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A meter is the distance light travels in a vacuum in approximately 300 millionths of a second.
Its exact conversion to US Customary units is not so simple.
This is because prior to the General Conference of Weight and Measures in 1960,
the meter was exactly 39.37 inches.
It is important to not only know the units in which a distance was measured,
but, the year it was measured as well.
After the conference in 1959 the conversion between the SI system and the US customary system became,
1 foot is exactly equal to 304.8 millimeters or .3048 meters
In order to prevent further confusion, the measurements prior to 1959 are called US Survey feet,
and those after 1959 are called International feet
For global applications in engineering, the ability to convert from one system to another and back is extremely important.
There are many ways to convert from one system of units to another.
Here are two simple methods.
The first conversion method is to multiply by 1,
also known as the conversion factor method.
The exact conversion is turned into a fraction equal to 1, which is called a conversion factor.
in which the desired units are in the numerator of the fraction.
The rules of multipication state that multiplying anything by 1 does not change the nature of what is being multiplied.
Multiply by the conversion factor to find the desired units
Make sure that the units cancel so that only the desired units are left.
Another method is the ratio method.
To use this method, the ratio of the exact conversion is set equal to
the ratio of the desired units over the known units.
In many cases the desired unit is labeled as a variable, such as x, for easier arithmetic.
Solve for the desired unit.
Make sure that all of the units cancel so that only the desired units are left.
Another key part of conversions are significant figures, which assures the accuracy of the measurement.
As seen on previous slides, the conversions were rounded from 6 digits to 3 digits
It is important to know what constitutes a good measurement,
How many significant figures are too many?
When looking at this ruler, it is clear that the measurement is over 7 feet but not quite 8.
Noting the tick marks of the ruler, it also can be deduced that the measurement is least 7 feet and 7 tenths of a foot.
However any accuracy beyond that is an estimation, and therefore questionable.
While judging the distance between increments can indicate more accuracy, this significant figure is still an estimate.
All numbers after that become absurd;
There is no way to verify that these additional numbers are valid or accurate.
They can just be crossed out.
7.72 feet is the most reasonable measurement for this example.
When converting this measurement,
remember to round the final solution to three significant figures.