Using conservation of angular momentum as a braking system

In summary: This is definitely what I was thinking. Basically, by storing the energy from braking, we can use it to help turn the flywheel back to its original position.Now that we understand how flywheels resist turning, what implications does that have for using them for a brake?From a technical standpoint, it doesn't really have any implications. If we want to use a flywheel for a brake, it's just like using any other counter-rotating flywheel. From a practical standpoint though, I'm not sure how feasible it would be to store significant amounts of energy in a flywheel for very long periods of time.
  • #1
dicvt
3
0
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.

http://i.imgur.com/dfU4REU.png

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?
 
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  • #2
dicvt said:
My questions are first off, does a device like this make sense?
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?
https://en.wikipedia.org/wiki/Regenerative_brake

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.
 
  • #3
billy_joule said:
What are you converting your kinetic energy to?

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.
 
  • #4
dicvt said:
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.

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.
 
  • #5
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...

https://en.wikipedia.org/wiki/Gyroscopic_exercise_tool

Once the gyroscope inside is going fast enough, a person holding the device can accelerate the spinning mass to high revolution rates by moving the wrist in a circular motion.

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

...an acceleration of the rotation will occur regardless of the direction of the applied torque...
 
  • #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.
 
  • #7
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...

http://www.flybridsystems.com/F1System.html
 
  • #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.
 
  • #9
The energy stored in an ideal flywheel is..

E = 0.5*I*ω2
where
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).
 

1. How does conservation of angular momentum work as a braking system?

Conservation of angular momentum is the principle that states the total angular momentum of a system remains constant unless acted upon by an external torque. In the case of using it as a braking system, the rotational motion of a spinning object is slowed down by applying an external torque in the opposite direction of its spin. This causes a decrease in the object's angular momentum, leading to a decrease in its rotational speed.

2. What types of systems can utilize conservation of angular momentum as a braking mechanism?

Conservation of angular momentum can be applied in a variety of systems, including vehicles, machines, and even celestial bodies. For example, bicycles use the principle to slow down by applying brakes to the wheels, which creates an external torque in the opposite direction of the wheels' spin.

3. Can conservation of angular momentum be used as the sole braking system in vehicles?

No, conservation of angular momentum is not typically used as the sole braking system in vehicles. While it can be effective in slowing down rotational motion, it is not as efficient in stopping a moving object as traditional braking systems, such as friction brakes. Therefore, it is often used in conjunction with other braking methods to ensure safe and efficient braking.

4. Are there any disadvantages to using conservation of angular momentum as a braking system?

One potential disadvantage of using conservation of angular momentum as a braking system is that it requires a spinning object, such as a wheel or flywheel, to be present. This can add weight and complexity to a system, which may not be ideal in certain applications. Additionally, the effectiveness of this method may be affected by external factors, such as air resistance or uneven terrain.

5. How is conservation of angular momentum different from conservation of linear momentum?

Conservation of angular momentum and conservation of linear momentum are both fundamental principles in physics, but they describe different types of motion. Conservation of angular momentum deals with the rotational motion of an object, while conservation of linear momentum deals with the linear motion of an object. In other words, conservation of angular momentum applies to objects that are spinning or rotating, while conservation of linear momentum applies to objects that are moving in a straight line.

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