravitational field. this will result in the wavelength of light getting longer or shorter (depending on which way it is traveling) and also changing its apparent color! (this property is partially responsible for the phenomenon known as red-shifting).

yes, einstein's theory of relativity states that everything is relative, well almost everything (the exception being light). the actual title of his paper on what we now call 'special relativity' was actually called something along the lines of "On the Invariance of the Speed of Light", which tells us what einstein really had in mind when he was developing his theory.

in any case, in order to illustrate this 'invariance', we'll look at two similar examples (which i'm sure you may have seen before):

example 1:

you are at one end a train which is moving at a constant velocity, let's say 30 km/hr. and your friend is standing at the other end (let's say he's at the front and you're at the back of the train). now, you have a frisbee and you throw it to your friend at 5 km/hr. both you and your friend see the frisbee traveling at 5 km/hr. however, an observer who is outside the train (on the ground) will observe the frisbee traveling forward at 35 km/hr! (and the train is still moving at 30 km/hr to him as well).

so how is it possible that we have 2 different measured speeds for an object? is the object actually traveling at 2 different speeds? is there an objective way to measure the absolute speed of an object, such that it has one speed all the time? this is where einstein comes in! einstein said that there is no way to objectively measure the speed of the frisbee. he says that it's speed is always measured relative to some other stationary object. for you and your friend, you measured the speed relative to the train (which you considered stationary because you were in it). for the outside observer, he measured the speed relative to the ground (which was stationary to him). (also, imagine the apparent speeds and path that the frisbee would seem to travel to someone on mars or on the moon! they would be very complicated indeed!)

now, example 2:

you are again on a train moving at a constant velocity, 30 km/hr again. but this time, you have a flashlight instead. so, from inside the train, you turn on your flashlight and you observe the light coming out the end of it. this light is traveling at a velocity c (about 300,000 km/sec). and this light will hit the front wall of the train in some time t (based on the length of the train). now, there is also an observer who is outside of the train. at what speed does he see the light travel? does the light travel at 300,000 km/s + 30 km/hr? no! again, einstein comes in and says that the light will travel at the same apparent speed regardless of the observer! so, the light will travel at c, but how long will it take to strike the end of the train? will it be the same time t? well this is a complicated answer, and this gives rise to the phenomenon known as time-dilation. this states that in order for the speed of light to be kept constant (to any observer), time will actually shorten to allow light to make the distance.

so, in short, what einstein is saying is that space and time (and velocities) are relative to their observers. however, the speed of light (the universal constant) is the only thing which will never vary (due to complex phenomena like time dilation and many others).

i know that i have introduced a lot of complicated topics here, and i hope that i haven't confused you (relativity is a very confusing topic anyway). but i hope that you've gained something from this!

please ask questions if i wasn't clear about something (which is likely the case haha)!

Y!A: College Algebra Question

this is another question that i answered on Y!Answers, which can be found at this url:

http://answers.yahoo.com/question/index;_ylt=An4wC0WopGR0xxZZcW2Zts3ty6IX;_ylv=3?qid=20090926133641AASsdGx&show=7#profile-info-7y2dxI6Iaa

Q:

John owns a hotdog stand. He has found that his profit is represented by the function p(x)=-x^2+66x+81, with p(x) being profits and x the number of hotdogs sold. How many hotdogs must he sell to earn the most profit.

my answer would be a million bajillion hot dogs lol. but my answer key says 33. help please

A:

haha yes, it would be nice if he could sell a million bajillion hot dogs, but look at your equation, do you notice that -x^2 term? that means that when he sells a lot of hot dogs, he'll be subtracting that huge number squared from his profits! that's really really bad for his profits!

also, if you'll notice, the formula for his profits is the equation of a parabola! not just any parabola, but one that is opening downwards. this means that there is maximum height that his parabola reaches, corresponding to his maximum profit, which makes this a maximization/minimization problem!

now, all that analysis aside, here's how you solve it:

since the slope (derivative, in other words) of his profits curve (the parabola) is zero at its maximim, we can take the dervative of the profits curve and set that equal to zero to solve the number of hot dogs:

