Mechanical  kinetic  energy  is  also  measured  in ft-lb.  The  amount  of  kinetic  energy  present  at  any one time is directly related to the velocity of the moving  object  and  to  the  weight  of  the  moving object. Mechanical potential energy can be changed into mechanical kinetic energy. If you push that 5-pound rock over the edge of the 100-foot cliff, it begins to fall, and as it falls, it loses potential energy  and  gains  kinetic  energy.  At  any  given moment,  the  total  mechanical  energy  (potential plus kinetic) stored in the system is the same—500 ft-lb. But the proportions of potential energy and kinetic energy are changing all the time as the rock is falling. Just before the rock hits the earth, all the  stored  mechanical  energy  is  kinetic  energy.  As the  rock  hits  the  earth,  the  kinetic  energy  is changed into energy in transition—that is, work and heat. Mechanical  kinetic  energy  can  likewise  be changed  into  mechanical  potential  energy.  For example, suppose you throw a baseball straight up in the air. The ball has kinetic energy while it is in motion, but the kinetic energy decreases and  the  potential  energy  increases  as  the  ball travels  upward.  When  the  ball  has  reached  its uppermost  position,  just  before  it  starts  its  fall back to earth, it has only potential energy. Then, as  it  falls  back  toward  the  earth,  the  potential energy  is  changed  into  kinetic  energy  again. Mechanical   energy   in   transition   is   called WORK.   When   an   object   is   moved   through   a distance against a resisting force, we say that work has been done. The formula for calculating work is W = F × D , where: W  =  work, F  =  force,  and D  =  distance. As  you  can  see  from  this  formula,  you  need to  know  how  much  force  is  exerted  and  the distance through which the force acts before you can  find  how  much  work  is  done.  The  unit  of force  is  the  pound.  When  work  is  done  against gravity,  the  force  required  to  move  an  object  is equal to the weight of the object. Why? Because weight is a measure of the force of gravity or, in other words, a measure of the force of attraction between  an  object  and  the  earth.  How  much  work will you do if you lift that 5-pound rock from the bottom  of  the  100-foot  cliff  to  the  top?  You  will do  500  ft-lb  of  work—the  weight  of  the  object (5 pounds) times the distance ( 100 feet) that you move it against gravity. We also do work against forces other than the force of gravity. When you push an object across the deck, you are doing work against friction. In this case, the force you work against is not only the  weight  of  the  object,  but  also  the  force required  to  overcome  friction  and  slide  the object  over  the  surface  of  the  deck. Notice  that  mechanical  potential  energy, mechanical   kinetic   energy,   and   work   are   all measured in the same unit, ft-lb. One ft-lb of work is done when a force of 1 pound acts through a distance   of   1   foot.   One   ft-lb   of   mechanical potential energy or mechanical kinetic energy is the   amount   of   energy   that   is   required   to accomplish  1  ft-lb  of  work. The amount of work done has nothing at all to do with how long it takes to do it. When you lift  a  weight  of  1  pound  through  a  distance  of 1 foot, you have done 1 ft-lb of work, regardless of whether you do it in half a second or half an hour.  The  rate  at  which  work  is  done  is  called POWER.  The  common  unit  of  measurement  for power is the HORSEPOWER (hp). By definition, 1 hp is equal to 33,000 ft-lb of work per minute or 550 ft-lb of work per second. Thus a machine that  is  capable  of  doing  550  ft-lb  of  work  per second is said to be a 1-hp machine. (As you can see,  your  horsepower  rating  would  not  be  very impressive  if  you  did  1  ft-lb  of  work  in  half  an hour. Figure it out. It works out to be just a little more  than  one-millionth  of  a  horsepower.  ) THERMAL  ENERGY Earlier in this chapter we discussed molecules. You  should  remember  that  all  substances  are composed  of  very  small  particles  called  molecules. The  energy  associated  with  molecules  is  called thermal energy. Thermal energy, like mechanical energy,  exists  in  two  stored  forms  and  in  one transitional   form.   The   two   stored   forms   of thermal energy are (1) internal potential energy and (2) internal kinetic energy. Thermal energy in  transition  is  called  HEAT. Although molecules are too small to be seen, they behave in some ways pretty much like the larger objects we considered in the discussion of mechanical   energy.   Molecules   have   energy   of position  (internal  potential  energy)  because  of  the 2-8