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Q: Astro Physics ( Answered,   1 Comment )
Subject: Astro Physics
Category: Science > Astronomy
Asked by: braggy-ga
List Price: $3.50
Posted: 17 Apr 2004 13:09 PDT
Expires: 17 May 2004 13:09 PDT
Question ID: 331811
A substance has a 1/2 life of 12 days.  At day 0 you have 28grams of
the material.  How much will be left. after 36 days pass?

Please explain how you reached your answer Be explicit.
Subject: Re: Astro Physics
Answered By: eiffel-ga on 18 Apr 2004 06:47 PDT
Hi braggy,

Because 36 days is an integer multiple of the half-life, we can work
out the answer by taking one half-life period at a time:

   At day  0, we have 28 grams
   At day 12, we have half of this (14 grams)
   At day 24, we have half again (7 grams)
   At day 36, we have half again (3.5 grams)

Alternatively, we can use a general formula for exponential decay
which will allow us to work out the amount of substance remaining
after any number of days.

The formula for exponential decay is

   s = s0 * e ^ (k * t)


   s is the amount of the substance at any time t
   s0 is the amount of the substance at day 0
   e is the base of the natural logarithms (2.718281828459...)
   k is some constant showing how quickly the substance decays
   t is the time since day 0
   * represents multiplication
   ^ represents exponentiation

First we must find the value of 'k' for this substance. What we do
know is that after 12 days, half of the substance remains (14 grams).
We plug these values into the formula:

   14 = 28 * e ^ (k * 12)

Divide both sides by 28:

   0.5 = e ^ (k * 12)

Take the natural logarithm of both sides:

   ln 0.5 = ln(e ^ (k * 12))

Cancel the 'ln' and the exponentiation on the right-hand-side, which
we can do because e^(ln x)=x for any x:

   ln 0.5 = k * 12

Solve for 'k':

   k = (ln 0.5)/12 = -0.057762265046662

Now, having found 'k', we can use our original formula to find the
amount of substance remaining after 36 days:

   s = 28 * e ^ (-0.057762265046662 * 36)

     = 3.5 grams

A worked example in more detail can be found here:

If you don't understand any part of my explanation above, please ask
for more details by requesting a clarification.

Google Search Strategy:

"exponential decay" formula

Subject: Re: Astro Physics
From: corwin02-ga on 17 Apr 2004 14:05 PDT
Ending amount = (beginning amount/2N) where 

N= elapsed time/halflife 


N= number of half lives

ergo 3.5 gr or 12.5 percent 

28 <12> = 14
14 <12>=  7
7 <12> = 3.5

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