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Fluid Mechanics - Drag (Bending Moment)

  1. Apr 3, 2013 #1
    A television transmitting antenna is placed on top of a mast which is 30m high and 300mm in diameter. The mast is on top of a tall building. The wind blows at 35m/s and at normal atmospheric pressure (1.225kg/m^3). If the drag coefficient for the mast is 0.2. calculate:

    (i) the total drag force on the mast.
    (ii) the bending moment at the base of the mast.

    (i) Fd=.5*Cd*ρ*U^2*A=.5*.2*1.225*35^2*(30*0.3)=1350.6N

    (ii) but how do you do this question?

    (the answer is M=20.26kNm but i do not know how to get there)

    thank you in advance for your help.
     
  2. jcsd
  3. Apr 3, 2013 #2
    I think they're assuming the drag force is an even distributed load over the entire mast. With that as the case, you can place the 1350.6 N force at the midpoint of the mast (15m). The FBD yields 1350.6 N * 15m = 20259 Nm.
     
  4. Apr 3, 2013 #3
    that was potentially the most stupid question i have ever asked. thanks for your help sir.
     
  5. Apr 3, 2013 #4
    Hahaha you're welcome. Glad I could help.
     
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