Physics for Scientists and Engineers 3E by R. Knight, P1.33

Estimate the average speed with which the hair on your head grows. Give your answer in both m/s and µm/hour. Briefly describe how you arrived at this estimate.

Solution:

My barber trims about an inch of hair when I visit him every month for a haircut. The rate of hair growth is

$\displaystyle \left(\frac{1\:inch}{1\:month}\right)\left(\frac{2.54\:cm}{1\:inch}\right)\left(\frac{10^{-2}\:m}{1\:cm}\right)\left(\frac{1\:month}{30\:days}\right)\left(\frac{1\:day}{24\:hr}\right)\left(\frac{1\:hr}{3600\:s}\:\right)$

$\displaystyle =9.8\times 10^{-9}\:m/s$ ☚

$\displaystyle \left(9.8\times 10^{-9}\:m/s\right)\left(\frac{10^6\:\mu m}{1\:m}\right)\left(\frac{3600\:s}{1\:h}\right)=40\:\mu m/hr$ ☚