is  Vj  feet  per  second,  and  if  w  pounds  of  water  are  ^^
discharged  per  second,  the  force  will  be  very
nearly  equal  to

g

Here  we  have  the  velocity  in  the  direction  of  the  jet  changed
from  Fj  feet  per  second  to  zero;  that  is,  there  is  a  retardation,  or
negative  acceleration,  of  V ^  feet  per  second;  consequently  the
force  is  measured  by  the  product  of  mass  and  the  acceleration,
g  being  the  acceleration  due  to  gravity.  A  force  exerted  by  a
jet  or  stream  of  fluid  on  a  plate  or  vane  is  called  an  impulse.    It


■I


474  STEAM-TURBINES

is  important  to  keep  clearly  in  mind  that  we  are  dealing  with
velocity,  change  of  velocity  or  acceleration,  and  force,  and  that
the  force  is  measured  in  the  usual  way.  The  use  of  a  special
name  for  the  force  which  is  developed  in  this  way  is  unfortunate
but  it  is  too  well  established  to  be  neglected.
If  the  plate  'or  vane,  instead  of  remaining  at  rest,  moves  with
the  velocity  of  V  feet  per  second,  the  change  in  velocity  or  negative
acceleration  will  be  Fj  —  F  feet  per  second,  and  the  force  or
impulse  will  be

P  ^^(V,  -  F).
g

This  force  in  one  second  will  move  the  distance  F  feet  and  will
do  the  work

J  (F,  -  F)  F  -  J  iV,V  -.  P)  .    .    .  (276)

foot-pounds.
