| Basic Thermodynamics ~ J. Pohl © | www.THERMOspokenhere.com (35-A345) |
When the English Engineering Unit System was specified (~1824) mass was set to be a dimension and defined as a certain quantity of matter. That quantity of matter was specific; scales and means of extension of quantification of mass were built. The "founding mass" was a cube of metal kept in a vault in France with replicas for distribution.
In those times "force" was an idea more subtle than mass. One immediate idea of force was the "effort" required by someone to hold some mass in his hand. Some clever lads supposed force to be a dimension as is mass. Going further, they gave quantification to force. They defined a "unit force" (one pound force) to equal in magnitude, "that force" required to support a unit mass (one pound mass) at sea level.
Thus to support or carry ten pounds mass of potatoes (in uniform motion) a person must exert a vertical force of ten pounds force. When we apply Newton's 2nd Law to the event of uniform motion (in the vertical direction, 0 = ΣF) a necessary identity among units (for potatoes and everything) results.
| (1)
Notice the "negative sign" belongs to the vector part of the gravity force. |
This equation seems peculiar because all quantities are known. What the equation reveals is: "Engineers who chose mass, length, time, and force as independent dimensions, then ascribe their units arbitrarily, will discover: i) the dimensions are not independent and ii) their units are related by a constant of proportion (or proportionality constant). In the case of the English Engineering System, this constant is 32.2 (to sufficient accuracy for our purposes).
Equation (1) yields the fact that in the English engineering system (where F, m, L and t defined) the relationship among units is:
| (2)
Force, mass, length and time are related. Should one arbitrarily specify units to all four; a proportionality constant arises. |
In contrast, the metric system defines only mass, length and time as dimensions. Units specified are the kilogram, meter and second. Force is left to be a "derived" entity. Were metric units used in the above consideration we would obtain:
| (3)
In the MKS system units for mass, length and time are specified. The unit for force is derived from them; thus the proportionality,"1." |
With length, the metric system is superior. The English Engineering System was pragmatic in locking the unit mass to be a unit force. It sure facilitates hydraulic engineering calculations. Both Metric and English Engineering Units are used in the United States.
When English Engineering units were specified (~1824) mass was set to be a dimension and defined as a certain, specific quantity of matter. The "founding mass" was a cube of metal kept in a vault in France with replicas for distribution. In those times "force" was an idea more subtle than mass. One immediate idea of force was the "effort" required by someone to hold or support some mass in his hand. Some clever lads supposed force to be a dimension as is mass. Going further, they gave quantification to force. They defined a "unit force" (one pound force) to equal in magnitude, "that force" required to support a unit mass (one pound mass) at sea level.