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Homework Help: Conservation of energy with springs and rotational motion

  1. Nov 14, 2012 #1
    1. The problem statement, all variables and given/known data

    2. Relevant equations
    T1 + V1 = T2 + V2

    T = kinetic energy
    V = potential energy

    3. The attempt at a solution
    I am only confused about how to get T2. Here is how the answer key gets T2:

    I don't get why Va = Vb = 0. The wheels have some velocity so it doesn't make sense how they are zero. I realize that they won't rotate because there is no friction but I still think there is a velocity.

    I also don't get why they can assume that the rods are pinned. How are they pinned when the spring stretches from √2 to 2m in length (this is from the solution as well)?
  2. jcsd
  3. Nov 14, 2012 #2


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    Homework Helper
    Gold Member

    Va=Vb=0 only when θ = 0. At this angle, AC and CB are horizontal and the maximum separation between A and B has been achieved.

    ''pinned" is not an assumption, it is expressly given--"All pin joints move without friction". Not sure what the answer key means by like "pinned case".
    Last edited: Nov 14, 2012
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