331.  The  Actmi  Baeru  {A.  M,,  547,  589)  of  a  moving  body  is
the  work  which  it  is  capable  of  performing  against  a  retarding
resistance  before  beine  brought  to  rest,  and  is  equal  to  the  eneigy
which  must  be  exerted  on  the  body  to  bring  it  itom  a  state  of  rest
to  its  actual  velodiy.  The  value  cf  that  quantily  is  Reproduce  of
the  Ufeight  of  the  body  into  the  height  fiom  which  it  must  faU  to
acquire  its  actual  velocity;  that  is  to  say,

2g  ^'-^

The  toted  actaal  energy  of  a  system  of  bodies,  each  moving  with
its  own  velocity,  is  denoted  by

~Tr' ^-^

and  when  those  bodies  are  the  pieces  of  a  machine,  whose  velocities


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374  DYNAUICS  OF  MACHINERY.
