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!

Find angular momentum

  1. Dec 25, 2016 #1
    1. The problem statement, all variables and given/known data

    upload_2016-12-25_15-43-27.png
    2. Relevant equations
    I= sum m r2
    L= r p
    or
    L=I W

    3. The attempt at a solution
    I= m1 r12 + m2 r22
    I= 5.20 (0.9)2+ 2.20(0.9)2= 5.994 kg.m2

    Then I used the second equation of second momentum
    L(Angular momentum) = I W
    L= 5.994 x 4.60

    In the solutions sheet, he used the first rule: L= r p and he got a different answer than mine: What did I do wrong?

    upload_2016-12-25_15-49-31.png
     
  2. jcsd
  3. Dec 25, 2016 #2

    gneill

    User Avatar

    Staff: Mentor

    "4.60" is the linear velocity. You need to use an angular velocity for W.
     
  4. Dec 25, 2016 #3
    How do I know if it's linear or angular velocity? He didn't mention if it's linear or angular.
     
  5. Dec 25, 2016 #4

    gneill

    User Avatar

    Staff: Mentor

    What are the units given for v? What are the units of linear velocity? How about angular velocity?
     
  6. Dec 25, 2016 #5
    Linear velocity has unit m/s
    Angular velocit has unit rad/s

    I got it :D. Thanks!!
     
  7. Dec 25, 2016 #6
    In the second equation L = rxp >> Is cross product between vectors so if we want the magnitude, we should use rxf sin(angle). Why he didn't do that?
     
  8. Dec 25, 2016 #7

    gneill

    User Avatar

    Staff: Mentor

    L = r x p is a vector expression. The magnitude of L is given by L = |r||p|sin(θ).

    In this instance the angle happens to be θ = 90° . Knowing that sin(90°) = 1 he wrote the simplified expression for the magnitude. Granted, to be technically correct he should have pointed this out in some fashion, but it's a common enough simplification that it shouldn't cause problems interpreting the solution.
     
  9. Dec 25, 2016 #8
    Can you tell me why the angle between vector r and vecor p is 90? isn't the angle between them =0? since they are in same direction
     
  10. Dec 25, 2016 #9

    gneill

    User Avatar

    Staff: Mentor

    p is a linear momentum of one of the particles. It would be co-linear with the velocity vector of that particle (p = mv). Since the particles are moving in a circle and thus velocities are tangential, the angle between the radius vector and the velocity must be 90°.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Find angular momentum
  1. Find Angular Momentum (Replies: 1)

Loading...