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: HELP! on tension and work!

  1. Nov 12, 2007 #1
    1. a cable lifts a 1200-kg elevator at a constant velocity for a distance of 35 m in 5.3 seconds.

    A.) What is the work done by the tension in the cable?
    B.) What is the work done by the elevator's weight?
    C.) What is the power provided by the tension in lifting the elevator?
  2. jcsd
  3. Nov 12, 2007 #2
    There are very simple equations one can use to solve these.
  4. Nov 12, 2007 #3
    draw a free body diagram.. and write down the relevant equations u need
  5. Nov 12, 2007 #4
    for A.) i did W=FxD so W=12000x35=420000
    for B.) i wasn't sure how to do because isnt it the same thing as A?
    and C.) P=W/T
    P=420,000/5.3= 79245.3
    not sure if they're right though..
  6. Nov 12, 2007 #5
    A) that looks fine
    B) no, the rope pulls up, what does the pull of gravity do to the elevator?
    C) looks fine
  7. May 2, 2010 #6
    when the rope pulls upwards and gravity pulls down, then gravity does negative work. Remember, gravity does negative work most of the time, since the rope is pulling up and gravity is pulling down, the direction of acceleration of gravity and acceleration of the elevator are not the same, therefore, negative work is being done by gravity. If the elevator was moving down then gravity would be doing positive work on the elevator.
  8. May 2, 2010 #7
    a) correct
    b)T=mg that means that w=mg and they are equal.
  9. May 2, 2010 #8
    This is a 2 and a half year old topic.:wink:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook