# Zwiebach quick calculation 8.7

1. Sep 28, 2007

### ehrenfest

1. The problem statement, all variables and given/known data
I am trying to do QC 8.7. If I use the equation given on page 127, l = 2/pi E/T0, I do not think that produces right answer. Is there another equation I should use
?

2. Relevant equations

3. The attempt at a solution

2. Sep 29, 2007

### Jimmy Snyder

You should use equations (3.88) on page 54 but replace the equation for G with the equation for $\alpha'$. Then follow the technique on that page.

3. Sep 29, 2007

### ehrenfest

Sorry, can you explain what you mean replace the equation for G with the equation for alpha' (8.63 I assume)? G and J/hbar do not even have the same units.

4. Sep 30, 2007

### Jimmy Snyder

There are three equations labeled (3.88). Replace all of the P subscripts with S. Toss out the equation that defines G and use equation (8.63) in its place. Express $\sqrt{\alpha'}$ in terms of ${l}_S, {m}_S,$ and ${t}_S$.

5. Sep 30, 2007

### ehrenfest

OK. I see why that works. My only concern is how do you know that you can replace the p subscripts with s subscripts. Those relations are only true in the Planckian system of units, right?