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KE Work-Energy Theorem Problem question

  1. Nov 6, 2006 #1
    I am compleatly lost on these two KE Work-Energy Theorem Problem questions any feedback or help would be greatly appreciated. Thank you in advance......

    1.)
    47.0 g golf ball is driven from the tee with an initial speed of 52.0 m/s and rises to a height of 25.0 m.
    (a) Neglect air resistance and determine the kinetic energy of the ball at its highest point.
    J
    (b) What is its speed when it is 10.0 m below its highest point?
    m/s

    2.)
    A fighter jet is launched from an aircraft carrier with the aid of its own engines and a steam-powered catapult. The thrust of its engines is 1.8 105 N. In being launched from rest it moves through a distance of 87 m and has a kinetic energy of 4.7 107 J at lift-off. What is the work done on the jet by the catapult?


    Thanks!
     
  2. jcsd
  3. Nov 6, 2006 #2

    PhanthomJay

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    You should show what you attempted or what you know before any responses will be of help to you.
     
  4. Nov 6, 2006 #3
    well for question 1.) i tried to use KE=.5mv2 and was not right so i tried
    .5mv2+mgh also not right i am lost at this point.....

    And for question 2 i tried to do
    4.7x10^7=1.8x10^5+x
    also not right any ideas????
     
  5. Nov 6, 2006 #4
    thats a simple view of what i did if you want i can type my numbers and calculations out
     
  6. Nov 6, 2006 #5
    are my equations on the right track or am i just misinformed????
     
  7. Nov 6, 2006 #6
    The point of conservation of energy is that at any point (of your choosing), total energy is conserved. In your case, theres no nonconservative forces so total energy will be a combination of PE and KE. The trick is to find a convenient point to choose
     
  8. Nov 6, 2006 #7

    PhanthomJay

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    When there's no friction or applied or other non-cconservative forces acting, energy is conserved. That's the case here, since only gravity acts, and that is a conservative force. Anyway, for the golf ball question, you can now use the conservation of energy principle, Initial KE plus initial PE equals final PE plus final KE. So what's the initial KE? Initial PE? Final PE? Solve for final KE. Use the same approach for part 2 of that question.
     
  9. Nov 6, 2006 #8
    so for the golf ball KE=1/2mV2 + mgh

    .5(.047*52^2)+.047*-9.8*25=52.029

    Tried it and it worked so thanks guy i was neglecting to make accel due to grav -.

    As far as the jet problem i am still lost....
     
  10. Nov 6, 2006 #9
    ok having problems with part b of golf ball problem

    47.0 g golf ball is driven from the tee with an initial speed of 52.0 m/s and rises to a height of 25.0 m.
    (a) Neglect air resistance and determine the kinetic energy of the ball at its highest point.
    J
    (b) What is its speed when it is 10.0 m below its highest point?
    m/s

    I have used
    KEi+PEi = KEf+PEf so
    52.029= .0235*x^2 + -6.909

    solve for x you get x= 50.07993

    However this is not right what am i doing wrong?
     
  11. Nov 6, 2006 #10
    bump.........
     
  12. Nov 6, 2006 #11
    bump...............
     
  13. Nov 6, 2006 #12

    PhanthomJay

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    If you used the KE at the top as your initial point, and PE initial equal to 0 at that point, then when you look at the final PE, you must use h = - 10, not -15 as you used. Alternatively, you could have started at the very beginning, where KE_i =1/2(.047)(52)^2, PE_i = 0, and PE_f = + .047(9.8)(15), which will yield the same result when you use the equation. You can choose any point as PE=O, but then you must be consistent thereafter.
     
    Last edited: Nov 6, 2006
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