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!

Skater. Angular Momentum. Velocity.

  1. Apr 3, 2012 #1
    1. The problem statement, all variables and given/known data

    10. A skater brings her arms in close to her body during a spin. What is the effect on her
    angular momentum and angular velocity?

    A) Angular momentum increases, velocity increases.
    B) Angular momentum decreases, velocity decreases.
    C) Angular momentum remains constant, velocity increases.
    D) There is insufficient information to tell what will happen


    2. Relevant equations

    L = Iω
    ω = v / r

    3. The attempt at a solution

    By bringing her arms in closer, she is decreasing the radius of her moment of inertia. As "r" gets smaller, v gets larger so ω becomes larger. When ω becomes larger, L is larger too because they're directly proportional. I choose "A." Do you all agree?
     
  2. jcsd
  3. Apr 3, 2012 #2

    tms

    User Avatar

    No. What external forces are at work here? (I assume friction is being ignored.)
     
  4. Apr 3, 2012 #3

    rcgldr

    User Avatar
    Homework Helper

    and what effect does that have on her moment of inertia?
     
  5. Apr 3, 2012 #4
    tms - Erm, gravity?

    rcgldr - I = Mr^2 for a point mass so that decreases I.

    ω * r = v

    I just said "r" decreased. Perhaps letter "B" is a better choice.
     
  6. Apr 3, 2012 #5

    rcgldr

    User Avatar
    Homework Helper

    Yes, but is that affecting the angular momentum? If no external force is affecting angular momentum, then what usually happens with angular momentum.

    So what do you think happens with angular momentum?
     
  7. Apr 4, 2012 #6
    rcgldr - When no external forces are acting, angular momentum is conserved. But if "I" decreases, "L" should too.
     
  8. Apr 4, 2012 #7

    rcgldr

    User Avatar
    Homework Helper

    If angular momentum is conserved and "I" decreases, then what should happen with ω ?
     
  9. Apr 4, 2012 #8
    Angular momentum must increase to compensate for that. I was thinking of "v" and not "ω." So if someone came along and pushed the skater, would that be an external force?

    "C."
     
  10. Apr 4, 2012 #9

    rcgldr

    User Avatar
    Homework Helper

    Did you mean that angular velocity must increase (since angular momentum is conserved)?
     
  11. Apr 4, 2012 #10
    C.is correct:angular momentum remains constant,velocity increase because radius decrease...
     
  12. Apr 4, 2012 #11
    rcgldr - Yes. That's what I should have typed. Sorry about that.
     
  13. Apr 4, 2012 #12

    tms

    User Avatar

    Which is exactly countered by the force of the ice on the skater, thus no net external force. Unless, perhaps, the skater is boring a hole through the ice, but I think we can ignore that possibility.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook