When to use L = r × p versus L = Iω for angular momentum?

[grief]

I have two formulas to find angular momentum, and I am not sure when to use one and when to use the other, or what the difference is between the two. One is L = r x p, and the other is L=Iw.

 

[dav2008]

The former is used for a the angular momentum of point particle about some point, while the latter is used for the angular momentum of an object rotating about a fixed axis.

 

[grief]

ok, I understand now. Thanks

 

[grief]

so, if it's a rod, for example, rotating about one of its ends, I should use L=Iw?

 

[dav2008]

Given that a rod is not a point particle, yes.

Just be careful to note that the moment of inertia (I) you are using is for that object about the given axis.