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Homework Help: Angular Velocity of the wheel

  1. Apr 18, 2014 #1
    A uniform Steel wheel of diameter, D=3m and mass, 8500kg is supported by a low friction bearing, shown as the large black spot in the drawing. Around the outer surface of the wheel, a light wire rope is wound. From the end of this wire rope, a mass, M=1000kg is attached

    Initially, both the wheel and the mass are at rest. The mass is released, and falls through a hight of 30m before hitting the earth's surface. As the mass falls, the tension in the wire rope turnes the wheel, i.e., the wire rope unswinds

    2. Relevant equations
    what is the speed of the mass just before it hits the ground ? and, what is the corresponding angular velocity of the wheel?

    (assume the acceleration owing to gravity is equal to 9.81m/s^2)

    3. The attempt at a solution

    I've had a go at both questions but I am not sure if they are right, could some one please let me know if this is correct or where I have gone wrong ?

    Third equation of motion
    initial velocity =U=0M/s
    uniformed accelleration= 9.81M/s
    distance traveled =t=30m



    angular velocity




    Attached Files:

    • disk.jpg
      File size:
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    Last edited: Apr 18, 2014
  2. jcsd
  3. Apr 18, 2014 #2
    The acceleration is not g. It would be if the steel wheel had negligible mass compared to the mass M.

    But imagine if the steel wheel were extremely large and massive and the mass M were very small. Would you think intuitively that the acceleration would still be g?
  4. Apr 18, 2014 #3
    Hi! I don't think your answers are right.
    According to the principle of conservation of energy, the the speed of the block will be as shown in the picture.
    The angular speed formula is correct, the problem is the speed.
    w=v/r <=> w=24.26/1.5=16.17rad/s

    Attached Files:

  5. Apr 18, 2014 #4
    Hope it helped!
  6. Apr 18, 2014 #5
    AAAHH i'm an idiot I forgot to square root the velocity!!! so this is correct then??


    angular velocity

  7. Apr 18, 2014 #6
    Yes! Now it is correct!
    Keep in mind that both velocities are squared in that particular equation
  8. Apr 18, 2014 #7
    Does the rotating wheel also have kinetic energy?
  9. Apr 18, 2014 #8

    I = moment of inertia= (1/2)mr^2
    w = angular speed = v/r


    When an object has translational as well as rotational motion, its total kinetic energy is the sum of the translational kinetic energy and the rotational kinetic energy.

    Please check my calculations
  10. Apr 18, 2014 #9
    Yes, but you didn't include this in your estimate of v for mass M.
  11. Apr 18, 2014 #10
    Ok let me try again.
    The energy of the system must stay the same, therefore, the variation of energy of the block must be the same (symmetric) as the variation of the energy of the wheel.
    The loss of potential energy of the block will result on the increase of the kinetic energy.

  12. Apr 18, 2014 #11
  13. Apr 18, 2014 #12
    Now what happened to the translational kinetic energy? Your first attempt included it but excluded the rotational component of kinetic energy. Now for some reason you are including rotational but excluding translational.

    Here is your own statement:
  14. Apr 18, 2014 #13
    But the wheel is attached isn't it? Therefore there would only be rotational kinetic energy..
  15. Apr 18, 2014 #14
    Attached to what?

    You have to look at the energy of the entire system. Or if you only want to look at the wheel then you have to consider all forces that exert work on the wheel.
  16. Apr 18, 2014 #15
    It says that the wheel is supported. There is no translational movement. Only rotational
  17. Apr 18, 2014 #16
    but the mass has translational velocity, yes?
  18. Apr 18, 2014 #17
    But the wheel as a whole does not move through space. It only spins around it's center of mass.
    Kinetic energy is only rotational.
  19. Apr 18, 2014 #18
    Yes, but it is attached to the falling mass which has a translational kinetic energy. you have to consider the system as a whole.
  20. Apr 18, 2014 #19
    Yes, but when the block comes to rest, all of its translational kinetic energy is transformed into rotational kinetic energy on the wheel. That is what my previous calculations demonstrated
  21. Apr 18, 2014 #20
    but that is not what the question asked.

    and even if it did you still calculated it wrong.
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