due  to  its  position.  This  energy  is  known  as  static  or  poten-
tial because  it  is  stationary.  When  the  weight  is  released
and  falls,  the  energy  is  called  kinetic  energy,  that  is,  energy
released  or  due  to  motion.  Potential  energy  is  sometimes
called  energy  of  stress;  for  example,  the  spring  in  a  spring
balance  is  under  tension  when  a  weight  is  suspended  from
the  hook.  Of  course  in  all  cases  the  weight  times  height
equals  the  energy  of  the  body,

E  (potential)  =  W  X  H,

although  sometimes   the   velocity  is   given   instead   of  the
height,    .Then  the:

WX  V2
E  (kinetic)  =  -

2  Force  of  gravity

or  WV2

K  =

2<7

This  is  obtained  by  substituting  in  the  formula  for  energy,
for  the  height  its  value

(Velocity)2
2  Force  of  gravity


64  APPLIED  SCIENCE

or  F2

20

Impulse  equals  force  times  time.     Impulse  may  be  defined
as  the  force  multiplied  by  the  length  of  time  it  acts.
Weight  X  Velocity
Momentum  =  —

32

The  energy  stored  in  a  revolving  fly-wheel  is  kinetic,  and
is,  therefore,  represented  by  the  formula

WV2

K  =

20

W  stands  for,  or  is  equal  to,  the  weight  of  the  wheel  in  pounds,
g  for  32,  attraction  force  of  gravity,  V  for  velocity  of  a  de-
finite point  in  the  iron  in  feet  per  second.  At  this
definite  point  the  whole  weight  is  assumed  to  be
collected.
