1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Angular Velocity on a circular platform

  1. Oct 23, 2008 #1
    A large horizontal circular platform (M=120.1 kg, r=3.45 m) rotates about a frictionless vertical axle. A student (m=73.3 kg) walks slowly from the rim of the platform toward the center. The angular velocity w of the system is 3.9 rad/s when the student is at the rim. Find w (in rad/s) when the student is 2.47 m from the center.

    Does the angular velocity w is not the same everywhere in the circular platform?...
    i put 3.9 rad/s and the answer was wrong
  2. jcsd
  3. Oct 23, 2008 #2
    i use de I1W1=I2W2

    if I= (1/2)Mr^2 then:


    finding out for... W2= [(R1^2) / (R2^2)]* W1

    W2= [(3.45^2)/(2.47^2)]*(3.9)
    W2= 7.61 rad/s

    the answer is wrongs....Can anyone help me plz...
  4. Oct 25, 2008 #3


    User Avatar
    Homework Helper

    Hi Naldo6,

    This equation is not correct. The angular momentum is not conserved for the student alone or the platform alone; it is conserved for the combination (platform+student). This means that the rotational inertia I has to be that of the combination (platform+ student).

    So for your equation I1W1=I2W2 :

    I1 = (initial I for platform) + (initial I for student)

    and a similar quantity for I2. Do you get the right answer?
  5. Oct 26, 2008 #4
    ty it gives me the correct answer with ur suggestion...
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook