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Bathroom scale being compressed but jumping on it

  1. Jul 17, 2009 #1
    1. The problem statement, all variables and given/known data
    If you stand on a bathroom scale, the spring inside the scale compresses 0.55mm, and it tells you your weight is 760N.
    Now if you jump on the scale from a height of 1.3 m, what does the scale read at its peak?
     
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  3. Jul 17, 2009 #2

    rock.freak667

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    What are your thoughts on the question? Post your attempt.
     
  4. Jul 17, 2009 #3
    well i tried a few things... i found the persons mass which is 77.55kg.Since F=ma,and we have m, i tired using kinamatics to find a but finding the velocity of the person just before impact. Then i got stuck. I dont know how to go about this question.
     
  5. Jul 17, 2009 #4

    rock.freak667

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    So when the person stands on the scale, they weigh 760N. This means that the spring compresses. Do you know a formula that relates force and extension?
     
  6. Jul 17, 2009 #5
    ya hookes law...F=kx.
     
  7. Jul 17, 2009 #6
    where k equals spring constant and x equals displacment
     
  8. Jul 17, 2009 #7

    rock.freak667

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    yes

    good.


    So can you find the spring constant,k using the information in the first line of the question?
     
  9. Jul 17, 2009 #8
    according to this the constant equals to 1381818.2 is this right
     
  10. Jul 17, 2009 #9

    rock.freak667

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    1381818.2 N/m yes, remember to put in your units.


    So second line now. If the person is at 1.3m in the air, what type of energy does he have? This energy is converted into the energy needed to compress the spring a displacement,x, at the peak.
     
  11. Jul 17, 2009 #10
    potential energy?
     
  12. Jul 17, 2009 #11

    rock.freak667

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    yes. What is the formula for gravitational potential energy? And what is the formula for the energy stored by a spring?
     
  13. Jul 17, 2009 #12
    The formula for gravitational potential energy is U=mgy where u equals the p.e, m equals mass and y is the displacement. the formula for the energy storedd in a spring is U=1/2kx^2
     
  14. Jul 17, 2009 #13

    rock.freak667

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    So if all the gravitational energy is converted to energy stored in the spring, that makes

    gravitational p.e. = energy stored in the spring.


    Can you find the compression,x for the scale now?
     
  15. Jul 17, 2009 #14
    ya i think i can..thanks for all the help..your a life saver
     
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