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Work - basic problem

  1. Oct 9, 2008 #1
    1. The problem statement, all variables and given/known data
    A 17000kg jet takes off from an aircraft carrier via a catapult. The gases thrust out from the jet's engines exert a constant force of 160 kN on the jet;the force exerted on the jet by the catapult is plotted in the figure b.

    a)Determine the work done on the jet by the gases expelled by its engines during launch of the jet.
    b)Determine the work done on the jet by the catapult during launch of the jet.

    [​IMG]

    2. Relevant equations
    W=Fdcos[tex]\theta[/tex]

    3. The attempt at a solution

    a)Since no displacement is stated outright, I assumed that it is 85m from the plot.
    160kN=160,000N
    [tex]W=160000*85*cos(180)=-1.36*10^7[/tex]

    This was an incorrect answer. What did I do wrong?
     
  2. jcsd
  3. Oct 9, 2008 #2

    LowlyPion

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    Homework Helper

    Since it is the Force over positive distance and the Force is in the direction of Positive distance, then the work is positive. You don't need Cosθ. The positive force goes to build positive KE which is increasing V2 not decreasing.
     
  4. Oct 9, 2008 #3
    Thanks, I was a little thrown off by the thrust out of the back of the plane I guess.
     
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