Why Does the Derivative of Momentum Appear in Angular Momentum Calculations?

[Xyius]

I do not understand this one step in deriving the angular momentum theorem for a system of particles.

The vector angular momentum about the point Q, not necessarily the origin is..
[PLAIN]http://img27.imageshack.us/img27/8196/pfquestion.gif

I do not understand why the difference in the velocities of the point Q and k equals the DERIVATIVE of the momentum. If you bring the mass term into the expression, then shouldn't it be equal to just p? Why is it dp/dt??

 

[Xyius]

Never mind, I just got it by differentiating. Now what I don't get is the next part. I will post a pic soon.

EDIT:
Okay so, when I differentiate the expression for angular momentum, I get the expression above. When THEY differentiate it, they get what I have plus this additional term.

[PLAIN]http://img24.imageshack.us/img24/9514/pfquestion1.gif

Maybe I am being oblivious again, but I cannot get this term to make sense!

 

[Xyius]

Yup! being oblivious again! I got it. I think on that note, I should finally stop studying haha.