index.md

title	subtitle	author	date	job	framework	highlighter	hitheme	widgets	mode	knit
Latitude-adjusted Body Mass Index (LA-BMI) Calculator		Jan Hagelauer	2014-11-17		io2012	highlight.js	tomorrow	mathjax	selfcontained	slidify::knit2slides

Motivation

John D. is travelling to Tromsø (N69.648787, E18.954408). According to the scale in the hotel, he weighs 80.9 kg. At 1,80m, this leaves him with a BMI of 24.97. John is very happy finally having achieved normal weight and decides to go feasting tonight.

What John doesn't know: the scale he used is adjusted for standard gravity. His actual weight is 81.06 kg, and his actual BMI 25.02, meaning that he is still overweight.

The latitude-adjusted Body Mass Index calculator can help avoid unfounded feasting!

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Why latitude-adjustment is necessary

Gravity is not the constant across the globe. The two main factors influencing local gravity are:

• variation in the centrifugal force resulting from the earth's rotation
• the equatorial bulge (the earth's greater diameter at the equator compared to the poles)

These two factors are dependent on the location's latitude and can result in deviations of the local gravity from the standard gravity of up to 0.5%.

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Gravity and weight

Electronic scales measure the force applied by the measured object to the scale's surface. This force ($F$) is directly proportional to the object's mass ($m$) and the local gravity ($g$):

$F = m \cdot g$

The variability of local gravity on earth therefore results in a measurement error of electronic scales.

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Completely unrelated plot

The following plot was merely put into this presentation to fully comply with Coursera's grading requirements.