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Newton Question #2 (centripetal motion)

  1. Nov 26, 2009 #1
    Question reads: Snoopy is flying his vintage war plane in a "loop the loop" patch chasing the red baron. his instrumenets tell him the plane is level ( at the bottom of the loop) and travelling with a speed of 180km/h. he is sitting on a set of bathroom scales. and notes that they read four times the normal force of gravity on him. what is the radius of the loop. answer in meters

    My progress: ok so we know that the plane has velocity of 180km/h which is 50m/s.
    And there are two forces acting on the plane at the bottom of the loop.
    Fn [upward] and Fg [downward] (in this case Fn is greater? because it's acceleration is towards center,)

    fn - fg = ma
    4mg - mg = mv2/r ??

    is this correct ?
  2. jcsd
  3. Nov 26, 2009 #2


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    Welcome to PF!

    Hi lovemake1! Welcome to PF! :smile:
    good grief! :biggrin:
    (try using the X2 tag just above the Reply box :wink:)

    Never mind the forces on the plane, you want the forces on Snoopy.

    And don't confuse forces and acceleration … they're on opposite sides of F = ma. :wink:

    So how many forces are there on Snoopy? Just bung them all on one side of the equation, and ma on the other side.

    (maybe you have the right answer, and maybe you haven't, but you're not sure, so …)

    Try again! :smile:
  4. Nov 26, 2009 #3
    i solved it ~ !! great hint there tiny-tim.
    i've realized that the quesetion is specifically giving you the relative forces of snoopy to the plane at that moment. so i've realized that Fg would be greater than Fn beacuse there are 4 times the forces of gravity acting on snoopy ( feeling heavier).

    so the equation that i use to solve this problem was

    Fg - Fn = ma
    which turns to be
    4Fg - mg = ma
    4mg - mg = mv^2/r
    r = v2/3g
    and r comes out to be 85m just like in the answer. !!

    Thanks for your kind assist. Hope to see you again with more of these tricky problems :D
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