连杆机构英文文献和中文翻译(12)

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1. 10 UNITS

There are several systems of units used in engineering. The most common in the United States are the U.S. foot-pound-second (fps) system, the U.S. inch-pound-second (ips) system, and the System International (SI). All systems are created from the choice of three of the quantities in the general expression of Newton's second law

* It is unfortunate that the mass unit in the ips system has never officially been given a name such as the term slug used for mass in the fps system. The author boldly suggests (with tongue only slightly in cheek) that this unit of mass in the ips system be called a blob (bl) to distinguish it more clearly from the slug (sl), and to help the student avoid some of the common units errors listed above.

Twelve slugs = one blob.

Blob does not sound any sillier than slug, is easy to remember, implies mass, and has a convenient

abbreviation (bl) which is an anagram for the abbreviation for pound (Ib). Besides, if you have ever seen a garden slug, you know it looks just like a "little blob."

where m = mass in Ibm' a = acceleration and gc = the gravitational constant.

The value of the mass of an object measured in pounds mass (Ibm) is numerically equal to its weight in pounds force (Ib/). However the student must remember to pide the value of m in Ibm by gc when substituting into this form of Newton's equation. Thus the Ibm will be pided either by 32.2 or by 386 when calculating the dynamic force. The

result will be the same as when the mass is expressed in either slugs or blobs in the F =

ma form of the equation. Remember that in round numbers at sea level on earth:

I Ibm = llbf I slug = 32.2 Ibf I blob = 386 Ibf

The SI system requires that lengths be measured in meters (m), mass in kilograms (kg), and time in seconds (sec). This is sometimes also referred to as the mks system. Force is derived from Newton's law, equation 1.1b and the units are:

kilogram-meters per second2 (kg-m/sec2) = newtons

Thus in the SI system there are distinct names for mass and force which helps alle- viate confusion. When converting between SI and u.s. systems, be alert to the fact that

mass converts from kilograms (kg) to either slugs (sl) or blobs (bl), and force converts from newtons (N) to pounds (Ib). The gravitational constant (g) in the SI system is ap- proximately 9.81 m/sec2.

The principal system of units used in this textbook will be the U.S. ips system. Most machine design in the United States is still done in this system. Table 1-4 shows some of the variables used in this text and their units. The inside front cover contains a table of conversion factors between the U.S, and SI systems.