1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Torque/moment of intertia questions

  1. Jan 22, 2007 #1
    i have two questions that i need help understanding

    1a. The problem statement, all variables and given/known data

    A majorette takes tow batons and fastens them together in the middle at right angles to make an "x" shape. Each baton was 0.6m long and each ball on the end is 0.50kg. (Ignore the mass of the rods.) What is the moment of inertia if the arrangement is spun around an axis formed by one of the batons?

    2a. Relevant equations

    I=(1/12)ML^2

    3a. The attempt at a solution

    Not sure how to apply the equation to the problem. And what does it mean when n the arrangement is spun around an axis?


    _______________________________________________

    1b. The problem statement, all variables and given/known data

    A tether ball is attached to a pole with 4.0m rope. It is circling at 0.20 rev per second. As the rope wraps around the pole it shortens. How long is the rope when the ball is moving at 6m/s

    2b. Relevant equations

    not sure

    but .20 rev/sec = 1.26 rads/sec

    3b. The attempt at a solution

    not sure what to do.. what equations should i be focusing on?
     
    Last edited: Jan 22, 2007
  2. jcsd
  3. Jan 22, 2007 #2

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    In problem one, the rod has no mass; you should focus your attention on the moment of inertia of the 'point' masses at the ends of the rods. If you consider one of the rods as the x axis, then the other rod is spinning clockwise or counter clockwise around that rod, as opposed to, for example, the case where both rods are rotating about an axis that is perpendicular to their plane of rotaion. You are looking at the first case. Identify the moment of inertia formula, and determine the distance of each ball mass as measured to the axis of rotation.
     
  4. Jan 22, 2007 #3
    if i do that (.5*.3^2) times to for each ball... so .09 kg-m^2... doest hat make sence?
     
  5. Jan 22, 2007 #4

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Makes a lot of sense, great. The other 2 balls don't contribute to the moment of inertia, since there is no distance of those masses to the axis of rotation. Now, page 2. Focus on conservation of angular momentum for the 2nd problem.
     
  6. Jan 22, 2007 #5

    well the conservation of angular momentum is L=Iw(w=omega)

    but i can't seem how to fit the givens in or come up with missing pieces like the moment of inertia
     
  7. Jan 22, 2007 #6

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    yeah, it's L_(initial) =L_(final). You can calculate I_(initial) now in terms of the ball's mass. You are given w_(initial). You are given the ball's final tangential speed. Try solving for R, the length of the shortened rope.
     
  8. Jan 22, 2007 #7
    well w=v/r

    so
    if L_(initial) =L_(final)
    and I=mr
    and L=Iw

    so

    mrw=mrw
    w=v/r
    mrw=mr(v/r)
    i canceled the m's out cause the mass is the same
    i also canceled the r's out on the right side
    r(1.26)=6
    so from that we get
    r=4.76
    but were not trying to get r were trying to get a length?

    mlw=ml(v/r)
    i canceled the m's out cause the mass is the same
    4(1.26)=l(6/4.76)

    l=4 again.. which doesn't make since.. ugh i am so lost
     
  9. Jan 22, 2007 #8

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    You said I =mr but of course you meant I =mr^2?
    Then r_(initial) is 4, and r_(final) is what you are trying to solve for (where
    r_(final) = length_(final). Give it another try....
     
  10. Jan 23, 2007 #9
    okay i got it 16(1.26)=r6 r=3.36

    thanks a lot phantomJay for all your help
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Torque/moment of intertia questions
  1. Moment of Intertia (Replies: 3)

  2. Moment of Intertia (Replies: 1)

Loading...