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Force of Wind on a moving object.

  1. Dec 6, 2009 #1
    Hi All,

    I have a problem in which I am trying to calculate the force of the windspeed, x, on a surface area of a moving object, y.

    I am applying this to a monorail model, where a monorail carriage (lets for this purpose assume its a rectangular box) with a surface area on one side is y, is travelling at s km/h, with a direct crosswind speed of x.

    I am trying to assertain if a) the weight of the carriage is important in finding out the total sideways force. We can assume that the monorail can support itself in regards to downwards force, but I am trying to find out the sideways force applied to the carriage.

    If I have missed anything feel free to add your input - I only have high-school grad level physics knowledge!

    Thankyou in advance,
  2. jcsd
  3. Dec 6, 2009 #2


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    Staff: Mentor

    You're looking to calculate drag. Here's the equation and an explanation of how to use it: http://www.grc.nasa.gov/WWW/K-12/airplane/drageq.html

    The problem you will run into is that the drag coefficient is very difficult to calculate so you'll probably need to research and find typical drag coefficients for trains.
  4. Dec 6, 2009 #3
    The weight of the carriage has zero effect on the force from the air resistance - or wind.

    However it is more likely the object will tip over if the position of its centre of mass is very low. But it's less likely to loose it's desired trajectory (it'll stay on the rail) if the mass of the object is very large.
  5. Dec 6, 2009 #4
    Thankyou for your reply's.

    So the drag equation is D = Cd x ((P x V^2)/2) x A

    With V^2 (Velocity squared) do take the sum of the wind speed and carriage speed? i.e. if wind and carriage are same direction, addition, and if in opposite directions, subtraction?

    The weight of the carriage will be high, maybe up to 30 tonnes. As it on a monorail I will need to find specifically the limitations on sideways force applied to the carriage that the monorail can handle. Does anyone know how I would be able to calculate this 'sideways' force due to the wind? Would I use the drag model again?
  6. Dec 6, 2009 #5
    At 30 tonnes I doubt the air resistance is going to have much effect on the behaviour of the carriage.

    But yes, add the velocities together, they're vectors them insert in the equation. ie If your travelling forward at 100km/hr and you have a 100km/hr tail wind, the force from air resistancwe would be just zero. If you had a head wind of 100km/hr then V is effectively 200km/hr (that's V = 27.78m/s).
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