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Work and force (my answers keep being wrong please someone explain)

  1. Oct 5, 2008 #1
    A plane weighing 220 kN (25 tons) lands on an aircraft carrier. The plane is moving horizontally at 62 m/s (139 mi/h) when its tailhook grabs hold of the arresting cables. The cables bring the plane to a stop in a distance of 88 m.
    (a) How much work is done on the plane by the arresting cables?
    MJ

    (b) What is the force (assumed constant) exerted on the plane by the cables?
    kN opposite the plane's direction of motion

    (Note: Both answers will be underestimates, since the plane lands with the engines full throttle forward; in case the tailhook fails to grab hold of the cables, the pilot must be ready for immediate takeoff.)

    how to do it

    mass of the plane m = m kg
    given velocity of the plane be = v m/sec
    initial kinetic energy of the plane
    K = mv^2 / 2
    the work done by the cables in bringing the plane to rest
    W = K
    the force excerted by the cables
    F = W / S
    where S is the disatance over which the plane brought to rest .


    My Work

    first I converted kN to Kg by multiplying 220 kN by 101.97 kg= 22433.4 kg

    then
    KE= (22433.4kg)(62m/s)^2 / 2= 43116994.8

    W=K but my answer was wrong

    and then

    43116994.8/88= 489965.85 which also was wrong

    PLEASE EXPLAIN
     
  2. jcsd
  3. Oct 5, 2008 #2
    Check your unit conversions. I believe your error exists there. Remember Force is mass times the acceleration due to gravity. This mass you will need to find the kinetic energy.
     
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