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Moment of Inertia? Potential and Kinetic Energy? Anybody awake?

  1. Oct 17, 2012 #1
    I don't know how to answer these - please please please help!

    Consider the following objects of mass m rolling down an incline of height h.

    (a) A hoop has a moment of inertia I = (1/2)mr2. What is the equation for the velocity vhoop of the hoop at the bottom of the incline? (Use the following as necessary: m, h, r, and g.)

    vhoop =

    (b) A solid cylinder has a moment of inertia I = (1/2)mr2. What is the equation for the velocity vcylinder of the cylinder at the bottom of the incline? (Use the following as necessary: m, h, r, and g.)

    vcylinder =

    (c) We know that the velocity of the sphere at the bottom of the ramp is √ [ (10gh)/7 ] from which we can conclude that the mass of the sphere does not affect the velocity of the sphere. Which of the following statements help to explain why the equations for the velocity in the case of the rolling cylinder and rolling hoop should be different from each other and from that of the sphere? (Select all that apply.)

    a) Even if all three objects had the same mass and the same radius, they would all have different moments of inertia and therefore different rotational kinetic energies, which will affect the velocity of the object as it rolls down the incline.
    b) The rotational kinetic energy of a solid depends on its moment of inertia I, which will affect the velocity of the object as it rolls down the incline.
    c) The moment of inertial of a sphere, cylinder and hoop are different because of how the mass is distributed in each of these objects. This would affect the velocity of the object as it rolls down the incline.
     
  2. jcsd
  3. Oct 17, 2012 #2

    SammyS

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    The moment of inertia for the hoop is mr2, not 1/2 there.
     
    Last edited: Oct 17, 2012
  4. Oct 17, 2012 #3
    Oh oops, you're right..
    Do you happen to know the answer to part a, b, and c though?
     
  5. Oct 17, 2012 #4
    also, isn't part c all three of them (choices a, b, and c)?
     
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