1 PhysicsChapter 5 Work & Energy Sections:15-1 Work 5-2 Energy 5-3 Conservation of Energy 5-4 Work, Energy & Power

2 Work  A force that causes a displacement of an object does work on that object  W= F x d  W is work, F is force, d is distance  Unit is a Newton-meter or Joule, J  Work is done on an object only if the object moves due to the action of an applied force  Consider the situations of a mom & child  Work is done on an object only when components of the applied force are parallel to the displacement of the object  If a force is applied at an angle , then only the vector component of the force parallel to the direction of displacement is the force that does the work on the object  W = F x d (cos  )  If  = 0, then cos 0  = 1 & so W=Fd; if  = 90, then cos 90  = 0 & W=0  If many constant forces act on an object, find the net work by finding the net force  W net = F net (d) (cos  )

3 Equations  W= F x d  W = F x d (cos  )  W net = F net (d) (cos  )

4 Work  Work is a scalar  Work is positive when direction of displacement is same as direction of applied force  Ex: lift a box (applied force up, direction is up)  Work is negative when direction of displacement is opposite that of applied force  Ex: friction of box sliding on floor (applied force moves box forward, but friction does work on box in opposing direction)

5 Kinetic Energy  Kinetic energy – energy of an object due to its motion  Depends on speed & mass of object  KE = ½ mv 2  KE is a scalar, unit is Joule, J

6 Potential Energy  Potential energy – energy associated with an object’s position  Gravitational potential energy – energy associated with an object due to its position relative to the Earth or some other gravity source  PE g = mgh  M = mass, g = 9.81m/s 2, h = height  Unit is Joule, J  FYI – electrical energy often measured in kW  h, 1 kW  h = 3.6 x 10 6 J

7 Elastic Potential Energy  Elastic potential energy – energy in a stretched or compressed elastic object  Relaxed length – term for the length of a spring when no external forces acting on it; when a force compresses or lengthens a spring PE is stored in spring (work is done to spring!)  The amount of PE depends on distance spring is compressed or stretched from relaxed length  PE e = ½ kx 2  k = spring constant, x = distance compressed or stretched  Spring constant = a parameter that expresses how resistant a spring is to being compressed or stretched, unit is N/m

8 Mechanical Energy  Mechanical energy – the sum of KE & all forms of PE  ME = KE +  PE Energy Mechanical Non-mechanical KineticPotential Gravitational Elastic

9 Conservation of Energy  ME i = ME f  Initial mechanical energy = final mechanical energy  Friction is absent  ½ mv 2 i + mgh i = ½ mv 2 f + mgh f  Above equation rewritten using KE & PE  In presence of friction, ME is not conserved; b/c ME converted to nonmechanical energy  Non-mechanical energy includes: electromagnetic energy, heat, light, nuclear energy, chemical energy

10 Work, Energy, & Power  Net work = change in KE  W n =  KE  Work done by friction:  W f =  ME(W f is work due to friction)  Power = W/  t  If W = Fd, then P = W/  t = Fd/  t  For work, remember there is a time interval involved, so work done is done in a unit of time.

11 Power  Since distance moved per unit time is speed then an alternative equation for power is:  P = Fv  Watt is unit for power; 1 W = 1J/s  1 horsepower = 746W  Wattage of light bulbs  Dim bulb ~40W  Bright bulb ~500W  Decorative light bulbs: indoor ~0.7W, outdoor ~7.0W  What about CFBs?