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2014-05-30
I am having trouble understanding the transfer of orbital angular momentum from a rotating body to a non-rotating body. In a very simple collision, for example between a disk and a sphere, the disk gains angular momentum from the sphere but in what direction? 
If it is perpendicular to the axis of the disk, the disk would be more dense and would therefore be more massive. But the actual disk is less dense, less massive. I know this was a simple example and it has not anything to do with a disk, but I am unsure about how to set up the differential equation to find the direction in which the disk gains angular momentum from the sphere. 
A:
The disk gains angular momentum from the sphere.
You get the angular momentum of the disk just by writing the angular momentum of the rotating disk. 
So the only thing you need is the formulae for the angular momentum of an object rotating around an axis, but there are more elegant ways to solve the problem, for example using conservation of angular momentum. 
And the figure shows you an example of the vectorial formulae for the angular momentum of a rotating object.
Q:
Generics: type erasure?
I'm trying to understand some code that is part of an API. It looks like this:
public class DBPersister {
    // and some methods that operate on T
}
public interface XmlPersister {
    // some methods
Is there a technical reason for doing this? I suppose the compiler (Java) could then assume that all of T's methods are present in XmlPersister. I'm pretty sure that this means that T would be an abstraction over XmlPersister, but I'd like to be sure.
No, there is not a technical reason. It's a new feature in Java 7 introduced as "generics unification" (JLS 9.1.3).
Basically, the language itself cannot distinguish between a class C and a method C. In both cases, the type parameter is constrained to be the same, in this case XmlPersister. This is allowed to happen, because the language is Turing complete, 3125841983
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