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What is the physical significance of work?

  1. Aug 24, 2010 #1
    As the title suggests, what is work? Or, what is the physical significance of work?
    My textbook define work as crossproduct of force and displacement.
    But why do we need that quantity?
     
  2. jcsd
  3. Aug 24, 2010 #2

    Dale

    Staff: Mentor

    Re: work

    It is the dot product of force and displacement, not the cross-product.

    The reason that we need it is because of its fundamental relationship to energy which is very useful in simplifying many problems.
     
  4. Aug 24, 2010 #3
    Re: work

    I made up this little cheat sheet that I have to refer to every time I get involved in working out work. Understanding (and remembering) the dimensional analysis (the basic MKS units of each measure, e.g., Meters per Second is speed) should help in getting the idea of each unit.

    Code (Text):
                     Getting Energy Straight

    Force -- Newton -- Mass times Acceleration ( F = MA )
                        Killograms times Meters per Sec^2: (Kg x M) / S^2
                        1 newton = 10^5 dynes
                        1 pound-force ~= 4.5 newtons

    Work --  Joule  -- Force times Distance ( W = FD )
     (aka Energy)       Newtons times Meters -- N x M:     (Kg x M^2) / S^2
                        1 Joule = 10^7 ergs
                                  .74 foot-pounds
                                  6.25x10^18 electron volts
                        1 BTU = 1 Kilo-joule

                        note:
                         Watt = volt x ampere
                         1 Columb -- amp-sec ~= 6.25 x 10^18 electron-second
                         Watt-seconds -- volt x coulmb
                         1 Joule = 1 Watt-second
                         1 KwHr = 3.6 Mega-joule

    Power -- Watt   -- Work per Time ( P = W/S )
                        Joules per Second -- J/S:          (Kg x M^2) / S^3
                        1 HP = 550 ft-lb/s = 745.7 watts
                        1 Kw = 1.34 HP
                        1 BTU/hour = .29 watts

    for extra credit:
    Pressure -- Pascal -- Force per Area ( P = F/A )
                           Newtons per Meter^2 -- N/M^2:    Kg / (M x S^2)
                    1 pound/sqin (PSI) = 6.9 Kpascal
     
  5. Aug 24, 2010 #4

    RonL

    User Avatar
    Gold Member

    Re: work

    Thanks, this might prove very helpful to me.

    Ron
     
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