P(x) = -x^2 +66x +81

so the derivative, P'(x) is

P'(x) = -2x +66

now we set that equal to zero (for the reasons mentioned above),

0 = -2x + 66

and solve for x (the number of hot dogs),

2x = 66

x= 33

voila!

to solve these problems in the future, try to analyze the problem as i did in the first two paragraphs. this will really help you get a grasp of the mathematics, and make solving these kinds of problems a breeze

Y!A: BIGGER IS ELECTRICITY!

this is a question i answered for some guy on Y!Answers who had this CRAZY theory that gravity was somehow based on electromagnetism and the E&M forces were somehow giving rise to mass... and some weird other stuff. (i think he was trying desperately to unify E&M and gravity, but not succeeding very well! oh well, at least he's got GUTS!). but i answered his question to try to give him some idea of why that wouldn't work, and so i thought i'd post it.

[also note that at some point before i answered there were some other responses which he apparently got rather peeved about, and he responded in the "Additional details section". so that's what that is in case you were wondering. and i did not include the other responses, but his additional explanation was, at the least, interesting and relevant, so i included that.]

http://answers.yahoo.com/question/index;_ylt=Aqg4W49nJOBNV0qoLYrfJ8Pty6IX;_ylv=3?qid=20091019134800AAooDPq&show=7#profile-info-UMVkZmYOaa

Q:

How does gravity really work?

I just want to know if there is a DEFINITE theory or property that says how gravity works. We know it is there, we know certain properties, but how EXACTLY does it work. I have asked around a little, and nobody knows. I have my own little thought that i think is wrong, however if so, i want somebody to prove me wrong. What if gravity is actually a subset of the force of electromagnetism. We know for a fact that atoms have charges (or at least ions do) but even when they are neutral, they still have charges, they just balance each other out. What if these charges of the protons and electrons still individually have a field/charge of their own that is able to reach out to other atoms. Meaning, that if you have one pos magnet, and on neg magnet, they would attract. However, if you have 10 magnets trying to pick up 10 other magnets, the force is much greater. Same goes with gravity, the more mass/matter you have, the stronger the force of gravity. So my question is, can gravity be a subset or sub-something of electromagnetism? it seems to have similar properties to it, just molded and applied differently.

Additional Details

No, sorry, apparently, people didn't get what i'm saying. I didn't say that you need an electric charge for gravity to work, i said you need mass. How this mass has gravity? i said the electric charges FROM THE PROTONS and ELECTRONS that make up this mass have a charge, and these charges COULD induce gravity e.g. the more matter/mass you have, the stronger the gravitational/magnetic pull. And please don't say this is not possible just because there is another theory saying gravity is already this, that. Ignore those theories, or better yet, incorporate this one, and say if it is possible that gravity is caused by these charges at the molecular (and maybe smaller ) level. Basically can gravity be a subset of MAGNETISM, not electromagnetism, and don't say it is not because modern science says so.

A:

i think i understand what you are saying. here goes:

are you trying to just identify an analogy between electromagnetism and gravity, or are you trying to say that gravity is created by some charges (whether they are on a charged particle or even inside a neutral one, like a neutron)?

judging by your reaction to the first two posts, i'm going to guess that you don't mean the second.

if this is the case, you are right in your thinking when you said that "the more matter/mass you have, the stronger the gravitational/magnetic pull is". this is a good analogy. in electromagnetism, the more charge you have, the bigger the electric/magnetic pull is; similarly, in gravitation, the more matter/mass you have, the more pull there is. if this is all you are trying to identify, you are correct.

however, there is no correlation between how much charge there is inside the atom (or particle) and how much gravitational attraction it experiences.

for example, let's consider a proton and a neutron. they have almost exactly the same mass, but there is no charge on the neutron and there is a +1 charge on the proton. no matter how much charge is on these two particles, they will still exhibit exactly the same gravitational pull.

let's look at another example on the atomic scale.

