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*From*: John Denker <jsd@av8n.com>*Date*: Mon, 18 Jan 2010 17:01:21 -0700

The following are mostly in the category of useful

"rules of thumb". In some cases checking the

validity is an exercise in unit conversions (but

sometimes not).

Speed of light = 1 foot per nanosecond.

Inverse speed of sound = 5 seconds per mile.

Room temperature = 25 meV (milli electron volts).

Inverse acceleration of gravity in aviation units:

9 feet per knot, per (hundred knots).

Another rule of thumb, literally: A thumb at arm's

length subtends 4 degrees. Seriously useful in

aviation.

http://www.av8n.com/how/htm/landing.html#fig-thumb-angle

Also useful in astronomy.

Comparing sizes: Earth is to ping-pong ball as

ping-pong ball is to atom.

Suppose you have a capacitor made of two cubes the

size of sugar cubes (1 cm by 1 cm) placed 1 mm apart.

What is the capacitance?

Suppose you have a one-turn inductor roughly the

size and shape of a stubby trash barrel: 1 m

in diameter and 1 m tall. What is the inductance?

What is the polar circumference of the earth in

meters?

Hint: Round number +- 200 ppm i.e. +- a few meters.

Hint: Not a coincidence.

How big is one metric ton of water?

Feynman's number: Calculate 1/243 to 30 decimal places

or more.

**Follow-Ups**:**Re: [Phys-l] Fun/cool unit conversion example?***From:*curtis osterhoudt <flutzpah@yahoo.com>

**References**:**[Phys-l] Fun/cool unit conversion example?***From:*Jeff Loats <jeff.loats@gmail.com>

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