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Homework Help: FBD representing a bird feeder.

  1. Feb 18, 2013 #1
    1. The problem statement, all variables and given/known data
    A bird sits in a birdfeeder suspended from a tree by a wire, as shown in the diagram below.
    Let WB and WF be the weight of the bird and the feeder respectively. Let T be the tension in the wire and N be the normal force between the bird and the feeder. Which of the following free-body diagrams best represents the birdfeeder? (The force vectors are not drawn to scale and are only meant to show the direction, not the magnitude, of each force.)

    The image and possible solutions are shown here: http://i48.tinypic.com/5zirzp.png

    2. Relevant equations

    None, its a conceptual question.

    3. The attempt at a solution

    I understand that there must be a force of tension and the weight of the feeder that acts on the birdfeeder. The correct answer is (e) in the picture provided... whereas I thought it was (f). I was just wondering if anyone can clarify the distinction between the N force and W{B} force and why it is the N force that must be considered rather than weight of the bird.
    Thank you.
  2. jcsd
  3. Feb 18, 2013 #2


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    Hello, Physicsdude123. Welcome to PF!

    The "weight" of the bird is the force of gravity that acts on the bird. It is a force that the earth mysteriously exerts on the bird through empty space ("action at a distance"). So, the weight of the bird is a force acting on the bird, not on the feeder.

    The bird happens to be in direct contact with the feeder. So, the bird exerts a "contact force" on the feeder where part of the surface of the bird presses on part of the surface of the feeder. This force is called a "normal" force since it acts perpendicularly to the surfaces in contact.

    Now, it might happen in a particular situation that the magnitude of the normal force that the bird exerts on the feeder equals the magnitude of the force that the earth exerts on the bird. But, even so, the weight of the bird and the normal force that the bird exerts on the feeder are distinct forces. When drawing the free-body diagram of the feeder, you would include the normal force that the bird exerts on the feeder but you would not include the force that the earth exerts on the bird (i.e., the weight of the bird).
    Last edited: Feb 18, 2013
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