Engineering Guesstimations

The word guesstimation according to Wikipedia is an "estimate made without adequate or complete information". It is also sometimes referred to as "educated guess" or in a more scientific terms "order of magnitude analysis". Many of us use this method in their everyday life without realizing it, because we all have to make decisions about things we do not fully understand or don't have complete information about. The subject is also marginally taught in many science and engineering courses, but as it comprises a significant amount of "feeling", many people think of it as compromising the solid mathematical foundations of those courses. Incredibly many people are lead to believe that science and engineering are subjects, where little intuition or creativity is required, and these people could not be more wrong. The goal of this book is to create a collection of examples of how using some common sense, some understanding of math and physics and some simple, "back of the envelope", calculations, we can make reasonable estimates about almost anything starting from everyday life and going to complex engineering problems. The book is separated in different chapters, depending on the area of application, and except for the introductory chapters does not have to be read in exact order. It is also meant to be fun, and to be seen as brain teaser. One of the best ways to kill the long hours traveling on a plane or bus is to look at the surrounding landscape and come up with interesting problems to solve.

Part 1: Introductory Examples

1. General

Engineering is the art of providing usable solutions to problems. Many of the problems for which engineers attempt to provide solutions have so many variables as to daunt the solver. A guess, often based on previous experience, is made to simplify the variables and allow a solution to be proposed. This is the art of guesstimation -- also known as the "SWAG" (Scientific Wild Ass Guess). This is a valid engineering technique rarely taught in schools.

Many times we will have data on (say) "average forces" action on an operation. We will not know the difference between maximum and average until we have done testing. It is often reasonably correct to assume that the maximum force will be some multiple of the average force (3X is often about right) and using that as an initial basis is a common guesstimation. One area or exception to the nominal maximum force = 3X average force rule lies with pyrotechnically actuated systems. In these instances a factor of 8X is not uncommon. This is a "practice and experience" arena.