Chronology Current Month Current Thread Current Date [Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

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

• 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.