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!

When is the net torque=0?

  1. Apr 24, 2017 #1
    1. The problem statement, all variables and given/known data
    A child is initially sitting near the outer rim of a revolving merry-go-round. Suddenly, the child moves towards the center of the merry-go-round (while it is still revolving). For the merry-go-round+child system, let the symbols L and K refer to the magnitude of the angular momentum (about the center of the merry-go-round) and rotational kinetic energy, respectively.

    Consider the following statements:

    Ia. L is conserved Ib. L increases Ic. L decreases

    IIa. K is conserved IIb. K increases IIc. K decreases

    Which of these statements are true? (The explanation is for the choice of ’II’)


    2. Relevant equations
    So, I know L is conserved/constant when dl/dt=0. And I know dl/dt=0 when net torque =0. But, how can I tell from reading this problem that the net torque is zero?
    when I draw a diagram of the merry go round and the child on it and make my axis the center of the merry go round, I get Net torque = -mgR (where m is mass of child and R is radius of merry go round). I just don't get how to tell that the net torque in certain problems =0 (and when it doesn't).

    Also, I thought for this problem that initially, the merry go round is rotating (has Krot), and the child is not moving w respect to merry go round (K=0). Then, the child is moving (has K trans). So, I don't see how K rotational of the system increases ?
    I'm just so confused :(

    3. The attempt at a solution

    Listed above
     
  2. jcsd
  3. Apr 24, 2017 #2

    kuruman

    User Avatar
    Homework Helper
    Gold Member

    The torque on the child is not mgR. It is generated by friction not by gravity. Regardless of that, your system is child + merry go round. There is no net torque acting on that system because it is isolated (assuming no friction, air resistance, etc.)

    From L = Iω and Krot = (1/2)Iω2, you can easily show that Krot = L2/(2I). What happens to I when the child moves towards the center while L stays constant?
     
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: When is the net torque=0?
  1. Net Torque (Replies: 5)

Loading...