Yet another question help please

[buc4933]

Suppose two galaxies move away from each other at 6,000km/s and they are 300,000,000 light years apart. if their speed has remained constant, how long has it taken these galaxies to move that far apart, express the answer in years. <br /><br />I don't know if this is a starting point or not but it takes 50,000 seconds at 6,000km/s to reach 300,000,000 light years........i think or do i have to convert light years into km or something

 

[buc4933]

kilometer = 1.05702341 × 10-13 light years is this right or do i not even need to do this for the problem

 

[buc4933]

1 light year = 9.4605284 × 1012 kilometers i think that what i need right?

 

[bobw]

Your example questions seem to boil down some variation of distance = rate X time so far. Miles is a distance, kilometers/second is a rate and minutes is a time. But miles are not equal to kilometers/second X minutes. All your units have to be of the same type for your answer to be correct, so yes you have to either convert the light years to kilometers or convert the kilometers/second to light years/second.<br /><br />When your teacher writes the equations on the chalkboard and shows the calculation does he cross out the units showing how the units of the answer are derived? Like this (ignore the periods..I need them to space out the gaps)?<br /><br />light years . . km<br />----------- . . X ---------- = seconds<br />km/sec. . . . . light year<br /> <div class="Discussion_UserSignature"> </div>

 

[buc4933]

this is an online class so thats why I've been asking for help

 

[bobw]

Basically in the example I showed above the light year in the top left and the light year in the bottom right will cancel out. It is sort of like algebra; light years/light years =1. Likewise the kilometers in the lower left and the kilometers in the upper right cancel out too. That leaves the seconds on the bottom of a division within a larger division and it ends up inverting the 1/sec in the bottom left. 1/3 = 1/3 but 1 / (1/3) = 3 and 1 / (1/sec) =sec. Maybe you need to e=mail the teacher and see if he can give you some kind of reference sheet on that type of calculation. <div class="Discussion_UserSignature"> </div>

 

[why06]

okay..okay...For not liking math you ask a lot of questions on it... <img src="/images/icons/wink.gif" /><br /><br />First: convert 300,000,000 ly into km <br />You need to find how many km exist in 300,000,000 ly.<br />take: 300,000km/sec. (the speed of light) <br />now: multiply this times 60 (to min.) -----18,000,000km<br />then: multiply that times 60 (to hr.)-------1,080,000,000km<br />next: multiply times 24 (to days)----------25,920,000,000km<br />now: multiply this times 365 days ( to find how far light travels in a year)-----9,460800,000,000km<br />finally: multiply this times 300,000,000ly (to get # of kilometers traveled)----2,838,240,000,000,000,000,000km---whew.<img src="/images/icons/crazy.gif" /><br /><br />Now heres the easy part... <img src="/images/icons/wink.gif" /><br />do: divide 2.83824*10^15 (or 16th i'm not sure) by 6,000km<br />now: divide out to seconds..<br /><br />you were on the right track, but were way off...I mean how can it take less time for the galaxies to seperate than it takes light... REALLY 50,000 sec. compared to 300,000,000 years for light....<img src="/images/icons/rolleyes.gif" /><br /><br /><br /><br /><br /><br /><font color="yellow">Finish<font color="white"> <img src="/images/icons/smile.gif" />, goddang man are you trying to kill me...</font></font> <div class="Discussion_UserSignature"> <div>________________________________________ <br /></div><div><ul><li><font color="#008000"><em>your move...</em></font></li></ul></div> </div>

 

[buc4933]

the answer needs to be in years so after i divide 2.83824*10^15 (or 16th i'm not sure) by 6,000km which is 4.7304x10^17 Final answer????

 

[buc4933]

Thank you so much for your help!!!!!!!

