Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: A crane on a building site.

  1. Feb 15, 2010 #1
    Hi, i was just wondering with this question because i'm unsure if im supposed to add both kinetic and gravitation energy into this answer? thanks.

    1 A crane on a building site lifts an 8.00 × 102 kg load from ground level to a vertical height of 90.0 m at a constant speed of 2.00 m s–1. Ignore the mass of the cable.

    A. How much work is done by the crane in lifting the load through this distance?
    m = 800kg h = 90m g = 9.8ms^-2

    E = mgh
    E = 800*9.8*90
    = 7.056*10^5 J

    Sirsh.
     
  2. jcsd
  3. Feb 16, 2010 #2

    rock.freak667

    User Avatar
    Homework Helper

    Yes you also need to add the kinetic energy

    Total work done = PE+KE = constant at all points in the motion (this is the law of conservation of mechanical energy)
     
  4. Feb 18, 2010 #3
    Are you sure? supposedly it's W = F/s therefore W = mg/s, W = 800*9.8/90 ??
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook