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Angular Momentum and Flywheel Energy problem?

  1. Feb 16, 2013 #1
    Hi guys, basically i have a few questions with regards to Prof Walter Lewin's videos. Firstly, this is the video of him explaining the flywheel concept with energy involved:

    My question is, why does a flywheel actually spin when the car goes downwards? how does the flywheel actually converts the translational KE to rotational KE? I'm guessing it's because of the relative motion of the air that spins the flywheel to convert it into rotational KE. However, if that is the case, wouldn't it not spin? As the air can push the wheel where it is in contact with half of it, causing both anti clockwise and clockwise moment. Am I right to say this? or have i mistaken how a flywheel works? Can't really imagine why it spins when it goes down as I have never seen one in reality..haha


    For this vid, starting from 1:10, you can see that the prof is making the spinning flywheel suspended on a string. while this was happening, why is it that he says the flywheel angular momentum which is pointing outwards, chasing the torque of the flywheel (which is perpendicular to the angular momentum direction)? This is known as precession right? In any case, i would like to know how does the direction of the angular momentum affect the motion overall, as i always thought that the direction of L (usually in the z-axis) is there for illustration purposes only since a curved arrow to indicate the rotating motion of a disk is too troublesome...

    Thanks a lot guys!!!
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Feb 16, 2013 #2
    1. He is talking about a hypothetical arrangement, when the flywheel is SOMEHOW made to spin up when the car needs to slow down. He is not proposing any particular mechanism for that. The task is not trivial, which is one of the reasons this is not done. What is done in some cars these days is regenerative braking, where the energy that would otherwise be wasted as heat is used to drive electric generators and then stored in the car's battery.

    2. This entire demonstration demonstrates that angular momentum is much more than just an arrow on a diagram. As to how it affects the motion, recall the definition of angular momentum.
  4. Feb 16, 2013 #3


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    This bicycle has flywheel based KERS...


  5. Feb 16, 2013 #4


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    If that's hard to follow... consider how car/truck drivers use engine braking when going down a very steep hill. They shift down the gears so that the wheels try to turn the engine faster than it wants to go. The bike puts energy into the flywheel the same way.
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