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Old Jan 19th 2010, 11:53 AM   #11
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"It is simply n / (sum(1/vi)) where sum(1/vi) is the sum of the inverse of (in this case speed) if the speed is slower, the speed is traveled at for a longer time hence this type of weighting is often used when calculating average speeds."

Maybe this is the rule that allows you to flip a fraction over.... I am not sure I cannot understand what is stated there very well...

but will try to study that sentence and look up information on it online
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Old Jan 19th 2010, 12:00 PM   #12
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OK, i see. from the beginning then:
A-B, we say is distance "D"
1)ab, 5m/s
2)ba, 3 m/s

V=Distance/time

time=D/V time(1)=d/5m/s time(2)=d/3m/s

Total time= d/5 + d/3= 5d+3d/15= 8d/15.
Total distance=d+d=2d

V(average)=total D/total T
=2d/(8d/15) = 30d/8d = 3,75.

That help? =)
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Old Jan 19th 2010, 12:16 PM   #13
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Average speed = Total Distance / Total Time

Total Distance = 2*d where d is the 1 wat distance

Total time = t1 + t2 where the time is unknown

Average speed = 2*d / (t1 + t2)

Substituting t1 = d1/v1 and t2 = d2 / v2

Average speed = 2*d / ( d1/v1 + d2/v2)

Average speed = 2*d / ((d1v2+d2v1)/(v1*v2))

Average speed = 2 *v1 * v2 / (v1 + v2)

Average speed = 2 * 1 / (1/v1+1/v2) which is the harmonic mean.

The other easier way to do it is simply to work out the unknown for time which is what physicsquest did earlier.

The only other advice I can give you is to repeat the problem over and over again, eventually it will become clear to you.
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Old Jan 19th 2010, 12:45 PM   #14
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Thanks

Thanks!

Stuck on a call right now at work (I work in PC IT HelpDesk) but I will read these soon as I can I get off work in 30 mins (thanks god!)

Appreciate taking the time to work with me on this as its been driving me absolutely crazy.

Brian
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Old Jan 19th 2010, 09:37 PM   #15
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Like to think everyone!

I like to thank everyone for the help! I think I have this figured out. I will be around for a lot more questions I am rusty on math and physics is very hard for me as I am not very good with word problems. Only thing I have going for me is I love to learn and I am a very curious person and I actually get excited about physics and math. I did buy the solution guide to my text book today and getting it sent to me air mail so that will help some!

Thanks,

Brian
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