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Quick Problem

  1. Jun 12, 2013 #1
    1. The problem statement, all variables and given/known data

    Johnny is swinging (on a swing) at a velocity of 4m/s when he is 56 cm above the ground. What is his maximum height?

    Johnny is 45 Kg.

    2. Relevant equations

    Pe = mgh
    Ke= (1/2)(m* v^2)
    Total energy= Pe + Ke

    3. The attempt at a solution

    I tried setting the Ke equal to the Pe, but I get 0.81 Meters. However, I know that the correct answer is around 1.4 Meters.

    I would like to know how to get 1.4 Meters.
  2. jcsd
  3. Jun 12, 2013 #2


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    At 45cm he has some PE and some KE.
    At the max height all of that is converted into PE.

    I get 1.37m if I use g = 9.81

    I suggest you show your working.
  4. Jun 12, 2013 #3
    Okay. So, what you are saying is:

    Pe: (9.81)(45)(0.56) + Ke: (1/2)(45)(4^2)

    Then you plug all of that in for Pe and solve for height.

    Thanks a bunch.
  5. Jun 12, 2013 #4


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