let's say you have some heavy atom - let's say it's Krypton for now. Krypton has 36 protons, 36 electrons, and 48 neutrons (in one of it's most stable isotopes). we'll call this Kr-84 (for it's mass number). now let's also consider an ion of Kr with all of its electrons stripped away, which would also be Kr-84. if we were to put both of these atoms in a stable environment with one other insignificant mass, they would both feel exactly the same gravitational attraction to it (because the masses of both Kr atoms are the same).

however, consider a similar scenario. if we place both of these atoms again in a controlled environment with one electron, the Kr ion would feel a very large attractive force towards it. on the other hand, the Kr atom (with all 36 of its electrons) would feel no attractive or repulsive force (assuming it was far enough away such that the electrons on the outside of the Kr atom won't repel the electron much more than its protons will).

as you can see, there is no correlation between how much charge an atom (or particle) has and how much gravitational force it experiences. although your theory is very innovative and cleaver, we did not find that the gravitational attraction increased when the charge increased.

note that this is true for magnetic as well as electric charges, but it is just much more difficult to give an example for magnetism because you can never have an isolated magnetic charge.

i hope this answers your question about your theory of gravity! although it is not correct in this case, i would still write to einstein to see if he has any ideas on it :P!

but to answer you first question about the theory of gravity, we still do not know exactly how gravity acts and behaves. physicists today are working very vigorously on this problem and we have an idea of what it should be like.

first of all, there is a theory in physics (more specifically, particle physics) which states that every force is carried by a particle (this law is called the force-particle duality). in electromagnetism's case, it is the photon; the strong nuclear force is carried by the gluon; etc... . if you do not know much about modern particle physics, some of these ideas may seem very foreign and strange to you, but i'll give them to you anyway.

in the case of gravity, the supposed particle which carries the force is the graviton. although this particle has yet to be observed, most physicists agree that it exists, and have many complicated theories as to why we haven't observed it yet.

this being said, the gravitational force is still a mystery to most. the common theory is still einstein's theory of general relativity, which states that matter in space puts a curve in the fabric space-time (which we cannot visualize very well - actually, it is also extremely difficult to visualize the shape of space itself!). then, it is this curvature which brings bodies together under the force of gravity.

now, it is interesting to think about why more matter creates a bigger curvature in space. this is the theory of why matter has mass, and it is also related to the theory of gravity. this is what most physicists have their eyes on now and we are currently testing a theory based on the Higgs boson, which is supposed to be the particle which gives all matter mass.

you may not be very familiar with these concepts, but i'm giving them to you just in case you are!

this may not answer your question of a definite theory of gravity, but keep in mind that today's most brilliant physicists are almost as dumbfounded as the rest of us about the nature of gravity.

hope this helped though (at least the first part about your theory)! any other questions about what i said?

collatz conjecture

So, I found out about this conjecture known as the 'Collatz conjecture'

It states the following: take any natural number, n. if n is even, divide it by 2 (n/2). if n is odd, multiply it by 3 and add 1 (3n + 1). Repeating this process over and over will eventually get you 1.

(Search it on wikipedia if you don't know it)

So, doing this for, say, n = 9, gives this:

9 -> 28 -> 14 -> 7 -> 22 -> 11 -> 34 -> 17 -> 52 -> 26 -> 13 -> 40 -> 20 -> 10 -> 5 -> 16 -> 8 -> 4 -> 2 -> 1 -> 4 -> 2 -> 1 -> 4 -> 2 -> 1

Now, initially this seems pretty interesting, and indeed it is!

Do it for a few more numbers (larger ones perhaps. although it's worth noting that a small number such as 9 got all the way up to 52!) and you'll see what i mean. It ALWAYS goes to 1, eventually!

This is actually an unsolved problem in pure mathematics, which I was surprised to find out. (Paul Erdos actually offered a $500 prize for the solution).

Like i was saying, it seems really complicated and important at first, but i'm not really sure it is.

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

Here's my take on it.

(Also, keep in mind that I'm not a degreed mathematician or anything like that. Tthis is just me thinking)

Again, the conjecture states that, for any natural number n, do the following: if n is even, divide it by 2; if n is odd, multiply it by 3 and add 1. Repeating this process over and over will eventually get you 1.

