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Homework Help: Lift and Drag of a Bird.

  1. Apr 1, 2013 #1
    1. A bird is gliding at an angle of 15° to the horizontal at a constant velocity. If the mass of
    the bird is 0.45 kg, calculate the lift and drag forces acting on the bird.

    2.A bird is gliding with a constant velocity. Its forward speed is 5 m/s and its sinking speed
    is 0.88 m/s. If the lift force is 29.43 N, calculate the drag force acting on the bird.

    So. I have two questions I don't really know what formulas to use or anything as i havn't been in a physics type class in a while, if someone could help answer these and explain them to me a bit or just explain them it'd be great :).

    I don't know what level this would be at in terms of introductory (which i'm assuming it is because the class i'm getting this in is not a physics class) or advanced considering this is a senior class.
  2. jcsd
  3. Apr 1, 2013 #2


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    Staff: Mentor

    Welcome to the PF.

    We do not provide solutions here, sorry. We require that you do the bulk of the work on your schoolwork questions.

    You would start each question by drawing a free body diagram (FBD) showing all of the vertical and horizontal forces on the birds. One key in the questions is they say "constant velocity", which means no net acceleration either vertically or horizontally. Since F=ma, what can you say about the balance of forces in each of the two dimensions?

    (And yes, these are Intro Physics questions. Here on the PF, Advanced Physics questions are for upper-division college and graduate-level questions.) :smile:
  4. Apr 1, 2013 #3
    Ahh, okay, well honestly, i don't think we've covered this in class. I have a free body diagram already.

    Do you have any ideas on what equations i could use to solve these ? I understand F=ma, but due to them having angles it messes me up. I know Drag is 15 degrees from the horizontal and Lift is 15 degrees from the vertical.

    Yeah honestly i didn't know what this would have been classed as it's a 4th year biomechanics class aha
  5. Apr 1, 2013 #4


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    Staff: Mentor

    If you can post your FBDs with all the forces and angles shown, that would help us to help you. You can post a (good quality) picture as an attachment, or scan the FBDs to PDF documents and add as attachments.
  6. Apr 1, 2013 #5
    I think i attached it.

    Attached Files:

    • FBD.png
      File size:
      4.5 KB
  7. Apr 1, 2013 #6


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    Homework Helper

    Note that lift to drag ratio is the same as glide ratio (forward speed / downwards speed) (for an aircraft or gliding bird at constant speed). I don't know if this was explained to you in class or in your text book.
  8. Apr 1, 2013 #7
    Nothing in class, and i didn't buy the textbook. But thanks, i'll try googling still. If anyone knows any equations that'd be grand though.
  9. Apr 1, 2013 #8


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    Staff: Mentor

    Sounds like it would have helped if you'd purchased the book. Is it available in your university library to borrow?

    We don't give you the equations here, you really need to be able to figure out which ones to use. But I've already given you a big hint about the sum of the forces in each dimension. In your first problem, what is the component of the gravitational force in the vertical direction? In the horizontal direction? What forces counter those forces....?
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