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Physics problem, missing equation? 
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#1
Jan2405, 04:14 AM

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Alan leaves Los Angeles at 8:00 a.m. to drive to San Francisco, 380mi away. He travels at a steady 45.0mph. Beth leaves Los Angeles at 9:00 a.m. and drives a steady 57.0mph.
they want to know how long does the first to arrive have to wait for the second? I know the method to go about solving this problem ( i think) but it is unusable unless i were to know how far beth is from san fran. this looks to me like an equation that i dont know, but im thinking something algebraic to solve for the missing distance. for alan i said that 380mi/45mph would equal 8 hrs and 20 mins. beth, i dont know 


#2
Jan2405, 04:24 AM

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Dude, notice that BOTH Alan and Beth leave for San Francisco *from LA*, so Beth is just as far away from Frisco as Alan is...the information you seek was given to you in the words.



#3
Jan2405, 04:33 AM

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#4
Jan2405, 05:03 AM

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Physics problem, missing equation?
The time difference (in hours) is simply
[tex] \frac{380M}{45Mph}\frac{380M}{57Mph} [/tex] Compute this number and express it in minutes... Daniel. 


#5
Jan2405, 05:50 AM

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^That's only the difference between their travel times, dexter. But Alan left an hour before Beth, and you're supposed to figure out who arrives first, and how long he/she has to wait for the other.
380/45 = 8.444444444 hours 0.444444444... = 4/9 4/9 hours * 60 min/hour = 26.66666666667 = 27 min 08:00hrs + 08:27hrs = 16:27hrs = 4:27pm < Alan's arrival time I wasn't trying to make fun of you. In the absence of any info regarding their respective locations in LA, it is reasonable to assume that Alan and Beth depart from more or less the same location. It's good that you are thinking above and beyond the problem, and outside the box. Your line of reasoning is realistic. But remember that physics often presents us with idealized cases and reasonable approximations. I agree that LA is very big, so the approximation is not that reasonable here, but they're trying to present you with a simple problem. Purposely witholding information to leave you guessing or force you to introduce new variables would be pretty sadistic. Beth: 380/57 = 6.66666666667 = 6 2/3 hrs 2/3 hrs = 40 min 09:00hrs + 06:40hrs = 15:40hrs = 3:40pm < Beth's arrival time Beth arrives first. Beth has to wait for Alan for 20 + 27 = 47 minutes. I hope this answer is correct. 


#6
Jan2405, 08:49 AM

P: 269

you all have been an outstanding help, the answer was 47.6 which is correct as far as this problem goes. i will surely be back for more questions.



#7
Jan2405, 08:56 AM

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#8
Jan2405, 09:58 PM

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ok...sorry I couldn't get back to you yesterday.
Alan's travel time is 380/45 = 8.444444444444... as I said. But 8.444444444...hours is an awkward way of expressing the time interval. We'd prefer to keep the eight hours, and express the remaining time in minutes. Just how many minutes is 0.44444.... hours? Well, we know that 0.5 hours is 30 minutes, so it has to be less than thirty, but close. Note that 0.44444.... = 4/9. So we have four ninths of an hour times sixty minutes PER hour (because that's how many minutes there are in an hour) gives you 26.6666....minutes, which we round up to 27. So Alan travelled for 8.444444...hours, which expressed more conveniently is approximately 8 hours and 27 minutes. 


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