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Homework Help: Work and Energy of fighter jet

  1. Jul 30, 2009 #1
    1. The problem statement, all variables and given/known data
    A fight 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 230,000 N. In being launched from rest it moves through a distance of 87 m and has a kinetic energy of 45,000,000 J at lift-off.
    What is the work done on the jet by the catapult?

    2. Relevant equations
    Work = (Force * cos(angle)) * displacement
    Work = Final Kinetic Energy - Initial Kinetic Energy

    3. The attempt at a solution
    My problem is that without the catapult's force, I can't do the first equation.
    In using the 2nd equation, initial velocity = 0m/s, which causes the initial kinetic energy to be 0 N. If that is so, then Work = Final Kinetic Energy and the catapult does no work.
    This one is driving me crazy...
  2. jcsd
  3. Jul 30, 2009 #2

    Doc Al

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    Staff: Mentor

    How did you deduce that the catapult does no work?

    Hint: What's the total work done? How much work does the engine thrust account for?
  4. Jul 30, 2009 #3


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    Staff Emeritus
    Science Advisor
    Gold Member

    Your conclusion that the catapult does no work does NOT follow from the statement that the work done is equal to the final kinetic energy (I fail to see what the latter has to do with the former).

    Here is an idea of how your thought process should go. How much work is done by the jet's engines alone? Is this amount of energy consistent with the stated final energy? If not, did the jet have more energy or less energy than what you expected in the end? If it had more energy than expected, then clearly the extra energy must have come from something other than the engines.

    So, in other words, you don't need to know the catapult's force on the jet, because you can DEDUCE how much work is done by the catapult by comparing the actual final energy to the final energy one would expect if the engines alone were doing the work.
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