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A generation is about one-third of a lifetime.

  1. Jun 25, 2014 #1
    1. The problem statement, all variables and given/known data

    A generation is about one-third of a lifetime. Approximately how
    many generations have passed since the year 0 AD?

    Let me first say that this is my first time taking physics and this is all new to me so I appreciate all the help.


    2. Relevant equations

    here is the answer I found by googleing but I don't quite understand it. I was wondering if someone could explain it to me.

    history x (10^11s / history) × (1 generation / 1/3 lifetime) × (0.5 lifetime / 10^9s) = 150


    3. The attempt at a solution

    How I approached this problem was that I chose 70 to be an average person's lifetime then I divided that by 3 to find what a generation is and then divide 2015 by that number which is not what the answer says. I think I just don't get the question.
     
  2. jcsd
  3. Jun 25, 2014 #2

    SammyS

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    Looking at that "Googled" solution:

    What time interval are they assuming for the "history" of whatever?

    What time interval are they assuming for a "lifetime" ?
     
  4. Jun 25, 2014 #3
  5. Jun 25, 2014 #4

    SammyS

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    Look at their expression.

    (1011s / history) is a conversion factor for units of "history" to seconds.

    How many years are there in 1011 seconds?


    Similarly, (0.5 lifetime / 109s) is a conversion from seconds to a "half a life(time)".

    How many years are in 109 seconds?
     
  6. Jun 26, 2014 #5
    A "Generation" is commonly 25 years. The Question is asking how much (unknown variable) multiplied by 25 gives 2015?
     
  7. Jun 26, 2014 #6

    SammyS

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    By the way, I should have said:

    Welcome to PF!


    AND --

    Your method is perfectly reasonable.

    I was merely trying to get you to understand some of the quantities in that expression you found elsewhere.
     
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