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Law Of Conservation Of Momentum Help

  1. Jun 15, 2015 #1
    1. The problem statement, all variables and given/known data

    A flywheel and shaft have a moment of inertia of 600kg and is rotating at 5 rev/s. Another flywheel and shaft, on the same axis, is suddenly connected by means of a clutch. If this flywheel and shaft has a moment of inertia of 620kg and is initially at rest, calculate:

    a) The common speed of rotation after the two are connected

    b) The loss in kinetic energy

    Answer is attached in document...

    But i need to describe the law of conservation of momentum and showing where it is used in this calculation and i need to describe what would the consequence for my calculations were the law of conservation not to be true....

    Its for a practice exam paper and i seriously need some help? thanks!!
     

    Attached Files:

    Last edited by a moderator: Jun 15, 2015
  2. jcsd
  3. Jun 15, 2015 #2

    berkeman

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    Staff: Mentor

    Welcome to the PF.

    First, there are typos in the units of Moment of Inertia (MOI) where you typed the problem in above. I've bolded the typos -- you can see what is missing by comparing to your attachment.

    Second, the attachment does a good job of working through the problem to get to the solutions. Is there some part that you don't understand? What is the definition of the angular momentum of a object in terms of its MOI and it's angular frequency? :smile:
     
  4. Jun 15, 2015 #3
    Heyy yeah i couldn't work out how to square the Kg,

    im just trying to work this out:

    describe the law of conservation of momentum and showing where it is used in this calculation?

    and

    to describe what would the consequence for my calculations were the law of conservation not to be true?

    But thanks for replying
     
  5. Jun 15, 2015 #4

    berkeman

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    Staff: Mentor

    There is an explicit step in that solution where conservation of angular momentum is used. That's why I asked you what the equation is for the angular momentum of a spinning object... :smile:
     
  6. Jun 15, 2015 #5
    The angular moment of a spinning object in this solution is I1W1 + I2W2 = I3W3?
     
  7. Jun 15, 2015 #6

    berkeman

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    Staff: Mentor

    Correct. That line is using the law of conservation of angular momentum. That is how this problem is solved, since kinetic energy is not conserved in this type of collision/rotation-clamping situation. :smile:
     
  8. Jun 15, 2015 #7
    So what would be the problem if my calculations were the law of conservation not to be true?

    Would i just receive a incorrect value?
     
  9. Jun 15, 2015 #8

    berkeman

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    Staff: Mentor

    Beats me. That's kind of a weird question. The world wouldn't work anymore... :woot:
     
  10. Jun 15, 2015 #9
    Yeah thats whats i thought, but cheers for the insight helps a lot!!
     
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