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Homework Help: Horizontal Force to Hold Pendulum Perpendicular

  1. Oct 15, 2011 #1

    Recently I got this problem: what sustained horizontal wind pressure/force is needed to make a pendulum swing up and stay at a certain angle?

    To solve this problem, I imagined applying a horizontal force FH to the bob (of mass "m"). I learned that the angle that the pendulum makes with the vertical (denoted as "theta") is found by solving:

    tan (theta) = FH / mg

    Is this correct? Does this mean that a sustained force of FH will keep the pendulum at the angle theta? Because the equation seems to imply that you need an infinite FH to make the pendulum horizontal, i.e. theta = 90 degrees. Weird...?
  2. jcsd
  3. Oct 15, 2011 #2
    Well, if theta = 90 degrees, then tan (theta) is infinite which suggests that the wind speed required would also have to be infinite.
    In reality, there is no windspeed that would make the pendulum line horizontal. The angle will always be less than 90 degrees because gravity is always pulling down on the bob.
  4. Oct 17, 2011 #3
    Electrical wires are also NEVER horizontal. They always droop. Even a taught string, though it appears perfectly straight, isn't. There has to be a vertical component to the force holding it up, and that vertical somponent comes from the angle.
  5. Oct 17, 2011 #4


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    You can allways shape the mass as a wing so the passing wind makes sub-pressure above it in order to lift the mass the last degrees untill the pendulum rest at 90 degrees. However, that will also mean that the wind change direction when passing the mass/wing - meaning the wind will not move stright horizontal, but slightly downwards around the wing.

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