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Re: [Phys-L] angular momentum

I am used to seeing and using...
L=r ×p = r × mv.
What happens when the object has:
v_y_ = +10
v_x_ = 0
And the original position of the object is (5,5) relative to my origin.
Then even though p is constant, the position vector r is constantly changing in both magnitude and direction. In this case, L is not constant.
No torques, no interactions with something outside of my system (the ball and my oddly chosen origin), and L is not conserved.
Why is that?
There seems to be some sort of a condition for the origin. I am not used to that.
Thanks again for the time help.

.:. Sent from a touchscreen .:.
Paul Lulai

-------- Original message --------
Date:07/21/2014 1:05 PM (GMT-06:00)
Subject: Re: [Phys-L] angular momentum

Consider the motion of a ball, free of all forces, to be the constant
velocity path: x=a (a constant), and y= vt (v is its constant speed).
Its angular momentum about the origin (0,0) is simply m*a*v, a constant in
time. In the same way, Its angular momentum about any fixed point is a
constant in time .

Bob Sciamanda
Physics, Edinboro Univ of PA (Em)<>
-----Original Message-----
From: Paul Lulai
Sent: Monday, July 21, 2014 1:36 PM
Subject: [Phys-L] angular momentum

I am finding I have some questions about conservation of angular momentum
that I hadn't considered in the past. If I am investigating the angular
momentum of a soccer ball about a point, is angular momentum only conserved
if the ball is orbiting about the center of a circular path or a foci of an
ellipse? Certainly a ball traveling directly west across a field does not
have its angular momentum conserved.
I am completely excluding the idea of impulse, torques, and isolated systems
at this point. I just found I haven't thought about this aspect before.
Thanks for your thoughts.
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