Angular Momentum and Flywheel Energy problem?

In summary: Precession is the term for the spinning motion of the flywheel. It happens because the torque applied to the flywheel is always perpendicular to the angular momentum of the flywheel. This is why the direction of the angular momentum affects the motion overall. If you look closely at the video, the angular momentum of the flywheel is always pointing in the same direction, which is why the precession happens.
  • #1
SecretSnow
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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

Next:

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!
 
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  • #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.
 
  • #3
This bicycle has flywheel based KERS...

http://www.gizmag.com/flywheel-bicycle-regenerative-braking/19532/

The Flywheel Bicycle has a continuously variable transmission in the rear hub. This is linked to a 6.8 kilogram (15 lb) flywheel from a car engine mounted in the middle of the frame. When the cyclist wishes to slow down, such as when they're going down a hill or coming to a stop, they shift the transmission to maximize the flywheel-speed-to-bike-speed ratio. This "charges" the flywheel with kinetic energy - effectively a mechanical version of what happens in an EV where a battery stores the scavenged energy.

Once they want to accelerate or climb a hill, they do the opposite - they shift the transmission to minimize the ratio. This let's the energy stored in the flywheel drive the transmission, giving the bike and its rider a boost.
 
  • #4
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.
 
  • #5


Hi there,

I can explain the concepts of angular momentum and flywheel energy in a simplified manner.

Firstly, angular momentum is a measure of an object's rotational motion. It is the product of an object's moment of inertia (its resistance to rotational motion) and its angular velocity (how fast it is rotating). In the case of a flywheel, when the car goes downwards, the force of gravity causes the flywheel to rotate due to its moment of inertia. This rotation results in the conversion of translational kinetic energy (movement in a straight line) to rotational kinetic energy (movement in a circular path).

You are correct in thinking that the air plays a role in spinning the flywheel. This is because the flywheel is designed in such a way that the air pushes against it in a particular direction, causing it to rotate. This is similar to how a windmill works.

In the second video, Prof Lewin is demonstrating the concept of precession. This is when a spinning object's axis of rotation changes direction due to an external force acting on it. In this case, the torque (a force that causes rotation) is acting on the flywheel, causing its angular momentum to change direction. This is why the flywheel appears to be chasing the torque.

The direction of angular momentum is important as it determines the direction of the object's rotation. In the case of the flywheel, its angular momentum is pointing outwards, so it rotates in that direction. This is why the direction of angular momentum is usually shown in the z-axis, as it is perpendicular to the plane of rotation. This is also important in understanding how gyroscopes work.

I hope this helps to clarify your questions about angular momentum and flywheel energy. Keep exploring and asking questions!
 

1. What is angular momentum and why is it important in the flywheel energy problem?

Angular momentum is a measure of the rotational motion of an object. In the flywheel energy problem, it is important because it determines how efficiently the flywheel can store and release energy.

2. How is angular momentum related to the moment of inertia in the flywheel energy problem?

The moment of inertia is a measure of an object's resistance to changes in its rotational motion. In the flywheel energy problem, the moment of inertia is directly proportional to the angular momentum of the flywheel.

3. Can the angular momentum of a flywheel be changed?

Yes, the angular momentum of a flywheel can be changed by altering its moment of inertia or its rotational speed. Increasing the moment of inertia or rotational speed will increase the angular momentum, while decreasing either will decrease the angular momentum.

4. How does friction affect the flywheel energy problem?

Friction can decrease the efficiency of the flywheel in storing and releasing energy. This is because friction converts some of the rotational energy of the flywheel into heat, which is not easily converted back into kinetic energy.

5. Are there any real-life applications of the flywheel energy problem?

Yes, the flywheel energy problem has several real-life applications, such as in energy storage systems, gyroscopes, and mechanical watches. It is also being researched as a potential method for storing renewable energy, such as wind or solar power.

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