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Rev/sec necessary to reach 20-g

  1. Sep 1, 2011 #1
    1. The problem statement, all variables and given/known data

    http://imgur.com/Frn6v


    2. Relevant equations

    Ac=((2pi*r/T)^2)/r

    Where T is the time period

    3. The attempt at a solution

    Since a g is 9.8m/s^2

    20(9.8)=(4*pi^2*29)/T^2

    1144.87/(20(9.8))=5.841

    And solved for T getting...

    sqrt(5.841)=2.417

    The book's answer at the back reads 0.749 revs/sec.

    Did I miss something or is there something else I'm supposed to do with the answer? Thank you for your time.
     
  2. jcsd
  3. Sep 1, 2011 #2

    PeterO

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    NASA scientists lost mars probe which crashed into the planet heading for the calculated orbit height. In Their calculation they forgot that our SI units have metres for length, and set the orbital radius at xxx feet, when it should have been xxx metres.


    You have used a radius of 29 in your calculation. The radius is 29 feet , NOT 29 metres.

    Either express the 29 feet in metres or express g as 32.2 - the feet equivalent of our 9.8 ms-2
     
  4. Sep 1, 2011 #3
    Thanks for the laugh. It did seem weird reading the word "feet" in my Physics book, but didn't think too much of it.

    However, after converting 29 feet to 8.84 meters, and substituting that into the equation I'm getting, after the sqrt, 1.3 for the answer.

    Is there something else I may be overlooking?
     
  5. Sep 1, 2011 #4

    PeterO

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    What does you answer of 1.3 mean. An approximate check [like assuming pi = 3 etc] gives an answer very much like that so I am pretty sure it is correct.
    Perhaps you have forgotten what quantity you have calculated and what quantity you were asked for.
     
  6. Sep 1, 2011 #5
    I'm to be calculating how many revolutions per second are required for a person to feel 20-g of acceleration. I was actually going over the quantities used to see what I was left over with.

    Which was 1.3 seconds. But they want the answer in Revolutions per second. I think this is where I'm going wrong as I don't know what to do to make it into rev/sec.
     
  7. Sep 1, 2011 #6

    PeterO

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    The unit for revolutions per second is Hz or s-1

    so how to you change s to s-1?

    if T = 5 seconds, you get 12 per minute or 0.02 Hz
    if T = 0.1 you will get 10 per second or 10 Hz

    Soooo ....
     
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