a constant speed has no acceleration. When the velocity of an object increases by the same amount each  second  or  minute,  you  have  uniform acceleration.  Uniform  deceleration  is  when  the decrease  in  velocity  is  the  same  each  second  or minute. MASS,  WEIGHT,  FORCE, AND  INERTIA Very few terms are used in physics with greater frequency and assurance than mass, and few are more  difficult  to  define.  Mass  is  often  confused with  weight.  This  is  a  mistake  not  helped  since the unit of measurement for both mass and weight is  the  gram.  The  MASS  of  an  object  is  the quantity  of  matter  that  the  object  contains. The  WEIGHT  of  the  object  is  equal  to  the gravitational  force  with  which  the  object  is attracted to the earth. FORCE is what makes an object  start  to  move,  speed  up,  slow  down,  or keep  moving  against  resistance.  Force  may  be either a push or a pull. You exert a force when you push against a truck, whether you move the truck or only try to move it. You also exert a force when  you  pull  on  a  heavy  piano,  whether  you move the piano or only try to move it. Forces can produce  or  prevent  motion. A tendency to prevent motion is the frictional resistance  offered  by  an  object.  This  frictional resistance is called frictional force. While it can never  cause  an  object  to  move,  it  can  check  or stop   motion.   Frictional   force   wastes   power, creates  heat,  and  causes  wear.  Although  frictional force  cannot  be  entirely  eliminated,  it  can  be reduced  with  lubricants. INERTIA is the property that causes objects at rest to remain at rest and objects in motion to remain in motion until acted upon by an outside force. An example of inertia is one body that has twice as much mass as another body of the same material   offering   twice   as   much   force   in opposition  to  the  same  acceleration  rate. Inertia in a body depends on its motion. The physical  principles  of  mass  and  inertia  are involved in the design and operation of the heavy machinery that is to be placed into motion, such as an engine’s flywheel and various gears that are at  work  in  the  ship’s  engineering  plant.  The  great mass of the flywheel tends to keep it rotating once it has been set in motion. The high inertia of the flywheel  keeps  it  from  responding  to  small fluctuations  in  speed  and  thus  helps  keep  the engine  running  smoothly. ENERGY Can  you  define  energy?  Although  everyone has  a  general  idea  of  the  meaning  of  energy,  a good  definition  is  hard  to  find.  Most  commonly, perhaps,  energy  is  defined  as  the  capacity  for doing  work.  This  is  not  a  very  complete definition.  Energy  can  produce  other  effects  which cannot  possibly  be  considered  work.  For  example, heat can flow from one object to another without doing work; yet heat is a form of energy, and the process  of  heat  transfer  is  a  process  that  produces an effect. A better definition of energy, therefore, states  that  energy  is  the  capacity  for  producing an  effect. Energy exists in many forms. For convenience, we  usually  classify  energy  according  to  the  size and nature of the bodies or particles with which it is associated. Thus we say that MECHANICAL ENERGY  is  the  energy  associated  with  large bodies  or  objects—usually,  things  that  are  big enough  to  see.  THERMAL  ENERGY  is  energy associated  with  molecules.  CHEMICAL  ENERGY is  energy  that  arises  from  the  forces  that  bind  the atoms  together  in  a  molecule.  Chemical  energy is  demonstrated  whenever  combustion  or  any other  chemical  reaction  takes  place.  Electrical energy   (light,   X   rays,   and   radio   waves)   is associated  with  particles  that  are  even  smaller  than atoms. Mechanical   energy,   thermal   energy,   and chemical energy must also be classified as being either stored energy or energy in transition. STORED  ENERGY  can  be  thought  of  as energy that is actually contained in or stored in a  substance  or  system.  There  are  two  kinds  of stored  energy:  (1)  potential  energy  and  (2)  kinetic energy. When energy is stored in a substance or system  because  of  the  relative  POSITIONS  of  two or more objects or particles, we call it potential energy. When energy is stored in a substance or system  because  of  the  relative  VELOCITIES  of two or more objects or particles, we call it kinetic energy. Mechanical energy in transition is called work. Thermal energy in transition is called heat. In the next   section   we   will   discuss   mechanical   and thermal  energy  and  energy  transformations. If  you  do  not  completely  understand  this classification,  come  back  to  it  from  time  to time  as  you  read  the  following  sections  on mechanical  energy  and  thermal  energy.  The examples  and  discussion  given  in  the  following sections will probably help you understand this classification. 2-6