# Modern Physics help.

1. Feb 7, 2008

### rock.freak667

1. The problem statement, all variables and given/known data
Analysis of potassium and argon atoms in a moon rock sample shows that the ratio of the number of stable Ar-40 atoms to the number of radioactive K-40 atoms is 10.3.
Assume that all the Ar atoms were produces by the decay of K atoms, with a half-life of 1.25x10^ years.
(i)Calculate:
a) the fraction of the original K-40 atoms remaining in the rock
b) the number of half-lives that has elapsed
c) the age of the rock.

(ii) From the answers above, deduce the age of the solar system.

2. Relevant equations

$$N=N_0e^{-\lambda t}$$

3. The attempt at a solution

Can someone guide me on how to start this?

"the number of stable Ar-40 atoms to the number of radioactive K-40 atoms is 10.3."

Now in the formula N=N_0exp(-$\lambda t$) N is the no. of radioactive atoms at time t, So I am a bit lost.

Last edited: Feb 8, 2008
2. Feb 7, 2008

### rl.bhat

If N is the no. of radioactive atoms at time t and N_o is the no. of radioactive atoms at time t = 0, then what is N_o - N? And what is (N_o - N)/N ?

3. Feb 8, 2008

### rock.freak667

Well $N_0-N$ would be the number of radioactive atoms left at time t.

But how do I find N_0 or N since I don't have the value of N at t=0 or at any value for t?

4. Feb 11, 2008

### rl.bhat

(N_0 - N) is the number of stable atoms.
In the problem (N_0 - N)/N is given. And that is equal to [e^(lambda t)] -1.