 

[bobw]

4.73 x 10 ^17 is the answer <b>in seconds</b>. To get the answer in years you need to divide by the number of seconds/year.<br /><br />You seem to be OK at the math but need some confidence with your unit conversions. A quick look around found this website showing how the units cancel when you convert them. It shouldn't take too long to read it. I hope this helps. It is the best I could find quickly, but the bottom part of the page gives you the idea of what I meant when I wrote about units canceling out. I would be surprised if your text book or online study materials don't show something like it.<br /><br />Always show your units at every step when you do your math on paper, that way you can be sure you don't forget that last step! It is easy to do after you get done figguring out the hard part. <br /><br />http://www.donnayoung.org/math/unit_multipliers.htm <div class="Discussion_UserSignature"> </div>

 

[why06]

Well lets see...2,838,240,000,000,000,000,000km/ 60,000km/s<br />equals..<br />39,720,666,666,666,666.6666....km/s<br />now:get it to minutes...divide by 60---- 662,011,111,111,111.1111....min.<br />next:..divide by 60...(to get hours)---11,033,518,518,518.5185185...hr.<br />then: divide by 24 (to get days)-----459729938271.6069382716 days<br />finally: divide by 365 (to get years)----- <font color="yellow">1259534077.4564518856756299678674 yr.<br /><br />Finish <img src="/images/icons/crazy.gif" /><br /><br /><font color="white"> By the way 4.7304 x 10^17 does not equal 6o,ooo. What conversion are you using man?! What units is 4.7304x10^17 in anyway...<br />well it doesn't matter what I just wrote should be the correct answer. Perhaps you should take a math course cuz this isn't even algebra man. Im sorry, but I can't do any more of your problems... your gonna have to ask someone else...<br /><br /><font color="yellow">I have a life, <img src="/images/icons/cool.gif" /></font></font></font> <div class="Discussion_UserSignature"> <div>________________________________________ <br /></div><div><ul><li><font color="#008000"><em>your move...</em></font></li></ul></div> </div>

 

[bobw]

1 light year = 9.4... x10^12 kilometers<br /><br />9.4... x10^12 kilometers/light year X 3x10^8 light years = 28x10^20 or 2.8x10^21 kilometers<br /><br />2.8x10^21 kilometers / 6 x10^3 kilometers/sec = .473x10^18 or 4.73x10^17 seconds <div class="Discussion_UserSignature"> </div>

 

[nexium]

Start processing the numbers given and you will often see the way to solve problems such as this: 6000 kilometers per second times 3600 = 21,600,000 kilometers per hour = 518,400,000 kilometers per day = 189,000,000,000 kilometers per year. divide ten trillion by 0.189 trillion = 52.91 years to move one light year times 300,000,000 = 15,870,000,000 = a little less than 16 billion years. Answer if I did not make an error. Neil

 

[bobw]

You make a good point about rounding off the number of kilometers/light year to 10 trillion for a quick estimate. Chosing a number larger than the actual number shows that the real answer should not be larger than the estimate. <br /><br />It is also a good idea to try the same problem in a different way, like you did, because any way you figgure it out should give the same answer. Your (correct) estimate of 16 billion years is about a billion times less than 4x10^17 so it is a good clue that something is wrong somewhere.<br /><br />I tried to find some tutorials to help buc4933 with dimensional analysis. While nothing is as good as having a real teacher for help, these might be better than nothing. Sometimes I read something in one place and don't get it, but read the same thing phrased another way and it clicks. I hope one of these makes buc4933 say "Oh! That's easy." <br /><br />This one is kind of funny.<br />http://wine1.sb.fsu.edu/chm1045/notes/Intro/Dimanal/Dimanal.htm<br /><br />Halfway down this page is a javascript dimensional analysis tutorial (I got 'em all right <img src="/images/icons/smile.gif" /> )<br />http://www2.wwnorton.com/college/chemistry/gilbert/tutorials/ch1.htm<br /><br />This one has some nifty red lines pop up when you mouse over the equations.<br />http://phoenix.liu.edu/~nmatsuna/dim/dim.anal.html<br /><br />Are Units Necessary?<br /><br />http://www.alysion.org/dimensional/fun.htm<br /><br />http://www.occc.edu/KMBailey <div class="Discussion_UserSignature"> </div>