1.3 Units, Coordinates and Numbers
        Units | Conventions | Coordinate System | Significant Digits

» Unit Systems
In Strength of Materials there are two SYSTEMS OF UNITS which you will need to be able to use and convert. The first is the U.S. Customary System (also known as: imperial, English, British, engineering, or American system). The other is the System International (International Standard System) often referred to as the metric system, or simply, S.I.. Conversion tables and modules are available by Clicking Here.

» Conventions
You will find that a CONVENTION is not always consistent from one type or set of engineers to another. The convention used in this text will be introduced as you work forward.

» Coordinate System
Generally, we will use the x-y-z Right-hand COORDINATE SYSTEM, and most problems can be considered as planar or two-dimensional. Generally speaking, in 2-d:
  • the positive x-direction points to the right, horizontal, and in the plane of the paper;
  • the positive y-direction points upward, vertical, in the plane of the paper; 
  • the positive z-axis is normal to these two, and points out of the paper. 
3-d and 2-d Cartesian Coordinate systems

» Significant Digits
SIGNIFICANT DIGITS provide a way of demonstrating the accuracy of a result. The accuracy of a calculation depends on the accuracy of the inputs (the loads, dimensions and other data such as material properties). For this class we will typically use 3 significant digits.

The main point:

A result can be no more accurate than the information from which it is derived.

Updated: 07/31/2006