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Centripetal force

  1. Jan 30, 2015 #1
    1. The problem statement, all variables and given/known data
    The centripetal force acting on an object is doubled. The radius is also doubled. The speed of the object:

    (a) increases by a factor of 4
    (b) increases by a factor of 2
    (c) decreases by a factor of 2
    (d) The change in the object's speed depends on its mass

    2. Relevant equations
    F = mv2/r

    3. The attempt at a solution
    So if I substitute 2's in for F and r in the above equation, my 2's cancel but the correct answer is a. ???
     
  2. jcsd
  3. Jan 30, 2015 #2

    BvU

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    Do a numerical example....
     
  4. Jan 30, 2015 #3

    haruspex

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    Not happy with the wording of the question. Centripetal force does not act. It is the force required to be supplied by forces which do act in order to produce a certain motion.
    Anyway, I don't get how the 2's would cancel. (Not that I necessarily agree with the supposed answer.). Please post detailed working.
     
  5. Jan 30, 2015 #4
    I am sorry; an erroneous math calculation. By substituting 2 for both the force and the radius, I get 4 which is (a). Thank you!
     
  6. Jan 30, 2015 #5

    BvU

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    Can you still show your calculation ? It doesn't fit at all.
     
  7. Jan 30, 2015 #6

    haruspex

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    As I hinted, I disagree with that answer. Looks like BvU disagrees with it too.
     
  8. Jan 30, 2015 #7
    Sorry everyone! That's what happens when you don't read the answer choices correctly! Okay here is the original question: the centripetal force acting on an object is doubled. The radius of the object's motion is also doubled. The speed of the object is:
    (A) increases by a factor of
    (B) increases by a factor of 2
    (C) decreases by a factor of 2
    (D) the change in the object's speed depends on its mass.

    Fc = mv2/r. If I plug in 2 for F and r, I get 2 = mv2/2 = 4 = mv2. Therefore, the correct answer is (B) increases by a factor of 2! Thanks everyone!
     
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