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Homework Help: Final Frequency and Kinetic energy change?

  1. Jun 20, 2016 #1
    1. The problem statement, all variables and given/known data
    Hi I know my picture has 2 problems, but lets just focus on the first problem parts a and b.
    So for part a it ask for the final frequency after the ant landed, and part b is the kinetic energy difference. Basically the
    ant=.01kg, diameter of cylinder is .115m,
    the weight of cylinder is .03kg,
    and its rotating at 300 rev/min.

    2. Relevant equations

    kfinal=.5*i*w where i is the inertia

    3. The attempt at a solution
    For part a I used the the rotational equation formulas, Im not sire if i picked the correct one though. For part b, its based on part a, basically find the kinital and final with the mass changing from .030g t0 .040 cause the ant has its own weight. Part b I understand its jsut part a I am dont really know where to start?

    Attached Files:

  2. jcsd
  3. Jun 20, 2016 #2

    Simon Bridge

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    The pic is about a bullet hitting a wooden block.
    Note: do not do physics by trying to guess the right formula. Instead, try to figure out what physical principle is at work ... i.e. some sort of conservation law.
    Guessing you have a rotating cylinder that an ant drops on ... part b is easier.
    Part (a) the frequency will be related to the angular velocity, which is related to both angular momentum and kinetic energy. The kinetic energy is also related to angular momentum.
  4. Jun 21, 2016 #3
    Omg sorry I uploaded the wrong picture, its this one. Please disreguard the image in my first post. Also for #1 part a, I need to get the final rev/min after the ant falls on it, so I am thinking, w=2*pi*f. But I have two unknowns because I don't know how to get f? unless w= winitial which is 300 rev/min?

    Attached Files:

    Last edited: Jun 21, 2016
  5. Jun 21, 2016 #4
    Sorry for double posting but I can't really find Ffinal because I don't see how I can use the initial frequency of 300 to get the final frequency?
  6. Jun 22, 2016 #5


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    Read Simon's post again. Don't start with equations, think about principles. See if you can reply to this without mentioning a single equation :-) If you get that right, then you can think about which equations might be relevant.

    So which principle should you apply here?
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