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Homework Help: 3 problems on Rotational Dynamics

  1. Nov 17, 2007 #1
    On this assignment I could do most of the work easily, I just have some questions on three problems

    3. The problem statement, all variables and given/known data
    A playground merry-go-round of radius R = 2.00 m has a moment of inertia I = 270 kg·m2 and is rotating at 9.0 rev/min about a frictionless vertical axle. Facing the axle, a 26.0 kg child hops onto the merry-go-round and manages to sit down on its edge. What is the new angular speed of the merry-go-round?

    3. Relevant equations
    [tex]\omega[/tex]= v/r

    3. The attempt at a solution

    I attempted to use conservation of momentum.

    Solve for M (mass of merry-go-round)
    M=135 kg

    Angular velocity
    Circumference of merry-go-round=12.5664 m
    12.5664 * 9
    Initial velocity=113.0976

    Initial Momentum

    So final momentum should be the same
    30536.352/ 2(162)=94.248

    This is wrong


    4. The problem statement, all variables and given/known data
    A wooden block of mass M resting on a frictionless, horizontal surface is attached to a rigid rod of length and of negligible mass. The rod is pivoted at the other end. A bullet of mass m traveling parallel to the horizontal surface and perpendicular to the rod with speed v hits the block and becomes embedded in it.

    (a) What is the angular momentum of the bullet- block system? (For the following answers, use M for the mass M, m for the mass m, and L for the length .)

    (b) What fraction of the original kinetic energy is converted into internal energy in the collision?

    4. Relevant equations
    ? If someone could just point me in the right direction

    4. The attempt at a solution


    5. The problem statement, all variables and given/known data
    Two astronauts, each having a mass of 70.0 kg, are connected by a 9.5 m rope of negligible mass. They are isolated in space, orbiting their center of mass at speeds of 5.50 m/s.

    (a) Treating the astronauts as particles, calculate the magnitude of the angular momentum.

    (b) Calculate the rotational energy of the system.

    (c) By pulling on the rope, one of the astronauts shortens the distance between them to 5.00 m. What is the new angular momentum of the system?

    (d) What are the astronauts' new speeds?

    (e) What is the new rotational energy of the system?

    (f) How much work does the astronaut do in shortening the rope?

    5. Relevant equations
    [tex]\omega[/tex]= v/r

    5. The attempt at a solution

    I have the answers for a-e, but I'm having a hard time with f
    A=3657.5 kg·m[tex]^{2}[/tex]/s
    B=2117.5193 J
    C=3657.5 kg·m[tex]^{2}[/tex]/s
    D=10.5 m/s
    E=7717.5 J

    *fixed momentums

    So F should be
    or 7717.5-2117.5193=5599.9807
    But this is the wrong answer

    *I figured it out, its supposed to be in Kj
    Last edited: Nov 18, 2007
  2. jcsd
  3. Nov 17, 2007 #2
    I hit post too soon on accident, here is the edit.
  4. Nov 17, 2007 #3


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    Homework Helper

    Some suggestions:

  5. Nov 17, 2007 #4


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    Homework Helper

    3.Angular velocity w = 2*pi*9/60 rad/s
    Angular momentum = Iw Using this try again.
    4.Using conservation of momentum find the velocity of the combined system of block and bullet. Then find the angular momenum. You can find the KE of bullet and bullet-block system.
  6. Nov 18, 2007 #5
    OK, for #5 I was being dyslexic when I found angular momentum and switched the 5 and the 7 at the end, and I was doing the right thing for finding the work done, I just forgot to convert to KJ.
    I'm working on the rest now.
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