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The Back of the Envelope - Purcell
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A column in the American Journal of Physics.

The Back of the Envelope
Purcell, Edward M.
A column in the American Journal of Physics.
Jan 1993 - July 1984. (or more?)

"A column called "The Back of the Envelope", for which I serve as editor, was a monthly feature of this journal from January 1983 through July 1984. Three new "order-of-magnitude" problems were presented each month, along with solutions of the problems of the preceding month. All told, there were 57 problems, the discussion of which fills something like 150 column-inches of text in Vols. 51 and 52 of the journal." [Purcell, Letters to the Editor, July 1985, v53n7 p615]

The American Journal of Physics (Am. J. Phys.) is a monthly publication of the American Association of Physics Teachers.

Columns:

• 1983 Jan (v51n1) p11 - `One page round-number handbook of physics'
Corrections and notes in n51p299, n51p1068, n52p301.
• 1983 Feb (v51n2) p107 - solutions to:
- "How big an asteroid could you escape from by jumping?"
- "If a tea cup could be made impervious to nucleons, how many neutrons could it hold at absolute zero, without running over? How many protons?" (typo correction in v51n4)
- "Rubber gloved, you are dribbling on an infinite court a basketball that is charged to 10 kV. How much energy is emitted as electromagnetic radiation, per bounce?"
• 1983 Mar (v51n3) p205 - solutions to:
- "Can a helium balloon lift the tank the helium came in?"
- "At room temperature in still air, how long can a pencil remain balanced on its point? How long at absolute zero?"
- "Estimate the length of the longest free path experienced by a nitrogen molecule in the lowest kilometer of the Earth's atmosphere during the last billion years."
• 1983 Apr (v51n4) p299 - solutions to:
- "Would an electron exposed only to solar radiation pressure and gravity be expelled from the solar system?"
- "What is the probability that a straight line drawn from the Earth in an arbitrary direction (excluding towards the sun) will hit a star in this Galaxy? Assume 1011 solar type stars and 10 kpc to Galactic center."
- "A ___ of water contains about as many molecules as there are ___s of water in all of the oceans. Is the correct substitution fro the blank drop, teaspoon, tablespoon, cup, quart, gallon, barrel, or ton?"
• 1983 May (v51n5) p391 - solutions to:
- "What is the largest number of airplanes that could be in blindly random flight, over the continental United States and between 15 000 and 30 000 ft altitude, without risking more than one fatality per billion passenger miles as a result of midair collisions?"
- "If the iron in the Earth's core were made into a wire as long as the radius of the visible universe, what would be the diameter of the wire?"
- "The "Planck mass" is defined as sqrt(h_bar c/G). As energy, what is this worth in gallons of gasoline?"
• 1983 June (v51n6) p494 - solutions to:
- "A star like the sun is just about visible to the naked eye if it is 50 light years away. If you want to put up an Earth satellite in the form of a reflecting sphere that can be seen as it passes over at night, what is the smallest diameter it may have?"
- "A paper cup on the table is empty, except for the air it contains. The surrounding air is perfectly still. What is the best guess for the time it will take diffusion to replace half the air molecules in the cup with new molecules: a second, a minute, and hour, or a day?"
- "Define a "sun day" as the ammount of energy received by the whole Earth in one day from the sun. The entire U.S. coal reserves have been estimated at 10 sun days. How many cubic miles of coal does that amount to? How does it compare with the amount of carbon in the Earth's atmosphere, of which about 1 molecule in 3000 is CO2?"
• 1983 July (v51n7) p586 - solutions to:
- "If the energy stored in the Earth's magnetic field could be drained with no losses and used to supply the world demand for electrical power, how long would it last?"
- "A guitar string tuned to G (392 Hz) sags in the middle, owing to its own weight, by what distance?"
- "If the electrons in one raindrop could be removed from the Earth without removing the protons, by how much would the potential of the Earth be increased?"
• 1983 Aug (v51n8) p685
• 1983 Sep (v51n9) p780
• 1983 Oct (v51n10) p874
• 1983 Nov (v51n11) p970
• 1983 Dec (v51n12) p1068-69 - Has corrections and comments.
• 1984 Jan (v52n1) p8
• 1984 Feb (v52n2) p107
• 1984 Mar (v52n3) p203
• 1984 Apr (v52n4) p301
• 1984 May (v52n5) p394
• 1984 Jun (v52n6) p490
• 1984 Jul (v52n7) p681
• 1985 Jul (v53n7) p615 - A letter to the editor regards getting the column indexed (it hadn't been), and commenting on the series.

## Notes...

I know of no other installments, but my search has been quite haphazard.

The Am.J.Phys.'s index seems to have seriously mangled entries for this column (as of 1997.Apr.25). I took the information above directly from journal photocopies.

 A View from the Back of the Envelope Comments encouraged. - Mitchell N Charity

```Notes:

Doables:

History: