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Calculate force from wind on kite

  1. Feb 3, 2012 #1
    1. The problem statement, all variables and given/known data

    A large kite of mass 3.8kg is flying through the air on a windy day. Currently, the tension from the string on the kite has a magnitude of 8.7N at an angle θ=33.7 degrees. The current acceleration of the kite has a magnitude of a=6.28m/s/s at an angle [itex]\Phi[/itex]=38.7 degrees. The only forces felt by the kite are its own weight, the tension from the string, and a foce from the wind. Find x-component of the force from the wind on the kite. The components of a are acos[itex]\Phi[/itex] in the negative x-direction, and asin[itex]\Phi[/itex] in the positive y-direction.

    2. Relevant equations
    ƩF=ma for the x and y directions


    3. The attempt at a solution
    ƩFx=m(acos[itex]\Phi[/itex])
    FTsinθ-Fx,WKcos[itex]\Phi[/itex]=m(acos[itex]\Phi)[/itex])
    Fx,WK=[-m(a)+FTsinθ]/cos[itex]\Phi[/itex]

    This doesn't seem to be working
     
  2. jcsd
  3. Feb 4, 2012 #2

    BruceW

    User Avatar
    Homework Helper

    FTsinθ-Fx,WKcosΦ=m(acosΦ))

    Here, you don't need to multiply Fx,WK by cosΦ because the fact that it is the x component 'takes into account' the angle of the wind force, if you see what I mean. Also, the angle wouldn't be the same as the angle of the acceleration of the kite anyway.

    Also, you wrote: ƩFx=m(acosΦ) But it says in the question that the component of a in the negative x direction is acosΦ, so shouldn't there be a negative sign here?

    Another possible reason that you might have the wrong answer is that you might have interpreted the angles wrong. I can't tell, because I don't know where the angles are being measured from / what direction, since I don't have the diagram.
     
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