WORK AND ENERGY

1
WORK AND ENERGY

• work
• is done when an object moves while force is
  acting on it W F d
• F (net) force acting on object
• d distance object moves while force is acting
• (note F is really the component of the force in
  the direction of motion)
• energy
• the ability to do work energy of a system
  amount of work that the system can do
• energy is stored work work done on a system
  ?system's energy increased ? system can give back
  energy by doing work.
• Power
• work done per unit time
• units of work and energy
• SI unit of work, energy 1 Joule 1 J 1 N m
• 1 calorie 1 cal 4.18 J
  (original definition 1 cal amount of
  energy necessary to increase temperature of 1 g
  of water by 1 degree Celsius) the calorie of
  dieticians is really a kilocalorie 1000
  calories
• English units foot-pound, BTU
• BTU amount of energy needed to raise
  temperature of 1 lb of water by 1 deg.
  Fahrenheit
• SI unit of power 1 Watt 1 W 1 J/s
• 1 kWh 1 kilo-Watt-hour 3.6 MJ

2
POTENTIAL AND KINETIC ENERGY

• lifting object
• work done against gravitational forceraised
  object can drop down and do work (e.g. pull a
  cart)
• i.e. raising object (doing work on it), increased
  its potential to do work ? gravitational
  potential energy
• falling of raised object
• object is accelerated -- loses potential energy
  -- gains energy of motion - kinetic
  energy
• object can do work by virtue of its motion.
• quantitatively
• W F h, F m g ? W m g h
• let object drop kinetic energy K mv2 /2
• conservation of (mechanical) energy
• when lifting the object, its gravitational
  potential energy is increased by the amount of
  work done lifting
• when the object falls, this energy is converted
  (transformed) into kinetic energy (energy of
  motion)
• gravitational potential energy Ug m g h
• kinetic energy K mv2 /2

3
TYPES OF ENERGY

• Many different kinds of energy can be
  transformed back and forth into each other
• kinetic energy energy of motion work that
  system can do because of its motion
  (translational or rotational)
• potential energy energy of position or state
  (gravitational, elastic, electric, chemical,
  nuclear)
• gravitational energy work system can do due to
  objects having been raised against gravitational
  force depends on reference level i.e. on how
  far object can fall down
• elastic energy due to ability of deformed
  (stretched, squeezed,..) system to snatch back
  (e.g. rubber band, spring..)
• thermal energy kinetic energy of random motion
  of molecules brought into system by heating
  different from other forms of energy - not all of
  it can be converted back.
• electromagnetic energy (electric energy)
  energy due to electromagnetic forces
• radiant energy energy carried by
  electromagnetic radiation
• chemical energy energy stored in molecular
  structure of chemical compounds can be
  liberated by chemical reactions converting
  compound into other compounds with less stored
  chemical energy.
• nuclear energy energy due to nuclear structure,
  i.e. how protons and neutrons are bound to each
  other to form nuclei.

4
CONSERVATION OF ENERGY

• Energy conservation
• the total energy of all participants in any
  process is unchanged throughout that process.
  Energy can be transformed (changed from one
  energy form to another), and transferred (moved
  from one place to another), but cannot be created
  or destroyed. In an isolated system the total
  amount of energy is conserved.
• Conservation laws in physics
• conserved quantities quantities that do not
  change - are conserved
• Conservation laws are related to symmetry
  property of system -also called invariance
  property.
• Every invariance property is associated with a
  conserved quantity.
• Energy conservation is related to invariance
  under translation in time (i.e. laws of physics
  do not change as time passes).
• Other conserved quantities
• momentum (invariance under translation in space)
  
• angular momentum (rotation)
• electric charge (gauge transformation)
• certain properties of subatomic particles
  (e.g. Isospin, color charge, ...)