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Don Y[_3_]

On 10/23/2015 10:21 AM, wrote:
On Fri, 23 Oct 2015 10:05:43 -0700, Don Y
wrote:

How many SECONDS will it take traveling at 10km/hour to travel 100km?

It helps a lot when you remember 100km/hr is about 60mph (a mile a
minute)

But that's the point -- familiarity teaches you how to make these
approximations and which approximations are "robust" (for a given
type of calculation).

For highway driving, I tend to average 60mph. I'm in no rush and
not keen on "pushing" the car any harder than necessary -- depart
10 minutes earlier instead of driving 10mph faster!

It's second nature for me to glance at the clock, convert miles to
minutes and arrive at an ETA (in my head). Thereafter, use the
clock as an indication of how much "longer" (in time and miles)
I have to drive. If I'm within 10%, I'm delighted -- no need
to drag out a calculator or rely on a navigation system to provide
me with updates, etc.

If I'm about to paint the roof and need to make a decision as
to whether or not I should lug the 5G container of paint up the ladder
with me -- or split it into smaller "loads", it's relatively easy to
figure 1G = 4qt = 8pt ~= 8lbs. So, 5G paint is AT LEAST 50 pounds
(ignoring the weight of the solids dissolved therein and the bucket
itself). The fact that it's not sold in liters -- which can "easily"
be converted into grams (Q: how many kg can you carry? do you know
this off-hand? Or, did you have to start with english units and
convert to metric in your head??) -- doesn't interfere with my
ability to estimate the weight.

When I was younger, I used to design speaker enclosures. Part of
that task is designing the crossover network (to route specific
frequency ranges to each of the different sized "drivers" in the
enclosure). There, "2 * pi * R" is a common factor in many calculations.
Solve this *once* and you'll never use a calculator again to solve it!

Just like 1G = 8lbs.

How many atoms in 12g of C-12? (Why 12g and not *10*?? : )
Unless you're using this number often, you won't have it
(or even an approximation of it) "handy". OTOH, if you *do*
use it often, that familiarity makes it seem far less of an
"unusual" quantity!