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Problem with acceleration

  1. Nov 11, 2016 #1
    • Moved from technical forums, so no template
    Hi , im trying to practice working with new exercises and i've met some problems.
    Lifting object with mass of 2kg to height of 10m we do 240J mechanical work.
    What is acceleration of lifting?

    My calculations:
    A=Fs=Fh
    F=A/h = 240/10 = 24N

    a= F/m = 24/2 = 12 m/s^2 , somehow it's not the right answer
     
  2. jcsd
  3. Nov 11, 2016 #2
    There is a downward mg which compensates the mechanical work done?

    your answer is correct if there is no mg force

    in fact,part of the mechanical work done is to compensate the work done by the mg force
     
  4. Nov 11, 2016 #3
    No , that's all what's written in question
     
  5. Nov 11, 2016 #4

    Dale

    Staff: Mentor

    The word "lifting" implies gravity.
     
  6. Nov 11, 2016 #5
    Lifting object with mass of 2kg to height of 10m we do 240J mechanical work.
    What is acceleration of lifting?

    the final answer is 2 ?
     
  7. Nov 11, 2016 #6
    2 m/s^2 yes
     
  8. Nov 11, 2016 #7
    in fact

    your answer is correct if there is no gravity.
     
  9. Nov 11, 2016 #8
    if there is gravity
    then you will feel a "resistance" from the gravity

    part of your 240J will try to compensate this "resistance" so that it is impossible that the force that you provide will be the same of the case of no gravity force
     
  10. Nov 11, 2016 #9
    How did you get 2? can you write the equation?
    Was it a=F/m - g?
     
  11. Nov 11, 2016 #10

    haruspex

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    I'm unsure how to interpret the question.

    If this is in a terrestrial setting, we can calculate that part of the work that went into overcoming gravity, i.e. into achieving the PE gain. That is less than 240J. Where did the rest go? Since it asks about acceleration, maybe we are supposed to assume there is residual KE, i.e. it is still moving upwards. If so, we can compute the gained velocity, but without knowing how long this took there is no way to find the acceleration.

    Alternatively, this is extraterrestrial, and all they want you to find is the local gravitational acceleration, but then that would be 12m/s2, not 2.
     
  12. Nov 11, 2016 #11

    NascentOxygen

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    The object was lifted 10m.
     
  13. Nov 11, 2016 #12
    Yes, although it isn't stated, I think maybe the solution should assume constant acceleration.
     
  14. Nov 12, 2016 #13

    haruspex

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    That does seem to be the most likely error/omission in the question.
     
  15. Nov 12, 2016 #14

    Dale

    Staff: Mentor

    Yes. That is it.
     
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