## 2010-10-14

### Going the distance

How far is it to the grocery store? What is the top speed of your favorite vehicle? How tall is the dinosaur chasing you?

We use units of distance/length everywhere; it's an essential dimension for other measures as well (e.g. speed = distance/time, area = length A * length B, etc.). Establishing a foundational understanding of SI distance units is paramount to a strong basis in Metrication as a whole.

In opposition to our efforts to see metric in everyday use is the fact that - when it comes to distance/length - Americans are unilaterally presented with US customary units. Some examples: the speed limit signs are given in miles/hr, our construction experts deal in feet and inches, we hike "14ers" (mountains approximately 14,000 ft in height), we watch American football players on the 100 yard field.

Whereas we see feet, inches, acres, miles, yards, and even the rare league; the SI measures of distance only have ONE...the meter. This is a distinct advantage over customary units.

As I presented in the post about temperature, using "key conversion points" is a helpful method. Here are some that I use, but I encourage you to find/create your own as well.

Distance Key Conversion Points (miles / kilometers)
* 100 mi ≈ 160 km
+ 88 mi ≈ 141 km
* 75 mi ≈ 120 km
* 62 mi ≈ 100 km
55 mi ≈ 88 km
*50 mi ≈ 80 km
40 mi ≈ 64 km
30 mi ≈ 48 km
*20 mi ≈ 32 km
*10 mi ≈ 16 km
*5 mi ≈ 8 km
3 mi ≈ 5 km
1 mi ≈ 1.61 km
Once again, I've placed asterisks next to the ones I think are nice to remember well. (I also placed a + next to the one that Marty and/or Doc Brown should watch for.)

Length Key Conversion Points (meters / feet)
*1000 m ≈ 3280 ft
*305 m ≈ 1000 ft
100 m ≈ 328 ft
*25 m ≈ 82 ft
10 m ≈ 32 ft
*5 m ≈ 16 ft
2.5 m ≈ 8 ft
2 m ≈ 6.6 ft
*1.5 m ≈ 5 ft
1 m ≈ 3 ft
Remembering the 1000 = 3280 rule sometimes makes other mental math easier. The 305 (roughly 300) to 1000 rule can be convenient in conversion; for example, "People enjoy hiking '14ers' in Colorado; peaks which are 14000 feet high or about... 4200 meters tall [ignoring the 5s and performing mental math... 300 x 10 = 3000, 300 x 4 = 1200, 3000 + 1200 = 4200].

Don't forget to use the prefixes where convenient!