1c. Units, Coordinates and Numbers  (SSES Ch. 1.7-1.9)
• Units • Coordinate System • Significant Digits 		Back | Index | Next
Unit Systems
In the United States, there are two unit systems that you will need to be able to use and convert between. 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) or S.I. system. Conversion tables and modules are available by clicking here, or at many online sources.
Coordinate System
Generally, we will use the x-y-z right-hand rectangular coordinate system, and most problems can be considered as planar or two-dimensional. Generally, in 2D:
  • the positive x-direction points to the right, is horizontal, and in the plane of the paper;
  • the positive y-direction points upward, is vertical, in the plane of the paper; 
  • the positive z-axis is normal to these two, and points out of the paper. 
3D and 2D cartesian coordinate systems

Significant Figures (Digits)

Significant digits indicate the precision of a numerical value. A number like 45.67 has 4 significant digits; 45.7 has 3 significant digits.

A result can be no more precise than the information from which it is derived. The precision of a calculation depends on the precision of the inputs (the loads, dimensions and other data such as material properties). In most texts, including this one, data is typically assumed to be good to 3 significant digits. In experiments and real-life situatiions, 2 digits may be all that is justifiable.

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Updated: 05/16/09 DJD