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 the mass of the merry go round: conservation of angular momentum?

  1. Nov 21, 2009 #1
    1. The problem statement, all variables and given/known data
    A child exerts a tangential 41.6 N force on the rim of a disk-shaped merry-go-round with a radius of 2.40 m.
    If the merry-go-round starts at rest and acquires an angular speed of 0.0850 rev/s in 3.50 s, what is its mass?

    2. Relevant equations
    torque = r * F
    (I + mr^2) ω / t = torque
    I of a solid disk = 1/2 mr^2

    3. The attempt at a solution
    I found the tangential torque to be 99.84 N/m, and set the momentum equation to it, plugging in the moment of inertia. I got 713.7, and the answer is 227 kg. What am I doing wrong?
     
  2. jcsd
  3. Nov 21, 2009 #2

    Nabeshin

    User Avatar
    Science Advisor

    Two things:
    i) Note that the angular speed is given in revolutions/second, not rad/s.

    ii) In your second equation, what are you assuming about the placement of the child? Does this assumption make sense?
     
  4. Nov 21, 2009 #3
    Once I convert the angular velocity, I can get the correct answer if I multiply it by 2:
    2 * [(torque * t) / (angular velocity)] = m*r^2
    Why does it work this way and not the other way?
     
  5. Nov 21, 2009 #4

    Nabeshin

    User Avatar
    Science Advisor

    The only moment of inertia in question here is just that of the merry go round (i.e 1/2mr^2) with m as the mass of the merry go ground. I think the way you were doing it, you put an extra object of mass m on the rim (so you had an extra +mr^2 term)!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Find the mass of the merry go round: conservation of angular momentum?
Loading...