text: be an unbalance of force to provide the ac- 
  celeration. Since there is only air within the 
  tube, the unbalance of force is provided by 
  the static pressure at station 1 being greater 
  than the static pressure at the constriction, 
  station 2. 
  
  (2) The total energy of the air stream in 
  the tube is unchanged. However, the air- 
  .’ stream energy may be in two forms. The 
  airstream may have a potential energy which 
  is related by the static pressure and a kimtic 
  energy by virtue of mass and motion. As 
  the total energy is unchanged, an increase in 
  velocity (kinetic energy) will be accompa- 
  nied by a decrease in static pressure (poten- 
  tial energy). This situation is analagous to 
  a ball rolling along-a smooth surface. As 
  the potential energy 
  the ball rolls downhill, 
  is exchanged for kinetic 
  due to position 
  energy of motion. 
  If .friction- were negli- 
  gibie, the change of potential energy would 
  equal the change in ki,netic energy. This- is 
  also the case for the airflow within 
  the tube. 
  The relationship of static pressure and veloc- 
  ity is maintained throughout the length of the 
  tube. As the flow moves past the constriction 
  toward station 3, the velocity decreases and 
  the static pressure increases. 