Class 2 – Thursday, September 8, 2011

Busy day.

Started with the survey – if you are in the course and didn’t answer it, please let me know.

Asked “how many seconds have you been alive?” and polled the class for answers. A few said thousands, a few said millions, most billions, a few trillions. Then we estimated with curly arithmetic: numbers have just one significant digit. So there are 400 days in a year, 3*4 = 10 (written with curly equals sign, for “approximately equal), 7*7 = 50. That way you can find the number of seconds in an hour (60*60=4000), a day (100000, using 25 hours per day), a year (4 with 2+5=7 zeroes, or 40 million). Then we knew most people had guessed correctly (billions).

I used the thousands, millions, billions, trillions list to introduce kilo, mega,  giga, tera. Having started on metric prefixes I digressed to actual metric units (for length).  Followed the text (chapter 2) in pointing out everyday approximations (“a meter is about a yard:). Google calculator (introduced for the first time) gives 39.xxxxx… inches, which led to a discussion of precision, useless accuracy. We also saw why “a meter is about 10% longer than a yard” is often more useful than “a meter is about three inches longer than a yard” – you could say right away that a 100 meter race was about 10% longer than a 100 yard race.

Note for class next time: interesting NY Times article on Google’s electricity use / carbon footprint, at I may decide to start a discussion of units there, even though power/energy doesn’t appear in the book until Chapter 6.

Ended with a few suggestions about how to read problems that ask whether numbers make sense, in the context of the hw problem on the number of internet ads people are exposed to.

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