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!

Angular Speed of a disk

  1. Nov 18, 2014 #1
    1. The problem statement, all variables and given/known data

    A disk of radius 0.41 m and moment of inertia 2.8 kg·m2 is mounted on a nearly frictionless axle. A string is wrapped tightly around the disk, and you pull on the string with a constant force of 52 N.

    11-108-rotating_disk.jpg

    What is the magnitude of the torque?
    torque = 21.32 N·m

    After a short time the disk has reached an angular speed of 4 radians/s, rotating clockwise. What is the angular speed 0.63 seconds later?
    angular speed = ??? radians/s

    2. Relevant equations

    torque = RFT

    Rotational angular momentum = (MR2/2)ω

    3. The attempt at a solution

    The new angular momentum is the old angular momentum plus the angular impulse, torque times time interval.

    (2.8/2)4 + (21.32)(.63) = 19.03 radians/s

    However, this answers is wrong, and I can't figure out what I am doing wrong.
     
  2. jcsd
  3. Nov 18, 2014 #2

    ehild

    User Avatar
    Homework Helper
    Gold Member

    The moment of inertia is given, it is 2.8 kgm2. Why did you divide it by 2?
     
  4. Nov 18, 2014 #3
    According to my textbook, Rotational angular momentum = (MR2/2)ω

    Therefore, I divided it by two. Is that incorrect?
     
  5. Nov 18, 2014 #4

    ehild

    User Avatar
    Homework Helper
    Gold Member

    MR2/2 is the moment of inertia of a homogeneous disk. The mass is not given. The moment of inertia is given as 2.8 kgm2. The angular momentum is moment of inertia times the angular speed.
     
  6. Nov 18, 2014 #5
    So is the Rotational angular momentum =
     
  7. Nov 18, 2014 #6

    ehild

    User Avatar
    Homework Helper
    Gold Member

    Yes, the angular momentum of a rotating body is Iω.
     
  8. Nov 18, 2014 #7
    So now I take (2.8)(4) + 21.32(.63) and divide that by moment of inertia?
     
  9. Nov 18, 2014 #8

    ehild

    User Avatar
    Homework Helper
    Gold Member

    Yes.
     
  10. Nov 18, 2014 #9
    Thank you for your help!
     
  11. Nov 18, 2014 #10

    ehild

    User Avatar
    Homework Helper
    Gold Member

    You are welcome:)
     
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: Angular Speed of a disk
  1. Angular Speed of disk (Replies: 2)

Loading...