You'll notice pretty quickly (it really only takes two or three runs of this algorithm to do so) that if we reach a power of 2, we're done pretty quickly, because we just keep dividing down by 2 until we hit 1.

So the question really is this (or can be stated as):

how long before we hit a power of 2?

It seems reasonable enough to me to think that given some arbitrary algorithm, you will eventually reach an equally arbitrary goal (assuming they're not contradictory, such as starting with an odd number and adding 2 until you reach an even number).
since 3n + 1 can reach odd numbers and even numbers, there is no reason it shouldn't reach a power of 2 at some point. that is to say, there's no reason that multiplying by 3 and adding 1 shouldn't eventually reach a power of 2 (ie they're not contradictory statements. For example, multiplying instead by 2 and adding 1, and reaching a power of 2 would obviously be contradictory statements). it would seem that the only thing that would need to be proved - if anything - is why that is true.

Consider the following:

Given any natural number:

If the number is not prime, divide it by its smallest prime factor;

If the number is prime, multiply it 2 and add 1, then go eat a cheeseburger.
(If, by chance, you get to 1, consider it prime. I say this just so this process is defined for every natural number.)

It is my contention that any number chosen will, after repeating this process, eventually converge at some small number (such as the 1 for Collatz); it turns out to be 3 for this problem (and indeed it does reach 3 every time). Also, you will soon become very fat.

(You can also see that it does here; and feel free to copy or rewrite this code and try different numbers for yourself):

You can also see that once it reaches 3, it will cycle back up to 7, and reach 3 again. This is much like the Collatz problem, in which it cycles 4-2-1-4-2-1-4-2-1, but never really stopping.

So does this question deserve as much, if any, attention as the collatz problem? Keep in mind, I just pulled this thing out of my rear-end.

Well, it actually seems harder to analyze than the collatz problem! As far as i know, it's much harder to put into an algorithmic form similar to the one for the collatz problem

(which is:

f(n) = n/2 (if n = 0 [mod 2]), 3n+1 (if n = 1 [mod 2])

)

The answer to me seems to be no, we shouldn't really spend much time analyzing this 'truly mysterious phenomenon'.

While, yes, it is very interesting (and fun to play with!), i don't see what the big deal is!

The Collatz problem says pretty much the same thing as 'if we take a number, and do some arithmetic to it, how long does it take until we get to a power of 2?' Actually, not even that, it only asks if we get to a power of 2, not even how long, or how many iterations. Kind of boring, no?

Actually, the 3n+1 just turns an even number into an odd number, and an odd number into an even number! and the n/2 will turn n odd or even. also note that this way, every odd number becomes even.

so we go... iterating...

odd.. even... even... even... odd... even... odd... even... even... odd... POWER OF 2!! YAY!! done!

All in all, this seems more like a statistics problem to me - seeing how long it takes to get a power of 2. Maybe it is interesting in analyzing some properties of the natural numbers; like the density of powers of 2, or even numbers or something like that. But again, don't we already know that? We know the density of powers of 2 because we have an algorithm to get powers of 2 (hint: start with 2 (or 1), multiply by 2). It's not like the density of primes, which we don't know (although we can approximate the number of primes with the prime-counting-function, which approaches ln(n) minus a small constant for sufficiently large n).

Also, notice that it doesn't matter how large these numbers get. You could iterate a thousand times and get a number in the trillions! but still, once you hit a power of 2, you come tumbling down to 1 very quickly. Granted that the powers of 2 are very sparse in the trillions, there is no way that you are going to stay up in the trillions for long, seeing as you are bound to encounter a number which is divisible by 4 or 16 or 256 soon, and come tumbling way down with the n/2 - try it.

In conclusion, it seems to be an arbitrary problem to me! Given an infinite amount of time and iterations, will this algorithm make power of 2, if you keep giving the algorithm even numbers?

But what do you think?

Is this a trivial problem?

(Also keep in mind that Paul Erdos thought this was a very difficult and complex problem. Ohhhh Paul Erdos! Are you intimidated? Can you feel your opinion changing?)