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How to find total force?

  1. Feb 24, 2012 #1
    An athlete whose mass is 86.5 kg is performing weight-lifting exercises. Starting from the rest position, he lifts, with constant acceleration, a barbell that weighs 470 N. He lifts the barbell a distance of 0.50 m in a time of 2.0 s.


    I cannot seem to find the answer to this. This is what i did:

    - I found the total force exerted on the feet by doing: 86.5*9.8 + 470N = 1317.7N
    - Then I used d = a*t^2 and got that the acceleration is 25m/s^2
    - Then i found the mass of the barbell by doing 470N/9.8 and got 47.96kg.
    - After using F=ma i added the two forces and got the answer 2516.7N.

    This isn't the right answer. What did i do wrong? Any help is appreciated.

    Thanks.
     
  2. jcsd
  3. Feb 24, 2012 #2

    tiny-tim

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    hi treyh! :smile:
    i don't understand the question :redface:

    is his body remaining stationary (apart from his arms)?

    or is his centre of mass also moving up 0.50 m ? :confused:
     
  4. Feb 24, 2012 #3

    PeterO

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    I just read the original post very carefully, but the only question mark I found was at the end of " What did I do wrong?"

    I am tempted to reply: " You didn't ask a question?"

    The line I highlighted red above is interesting. If an object falls under the influence of gravity - with an acceleration a little under 10 m/s^2 - it covers just under 20m in 2 seconds. This barbell covered only 0.5 m in 2 seconds yet you said the acceleration was 25 m/s^2 - more than double the acceleration due to gravity ??
     
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