MECHANICAL   ENERGY Let’s   consider   the   two   stored   forms   of mechanical  energy.  Mechanical  POTENTIAL energy exists because of the relative positions of two or more objects. For example, a rock resting on  the  edge  of  a  cliff  in  such  a  position  that  it will   fall   freely   if   pushed   has   mechanical potential energy. Water at the top of a dam has mechanical potential energy. A sled that is being held  at  the  top  of  an  icy  hill  has  mechanical potential energy. Mechanical  KINETIC  energy  exists  because of the relative velocities of two or more objects. If you push that rock, open the gate of the dam, or  let  go  of  the  sled,  something  will  move.  The rock  will  fall;  the  water  will  flow;  the  sled  will slide down the hill. In each case the mechanical potential  energy  will  be  changed  to  mechanical kinetic energy. Another way of saying this is that the  energy  of  position  will  be  changed  to  the energy  of  motion. In  these  examples,  you  will  notice  that  an external  source  of  energy  is  used  to  get  things started.   Energy   from   some   outside   source   is required  to  push  the  rock,  open  the  gate  of  the dam, or let go of the sled. All real machines and processes  require  this  kind  of  boost  from  an energy  source  outside  the  system.  For  example, a  tremendous  amount  of  chemical  energy  is  stored in fuel oil; but this energy will not turn the power turbine until you have expended some energy to start the oil burning. Similarly, the energy in any one   system   affects   other   energy   systems. However, it is easier to learn the basic principles of energy if we forget about all the energy systems that  might  be  involved  in  or  affected  by  each energy  process.  In  the  examples  given  in  this chapter,   therefore,   we   will   consider   only   one energy  process  or  energy  system  at  a  time, disregarding both the energy boosts that may be received  from  outside  systems  and  the  energy transfers  that  may  take  place  between  the  system we  are  considering  and  other  systems. Notice that both mechanical potential energy and  mechanical  kinetic  energy  are  stored  forms of  energy.  It  is  easy  to  see  why  we  regard mechanical potential energy as being stored, but it  is  not  so  easy  to  see  the  same  thing  about mechanical  kinetic  energy.  Part  of  the  trouble comes  about  because  mechanical  kinetic  energy is often referred to as the energy of motion, thus leading  to  the  false  conclusion transition  is  somehow  involved. case,   however.   Work   is   the that  energy  in This is not the only   form   of mechanical energy that can be properly considered as energy in transition. If  you  have  trouble  with  the  idea  that mechanical kinetic energy is stored, rather than in  transition,  think  of  it  like  this:  A  bullet  that has been fired from a gun has mechanical kinetic energy because it is in motion. The faster the bullet is moving, the more kinetic energy it has. There is no doubt in anybody’s mind that the bullet has the capacity to produce an effect, so we may safely say that it has energy. Although the bullet is not in  transition,  the  energy  of  the  bullet  is  not transferred  to  any  other  object  or  system  until  the bullet  strikes  some  object  that  resists  its  passage. When  the  bullet  strikes  against  a  resisting  object, then,  and  only  then,  can  we  say  that  energy  in transition  exists,  in  the  form  of  heat  and  work. In this example, we are ignoring the fact that some work is done against the resistance of the air and that some heat results from the passage of  the  bullet  through  the  air.  But  this  does  not change  the  basic  idea  that  kinetic  energy  is  stored energy rather than energy in transition. The air must  be  regarded  as  a  resisting  object,  which causes  some  of  the  stored  kinetic  energy  of  the bullet  to  be  converted  into  energy  in  transition (heat and work) while the bullet is passing through the  air.  However,  the  major  part  of  the  stored kinetic  energy  does  not  become  energy  in transition until the bullet strikes an object firmer than air that resists its passage. Mechanical  potential  energy  is  measured  in foot-pounds  (ft-lb).  Consider,  for  example,  the rock  at  the  top  of  the  cliff.  If  the  rock  weighs 5 pounds and if the distance from the rock to the earth at the base of the cliff is 100 feet, 500 ft-lb of mechanical potential energy exists because of the relative positions of the rock and the earth. Another  way  of  expressing  this  idea  is  by  the following    formula: P E = W x D , where: PE = total potential energy of the object (in  ft-lb), W   =   total   weight   of   the   object   (in pounds),  and D = distance between the earth and the object  (in  feet). 2-7