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.