"Work is of two kinds: first, altering the position of matter at or near the earth's surface relatively to such other matter; second, telling other people to do so.  The first kind is unpleasant and ill paid; the second is pleasant and highly paid"
Bertrand Russell

                                WORK      

• Definition
 
• Qualitative:  Work is a measure of the "change" a force gives rise to when it acts on an object.
 
• Quantitative:  The work done by a force acting on an object is equal to the magnitude of the displacement of the object multiplied by the component of the force in the direction of that displacement.
 
•    If an applied force causes no displacement, then no work is done.

This is in contrast to your everyday understanding of work.  If you spend all day pushing on a wall, you will feel as if you have done a lot of work - in fact no work will have been done, since the wall did not move.
 
• When force, F, and displacement, x, are parallel then,

• When force and displacement are at an angle theta,

• Units of work
   
  • SI            -    1 Joule = 1 Newton . 1 metre
  • British    -    1 ft.pound = 1 pound . 1 foot
  
  
James Precott Joule


• Work performed against gravity, when lifting an object through height, h.

where mg is the force due to gravity (also equal to the weight of the object)

Work done against gravity is dependent only on height and weight.  Same amount of work is done against gravity when climbing one floor via stairs and climbing one floor via elevator.  Unfortunately you do the work when you use the stairs, whereas the elevator motor does the work when you use the elevator !

           POWER    

• Definition:  "Power is the rate at which work is performed"
  
  
  Power = Work / Time
  
  If a force, F, causes an object to move with velocity, v , where the force and velocity are at an angle theta, then,
  
  
  

• Units of Power
• SI            -    1 Watt = 1 Joule / second  (J/s)
• British      -    1 ft.pound = 1 foot. pound/second  (ft.lb/s)
• Conversions
  1 kilowatt (kW) = 1000  Watts
  1 horse power (hp) = 550 ft.lb/s = 746 Watts
  
James Watt

"Prison van and concrete mixer in head on collision.
Police looking for 18 hardened criminals"
Ronnie Barker


 

Dr. C. L. Davis
Physics Department
University of Louisville
email: c.l.davis@louisville.edu