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Using conservation of angular momentum as a braking system

  1. Feb 25, 2016 #1
    Hello, I have a question about using the properties of conservation of angular momentum to provide mechanical resistance. Basically, I'd like to create a device that spins a disk similar to a gyroscope. The device has an external input that, depending on the configured orientation of the disk, will spin the disk either in line with its axis of rotation to provide no resistance, or against it to provide resistance.


    Take the above crappy drawing as a basic example. As the input (at the bottom of the image) turns about, it will turn the disk in such a way that it will provide resistance because of the properties of conservation of angular momentum. To control the amount of resistance the system provides, the disk chassis can be rotated as the arrows on the left of the image demonstrate. If the disk chassis is rotated 90 degrees, the disk would be spinning such that its axis of rotation would be in line with the input of the device, providing no resistance.

    My questions are first off, does a device like this make sense? More to the point though, I'm wondering what the properties are of conservation of angular momentum, in layman's terms. When providing resistance, will the disk slow it's rotation speed? Is the energy required to keep the disk spinning equal to the resistance it provides? How much resistance can be expected depending on the mass of the disk? Other considerations?
  2. jcsd
  3. Feb 25, 2016 #2


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    Think about the fundamental requirement of braking: converting kinetic energy into something else.
    Traditionally, braking has converted this kinetic energy to heat via friction, drum & disc brakes work this way.

    Now ask yourself, how does your braking system work? What are you converting your kinetic energy to? Rotational energy?
    What does your system do that improves on existing regenerative braking systems?

    I understand large flywheel based regenerative braking systems used in vehicles such as buses already use gyros, something to do with reducing the effect of the large rotating mass on the handling characteristics.
  4. Feb 25, 2016 #3
    This is definitely what I'm wondering, from a technical standpoint anyways. From a non-technical standpoint, I'm just wondering how exactly conservation of angular momentum behaves in practice. What happens when a gyroscope is forced to rotate against its axis of rotation? I'm assuming that the spinning disk will slow down, but I've also heard that it won't.
  5. Feb 29, 2016 #4
    If you are 'forcing' the gyroscope to do anything then you are transferring momentum to it. You need to account for this momentum transfer when you do a momentum balance on the system.
  6. Feb 29, 2016 #5


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    Personally I would forget about using a gyroscope and just look at a flywheel as its much easier to understand.

    Meanwhile.. The rotor of a gyroscope can certainly be accelerated...


    However that has a particular design feature which means...

  7. Feb 29, 2016 #6
    So, CWatters, if you force a gyroscope to turn in a way it doesn't want to it will speed up the disk? This makes sense and I believe invalidates the idea of using a gyroscope for a brake. It would not be feasible to continue to speed up a disk over long periods of time until it is revolving in the hundreds of thousands of RPM. I'd still like to play around with the idea though, maybe use an air brake to slow it down or something.
  8. Feb 29, 2016 #7


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    Just for info.. When KERS was introduced to F1 cars a few years back at least one team used a flywheel to store energy recovered during braking. The stored energy was used to help accelerate the car again later...

  9. Mar 1, 2016 #8
    Let's start by considering a stack of two counter spinning flywheels. That stack does not resist turning, it doesn't do anything special, because it's angular momentum is zero.

    Now let's consider what it's like to be one of the above mentioned flywheels. It's just like being a single flywheel, because why would a flywheel behave any differently near another flywheel. So I guess we must conclude that flywheels don't resist turning.

    That may be a surprising conclusion, but it would also break the laws of physics if a flywheel resisted turning like car wheels do when braking is happening.

    I guess the idea was to get rid of lot of unwanted energy without storing the energy anywhere, and it kind of seems like a flywheel could do the trick, but I'm sure it doesn't work.
  10. Mar 2, 2016 #9


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    The energy stored in an ideal flywheel is..

    E = 0.5*I*ω2
    I is the moment of inertia (a constant that depends on the mass and shape of the flywheel) and
    ω is the angular velocity

    This tells you...

    a) If you put more energy in the angular velocity must increase which means it must accelerate and
    b) If the angular velocity is a constant then the energy stored is also a constant (eg You don't have to add energy to maintain a constant angular velocity).
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