In order to be able to compare dimensions of something, such as length or weight, it is important that the quantity is measured or described in the same units.
There are several famous examples of unit conversion mistakes that have led to disasters, such as the metric conversion disaster that resulted in a NASA orbiter drifting off-course. Therefore, understanding unit conversion and how to check one's work can help reduce frustrating errors or even potential disasters!
How to Convert Grams to Pounds
First, grams is a unit of measure for mass, and pounds is a unit of measure of force. Often it describes the force of gravity on an object with some mass. Not only are grams and pounds different units, but they are also different quantities all together.
Mass is the amount of matter in an object, whereas a force is determined by the acceleration of that object. Don't forget, we are constantly rotating around the Earth's axis, and the Earth around the sun; this results in an acceleration that gives us weight on Earth. This also means that in different parts of the solar system, an object with some mass, m1, can weigh more or less depending on the local acceleration of gravity.
In the Imperial system, the acceleration of gravity, a, must be defined in units of feet/sec2, and the mass, m, in slugs, in order to use for formula F = m a to arrive at a net force, F, in pounds. In the metric system, for a mass in grams, and acceleration in meters/sec2, the resulting force has units of newtons.
However, due to the knowledge of an average acceleration of gravity on Earth, a simple conversion factor between grams and pounds exists: 1 pound = 453.59 grams. The nuance of units is embedded into this conversion factor.
The General Concept of Unit Conversion
In order to convert one unit to another, we need to be able to transform the quantity into another unit, without changing the quantity represented. Therefore, the most important part of unit conversion is knowing the conversion factor between two units. For example, there are 12 inches in 1 foot, and 100 centimeters in 1 meter; these lengths are equivalent, therefore 12 inches = 1 foot is an accurate equation.
The reason knowing the conversion factor is the most important, is because it is a form of the number 1; and multiplying a number by 1 does not change the quantity. In the case of conversion, the conversion factor is the multiplicative factor that equals one.
Conversion With Metric Prefixes
We have already covered the grams to pounds conversion: 1 pound = 453.59 grams. However, how can we convert kilograms to pounds?
Quite often, quantities in the metric system are described by prefixes that are used to signify the order of magnitude of the number, such as millimeters, microseconds or picograms. The standard unit of mass in the metric system is a gram; therefore, a kilogram is 1,000 grams, where the prefix kilo- means 103. So we immediately know the conversion from kilograms to pounds: 0.453 kg = 1 pound.
Another unit of mass in the imperial system is an ounce, which is 1/16 of a pound. Therefore, to convert ounces to grams, we can use our previously known conversion factor and divide it by 16, resulting in: 1 ounce = 28.35 grams.
The prefix system does not work in imperial units. Instead, small quantities are often rewritten in scientific notation.