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Circular Motion

  1. Nov 10, 2008 #1
    1. The problem statement, all variables and given/known data

    Snoopy is flying his vintage war plane, chasing the Red Baron. His instruments indicate that he is at the bottom of the loop and is traveling 180km/h (50m/s). He is, of course, sitting on a bathroom scale that reads 4 times his normal weight. What is the radius of the loop?

    2. Relevant equations

    Fc = mv^2 / r

    3. The attempt at a solution

    What force would lead to the centripetal force? I know that Fg is acting away from the center. What does his weight being 4 times larger affect?
  2. jcsd
  3. Nov 10, 2008 #2


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    At the bottom of the loop he has 2 forces acting on him.

    Radial acceleration and gravity. So where are the other 3 g's of acceleration coming from?
  4. Nov 10, 2008 #3
    Would they be a result of the air pushing up on the plane (lift) ?

    Would the force towards the center be 4Fg ?
    Last edited: Nov 10, 2008
  5. Nov 10, 2008 #4


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    The question is wanting you to figure on the basis of the speed and its circular motion what the radius is. The net force coming from radial acceleration will be 3*m*g.
  6. Nov 11, 2008 #5
    Your saying that the net force is 3mg, and the force due to gravity is mg

    Fup - Fg = Fnet

    So there must be a force of 4mg acting upwards on the plane at the bottom of the loop.

    What force is this?
  7. Nov 11, 2008 #6


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    The lift of the plane is not the issue. What you are measuring on the scale is the weight of the pilot. What you also have pressing the pilot into his seat is his mass being accelerated outwardly (you know that centripetal thing) plus the fact that he is also at the bottom of the loop and there is his weight also added to this force.
  8. Nov 11, 2008 #7
    Since 3mg is the net force, this would be pointing towards the center?

    So would..
    3mg = mv^2 / r

    And rearrange to solve for r ? and the m should cancle out
  9. Nov 11, 2008 #8


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    I think you are on to something. That should do it.
  10. Nov 11, 2008 #9

    Just trying to understand how you got the net force to equal 3mg

    Fnet = ma

    so would the radial acceleration be 3g ?

    Then the Fnet would equal 3mg

    But how would you determine that the acceleration is 3g.

    How would you present this information if you were being marked on your method and not on the final answer.
  11. Nov 11, 2008 #10


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    Write out the equation for what the net force is.

    What is it made up of. What does it equal.
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