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Cade
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 Posted: Sun Sep 09, 2007 1:50 pm Post subject: AP Physics much? |
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We're on the very first chapter of the textbook, going over dimensional analysis and conversions, and I'm already in trouble over the homework. Could you give me a hand with these problems?
"Suppose your hair grows at the rate of 1/32 inch per day. Find the rate at which it grows in nanometers per second."
Now, I'm capable of converting inches into nanometers, and days into seconds, and then putting one on top of the other, but I don't think that's the way my teacher wants it done.
"The speed of light if about 3.00 x 10^8 m/s. Convert this figure to miles per hour."Same problem. I know how to get meters into miles and seconds into hours, but I don't know how to do it the "right" way.
Thanks, guys. I'd really appreciate some help. |
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Snoink
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| Posted: Sun Sep 09, 2007 6:15 pm Post subject: |
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Hmmm... are you talking about the dimensional analysis that means putting your stuff in fractions and then multiplying it to each other? In any case, figure out what you want first (nm/s) and then get what you have (in/day). So then, to get rid of days, you would turn it into (day/24hour) and multiply it together with the other guy so you can cancel out days with days and then have it in hours. Then you figure out how many minutes are in an hour, how many seconds are in an hour, how many inches are in a meter, etc., and just string it together in a large fraction, and then you multiply all the fractions together and prettiness (or the answer) comes out! Voila!
That didn't make any sense.
Okay, because you have (1/32in)/(1 day) and you want to have an equivilient fraction that is expressed in nanometers per second, you can't actually change the numbers of (1/32 in)/day. That would be bad because then we would have a number that we don't really want to find. So we solve this by multiplying our first expression by 1. You already know that 2/2 = 1, 3/3 = 1, so you know that fractions can express the whole number of 1. But we can be smart and multiply it by different 1's, such as (24 hours)/day. 24 hours = 1 day, so it's still 1, but it's better to multiply.
So you string together a large blob of fractions that are equal to 1 that cleverly cancel out all the units you DON'T want into the units you do. Then multiply it all together and you should get a lovely equiviliency statement, in the desired terms. |
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Evangelina
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| Posted: Sun Sep 09, 2007 6:17 pm Post subject: |
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| were doing dimensional analysis too, and Im pretty much as lost as you are. |
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Cade
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| Posted: Mon Sep 10, 2007 1:01 am Post subject: |
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Thanks, Snoikus, and gosh-do-I-know what you're talking about, Evangelina. I figured it out with the help of the blessed internet this morning, and I just want to make sure I did it right, so here's a screenshot of my problem:
So I got that the answer is about 9.12 nm/s. *crosses fingers and hopes it's right*
I did the same thing for the speed of light problem, and then I looked up the real speed of light in mph online, so it looks like I did that right, at least.
 Colly |
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Snoink
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| Posted: Mon Sep 10, 2007 2:31 am Post subject: |
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| You're doing it right! |
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Cade
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| Posted: Tue Sep 11, 2007 12:03 am Post subject: |
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Thanks, Snoink! I think I've got dimensional analysis down, but now I'm angry at vectors...we'll see how that works out.  |
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