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!

Toy car conservation of angular momentum

  1. Apr 18, 2008 #1
    1. The problem statement, all variables and given/known data
    A 200g toy car is placed on a narrow 60 cm diameter track with wheel grooves that keep the car going in a circle. The 1 kg. track is free to turn on a frictionless, vertical axis. The spokes have negligible mass. After the car's switch is turned on, it soon reaches a steady speed of .75 m/s relative to the track. What then is the track's angular velocity in rpm?


    2. Relevant equations
    L=I*angular velocity
    L=mrvsintheta
    Moment of Inertia for a hoop: I=MR^2

    3. The attempt at a solution

    I found the angular momentum of the car and added it with the expression for the angular momentum of the track. The two combined should have a momentum of 0.
    (.200kg)(.30m)(.75m/s) + (1kg)(.30m)^2*angular velocity = 0

    angular velocity = -.5 rad/s = -4.77 rpm

    According to my book the answer is 4 rpm and I was just wondering what I am doing wrong.
     
  2. jcsd
  3. Apr 18, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    .75 m/s is the speed of the car with respect to the track, not with respect to the lab frame.
     
  4. Apr 18, 2008 #3
    The .75 m/s and the angular velocity you compute from that is relative to the track. The angular velocity of the track you want to compute is relative to the earth. You need to transform the angular velocity of the car to the non-rotating frame.
     
  5. Apr 18, 2008 #4
    Ok, I see. Can you help me with that transformation? Relative velocity is one of those things that makes a little uncomfortable. Would you just subtract the velocity of the car relative to the track from the answer that I got up above?
     
  6. Apr 18, 2008 #5

    Doc Al

    User Avatar

    Staff: Mentor

    Try this:
    speed of car w.r.t ground = speed of car w.r.t track + speed of track w.r.t ground
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Toy car conservation of angular momentum
Loading...