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Conservation of Angular Momentum & Energy question

  1. Mar 20, 2015 #1
    A streetcar is freely coasting (no friction) around a large circular track. It is then switched to a small circular track. When coasting on the smaller circle its speed is:

    a) greater
    b) less
    c) unchanged


    w = v/r
    KE = 1/2mv2

    My teacher said the normal force from the track on the train doing work as the train switches from the larger to the smaller circle and something about how the conservation of momentum is not valid in this situation. And when I looked this question up online, they saw that energy is conserved. However, I'm still confused on how to solve this problem and what the answer is (I think it's unchanged?).
  2. jcsd
  3. Mar 20, 2015 #2

    Doc Al

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    Staff: Mentor

    Does the force from the track do work on the train? That's key.
  4. Mar 24, 2015 #3
    I think so? I remember my teacher saying something how the normal force does work on the train but I'm confused as to how it does.
  5. Mar 25, 2015 #4


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    Science Advisor
    Homework Helper
    Gold Member

    You are right to question it. What direction will the force from the rails be in, in relation to the direction of the train's velocity?
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