Re: [Phys-l] Fun/cool unit conversion example?

[John Denker <jsd@av8n.com>]

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.