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MIT OpenCourseWare http://ocw.mit.edu 6.055J / 2.038J The Art of Approximation in Science and Engineering Spring 2008 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms .
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Back-of-the-envelope numbers Symbol What Value Units π pi 3 G Newton’s constant 7 · 10 11 kg 1 m 3 s 1 c speed of light 3 · 10 8 m s 1 k B Boltzmann’s constant 10 4 eV K 1 e electron charge 1 . 6 · 10 19 C σ Stefan–Boltzmann constant 6 · 10 8 W m 2 K 4 m sun Solar mass 2 · 10 30 kg R earth Earth radius 6 · 10 6 m θ moon / sun angular diameter 10 2 ρ air air density 1 kg m 3 ρ rock rock density 5 g cm 3 ~ c 200 eV nm L water vap heat of vaporization 2 MJ kg 1 γ water surface tension of water 10 1 N m 1 a 0 Bohr radius 0 . 5 Å a typical interatomic spacing 3 Å N A Avogadro’s number 6 · 10 23 E fat E bond combustion energy density typical bond energy 9 4 kcal g 1 eV e 2 / 4 π 0 ~ c fine-structure constant α 10 2 p 0 air pressure 10 5 Pa ν air kinematic viscosity of air 1 . 5 · 10 5 m 2 s 1 ν water kinematic viscosity of water 10 6 m 2 s 1 day 10 5 s year π · 10 7 s F solar constant 1 . 3 kW m 2 AU distance to sun 1 . 5 · 10 11 m P basal human basal metabolic rate 100 W K air
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