1 / 9

Guesstimation: Solving the world’s problems on the back of a cocktail napkin

Guesstimation: Solving the world’s problems on the back of a cocktail napkin. Larry Weinstein Old Dominion University. Princeton University Press, 2008. Why estimate?. Answers lead to actions. Answers fall in one of three ‘Goldilocks’ categories: Too big Too small

ernie
Download Presentation

Guesstimation: Solving the world’s problems on the back of a cocktail napkin

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Guesstimation: Solving the world’s problems on the back of a cocktail napkin Larry Weinstein Old Dominion University Princeton University Press, 2008 Jefferson Lab, 17 Sept 2008

  2. Why estimate? Answers lead to actions. Answers fall in one of three ‘Goldilocks’ categories: Too big Too small Just right (in the middle) ‘Too big’ or ‘Too small’ lead to obvious actions. ‘In the middle’ requires more thought (and perhaps even actual calculation). “An approximate number sense is essential to brute survival”, Gut Instinct’s Surprising Role in Math, NY Times, 16 Sept, 2008. Jefferson Lab, 17 Sept 2008

  3. How to estimate (almost) anything • Write down the answer (to within a factor of 10). If you can establish lower and upper bounds, take the geometric mean for your answer. How many clowns can fit in a VW Bug? More than 1 and less than 100. Estimate = This ensures that your estimate will be within a factor of 10. • If you can’t write down the answer, break the question into smaller pieces and go to step 1 Dare to be imprecise! Jefferson Lab, 17 Sept 2008

  4. How much individual trash do Americans generate every year? Reality: 2108 tons of Municipal Solid Waste How much landfill space will we need for the next century’s trash? V = 2108 tons/yr  102 yr / (1 ton/m3) > 21010 m3 Use V = 1011 m3. Landfill is 100 m high and 109 m2 = 103 km2 in area That is Los Angeles or Virginia Beach. Jefferson Lab, 17 Sept 2008

  5. Paper or plastic? What is the mass of all the flimsy plastic bags you use each year? How does it compare to the mass of gasoline you burn each year? Jefferson Lab, 17 Sept 2008

  6. If the Sun were made of gerbils, the Earth would be incinerated. Compare the power density of mammals and the sun.* Humans: P = 2500 kCal/day  (4103 J/kCal) / 105 s/day = 100 W Power/mass = 100 W / 100 kg = 1 W/kg Sun: P = 103 W/m2 4(1.51011 m) = 31026 W M = 21030 kg (from orbital period of Earth) Power/Mass = 10-4 W/kg *Thanks to Brian Quinn for the question Jefferson Lab, 17 Sept 2008

  7. What is the energy density of gasoline? Reality = 45 MJ/kg What is the energy density of batteries? Reality = 0.2 to 0.5 MJ/kg What is the energy transfer rate of gasoline (when fueling your car)? Jefferson Lab, 17 Sept 2008

  8. How much power can a windmill produce? Reality: 92 m blade diameter wind turbine in a 11 m/s wind would generate 2 MW (Danish Wind Energy Assc) But: Power  wind speed cubed. Very variable! Jefferson Lab, 17 Sept 2008

  9. Summary • Write down the answer. • Break up the question if you need to. • Dare to be imprecise! • Estimate to within a factor of 10 • Take the geometric mean of lower and upper bounds • Remember a few key numbers: • 1 m/s ~ 2 mph • Water density = 1 ton/m3 = 1 kg/liter • Avogadro’s number = 61023 • US population = 3108 Jefferson Lab, 17 Sept 2008

More Related