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Homework Help: Air resistance on a suspended object in motion

  1. Jan 25, 2012 #1
    1. The problem statement, all variables and given/known data
    A fire helicopter carries a 625 kg empty water bucket at the end of a cable 21.7 m long. As the aircraft flies back from a fire at a constant speed of 36.6 m/s, the cable makes an angle of 35.3° with respect to the vertical. Calculate the force of air resistance on the bucket.

    2. Relevant equations

    3. The attempt at a solution
    I started with the rationalization that constant speed implies 0 acceleration, meaning Fnet=0 in both the x and y direction.
    Dividing the forces up into Fnety=T(tension in the cable)-W(mg)
    and Fnetx is where I get stuck. If the acceleration is 0, then which force is opposing air resistance?
    Should I be calculating the horizontal component of the displacement from the vertical?
    Help, please!
  2. jcsd
  3. Jan 25, 2012 #2


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    You don't need to get into wire tension. The weight (W) acts straight down, and the horiz air resistance force (Fx) acts left. These horiz and vert forces are trig related , using the given angle. The resultant of these 2 forces, W and Fx, must lie in the direction of the cable, from equilibrium consideration.
  4. Jan 26, 2012 #3
    If you want to use tension you can and then just cancel it out by dividing the x forces equation by the y forces equation.

    You'll have the upward Ty = F wt and the horizontal Tx = F air resistance
    The y component of T is of course Tcos of your vertical angle and the x component, Tsin of the vertical angle.

    When you divide T's cancel and you have the same equation as the one you'll get if you use PJ's faster method.
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