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How long has the tree been dead?

  1. Jan 5, 2015 #1
    1. The problem statement, all variables and given/known data
    A tree contains a known percentage p0 of a radioactive substance with half-life tau. When the tree dies the substance decays and isn't replaced. If the percentage of the substance in the fossilized remains of such a tree is found to be p1, how long has the tree been dead?

    2. Relevant equations
    I don't know what formula to use but I know that I have to solve for t in the formula. So I just need to know the formula.

    3. The attempt at a solution
    p1=(1/2)^(-t/tau)
    This is all I can come up at this moment. I don't know if this is the right formula.
     
  2. jcsd
  3. Jan 5, 2015 #2
    You can use a positive exponent with (1/2) as your base or you can use a negative exponent with 2 as the base. And p0 should be in there somewhere.
     
  4. Jan 5, 2015 #3
    I got it! Thanks for the help!
     
  5. Jan 5, 2015 #4

    epenguin

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    If you have got it why not tell us the answer? The last poster who told me they'd got it, when I asked, hadn't, and I think this is true of quite a lot who say that.
     
  6. Jan 6, 2015 #5
    The answer is (tau)*ln(p0/p1)/ln(2).
     
  7. Jan 7, 2015 #6

    epenguin

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    Right
     
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