Hello,

The problem is given:

I first found v0(final) from:

$\displaystyle v0=sqrt(\frac {2*g*h}{lambda*\frac {l}{d}+1}) $

$\displaystyle v0=sqrt(\frac {2*9.81*18}{0.03*\frac {400}{2}+1}) $

v0 is 7.1 m/s

90% of v0 is v= 6.39 m/s.

Time to reach v=6.39 m/s:

$\displaystyle t=\frac {l*v0}{2*g*H}*log(\frac {v0+v}{v0-v})$

$\displaystyle t=\frac {400*7.1}{2*9.81*18}*log(\frac {7.1+6.39}{7.1-6.39})

$

$\displaystyle t=10.28 $ s

But the answer key says it is 1 mins 20 secs

Where may I go wrong?

Thanks a lot