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Work and spring constant questions?

  1. Jan 10, 2012 #1
    1) If a man lifts a 11.0 kg bucket from a well and does 7.00 kJ of work, how deep is the well? Assume that the speed of the bucket remains constant as it is lifted.

    2) Hooke's law describes a certain light spring of unstretched length 36.0 cm. When one end is attached to the top of a door frame, and a 7.60-kg object is hung from the other end, the length of the spring is 45.50 cm.

    (a) Find its spring constant.
    (b) The load and the spring are taken down. Two people pull in opposite directions on the ends of the spring, each with a force of 170 N. Find the length of the spring in this situation.




    work=fd
    PE=(1/2)k(x^2)




    ok for the first one i did 7000J/11= 636.36 and it says that's wrong
    and for the second one im not even sure where to start
     
    Last edited: Jan 10, 2012
  2. jcsd
  3. Jan 10, 2012 #2

    Redbelly98

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    You are using the correct equation, work = fd. So, what is f, the force required to lift a mass of 11 kg? Hint: it is the same as the force of gravity exerted on the mass.

    There is another useful equation for springs, relating force and displacement (amount of stretch) of the end of the spring. Find that equation and try to apply it here.
     
  4. Jan 10, 2012 #3
    the force of gravity is 9.81 so that would be the force. so 7000/9.81= 713.56 m ?

    and for the second one, i guess we dont have to do it. the answer in the website is wrong or something.
     
  5. Jan 10, 2012 #4
    oh wait. you need to find force using f=ma so 11*9.81=107.91
    and then plug that in which would be 7000/107.91= 64.87m
    i already got it wrong on the homework but thanks. At least I know how to do it now :)
     
  6. Jan 11, 2012 #5

    Redbelly98

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    Looks good. Too late for the homework, but remember it for the test :smile:
     
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