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Power and WORk need help on a question

  1. Feb 18, 2009 #1
    A 2.8e3 kg elevator carries a maximum load of 768.1 kg . A constant frictional force of 2.1e3 N retards the elevators motion upward. The acceleration of gravity is 9.81m/s^2. what is the minimum power must the motor deliver to lift the fully loaded elevator at a constant speed of 3.85m/s. answer in units of kW. please i need help on the question. if possible i need both the answer and a clear method on what was done to solve the problem

    if u can i need help on a another problem.

    a 35.0kg child is in a swing that is attached to ropes 2.20m long. The acceleration of gravity is 9.81m/s^2. Find the gravitational potential energy associated with the child relative to the lowest position when the ropes make a 31.0 degree angle with the vertical. awsner in units of J. please both method and answer would be appreciated
     
  2. jcsd
  3. Feb 18, 2009 #2
    Well in the first sitaution you can use a pretty handy formula which says that

    P=Fv

    Here P is the "power" and F is the sum force acting on the body (in the direction of the movment). v stands for the velocity.

    The tricky bit is to get F right. Forces opposing the movment is the totalt gravitational force and frictional force.

    << too much help given -- removed by berkeman >>
     
    Last edited by a moderator: Feb 18, 2009
  4. Feb 18, 2009 #3

    berkeman

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    Staff: Mentor

    Thread moved to Homework Help.

    You need to show us the relevant equations and your attempt at the solutions before we can offer any tutorial help. Please be sure to read the Homework Help rules at the "Rules" link at the top of the page.
     
  5. Feb 18, 2009 #4

    berkeman

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    Welcome to the PF. This thread was originally misplaced in the general technical PF forums. It is clearly homework/coursework, though, so the Homework Help rules still apply, even before it is moved here to the HH forums. Please only offer tutorial help (not working out the problem for the OP), and only after the OP has shown some effort at working out the problem themselves. Thanks.
     
  6. Feb 18, 2009 #5
    i have been trying it
    what am i supposed to post the wrong answer too huh
    am asking for help if u can then please otherwise i am not asking for the answer just the way and the formula if not please explain to me how am i supposed to show effort. i dont get that part
     
  7. Feb 18, 2009 #6

    Tom Mattson

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    Don't get snippy. You need our help, not the other way around. When you registered you were presented with the rules of Physics Forums, and you clicked "I Agree".

    From the Rules:


    Homework Help:
    On posting questions: Any and all high school and undergraduate homework assignments or textbook style exercises for which you are seeking assistance are to be posted in the appropriate forum in our Homework & Coursework Questions area. This should be done whether or not the problem is part of one's coursework. The reason for this is that the scientific and mathematical sections of Physics Forums are to be reserved for discussions and not academic assistance. Since graduate level assignments are meant to be more thought provoking (and hence more worthy of discussion), graduate level questions will be allowed in the relevant part of the main section of PF, provided that the graduate student attempts the problem and shows his work. NOTE: You MUST show that you have attempted to answer your question in order to receive help. You MUST make use of the homework template, which automatically appears when a new topic is created in the homework help forums.



    You are supposed to type out what you've done for this problem. We can't possibly tell you what your error was if you don't show it to us.
     
  8. Feb 19, 2009 #7
    ok im just saying , i dont know your protocol so its OK if you do not wanna help
     
  9. Feb 20, 2009 #8

    berkeman

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    Doing your homework for you is not going to be helping you to learn. Show us your attempt at solving the problems, and we can provide tutorial assistance. You will do the bulk of the work, as you should on your homework and coursework.
     
  10. Feb 20, 2009 #9
    OK your probably right but for this problem i didnt know where to start. If i just knew what formula to use i could probably do the rest. I didnt really understand how to go about this problem so i just needed help in the basic concepts not the really the calculations.
     
  11. Feb 20, 2009 #10

    berkeman

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    Power is force divided by time. P = F / t

    The total force in the first question is the force to lift the elevator and to oppose the friction. Use the basic equation

    F = ma

    and figure out what all of the forces are on the elevator. Then you can use the time specified and the first equation to get much farther along toward the answer.

    Please show us your work if you still get stuck...
     
  12. Feb 20, 2009 #11
    thanks for the help but it a little to late i already solved the problem but thanks anyways. sorry if i seemed in a bad mood just a little stressed and sorry im new to the forum so i didnt know your protocol
     
  13. Feb 20, 2009 #12

    berkeman

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    No worries. Glad you solved the problems. Get some sleep!
     
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