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!

Conservation of Angular Momentum

  1. Oct 31, 2012 #1
    1. The problem statement, all variables and given/known data
    A solid cylinder merry-go-round of mass 250kg and radius of 1.9m spinning at 1 revolution every 5 seconds has a 40kg child sitting at 1.1m from the axis. A 50kg child, running tangentially at 3m/s, jumps on the merry go round at the outer edge. What is the new rotation rate in rev/s?


    2. Relevant equations
    I1ω1=I2ω2


    3. The attempt at a solution
    (1/2MR^2 +m1r^2)ω1 + (m2r^2)(v/r)=(1/2MR^2 +m1r^2 +m2r^2)ω2
    I plugged in all the variables and solved for ω2 and got 0.214 rev/s. Did I set up the equation correctly and come about the right answer?
     
  2. jcsd
  3. Oct 31, 2012 #2

    ehild

    User Avatar
    Homework Helper
    Gold Member

    The radii are different for the children.

    ehild
     
  4. Oct 31, 2012 #3
    yes I plugged in the different radii. I just forgot to put a 1 and 2 behind them in the equations.
     
  5. Oct 31, 2012 #4

    ehild

    User Avatar
    Homework Helper
    Gold Member

    Could you please show your calculations in detail?

    ehild
     
  6. Oct 31, 2012 #5
    (1/2(250kg)(1.9m)^2)(1.256637061rad/s)+((50kg)(1.9m)^2(3m/s/1.9m)=(1/2(250kg)(1.9m)^2+(40kg)(1.1m)^2+(50kg)(1.9m)^2)w2.

    solved for angular velocity(w2) and got 0.214rev/s. is this the right set up?
     
  7. Oct 31, 2012 #6

    ehild

    User Avatar
    Homework Helper
    Gold Member

    You left out the first child on the left hand side.

    ehild
     
  8. Oct 31, 2012 #7
    I did it on paper just forgot to type it on here. When I include the first child on the left hand side, is that the right setup?
     
  9. Oct 31, 2012 #8

    ehild

    User Avatar
    Homework Helper
    Gold Member

    It is, but I got different result. Show calculations in detail.

    ehild
     
  10. Oct 31, 2012 #9
    Did you change the initial angular velocity from revolution/sec to rad/sec and then change the final answer back to revolutions/sec
     
  11. Oct 31, 2012 #10

    ehild

    User Avatar
    Homework Helper
    Gold Member

    Ops! You are right. I calculated w. So your result is correct.

    ehild
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook