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# Re: [Phys-L] angular momentum

Right, the position vector is changing. In particular, for your
example, the magnitude of r is growing, and its angle with p is
decreasing. Angular momentum is the cross product of r and p, so that
angle matters. The decrease in angle counteracts the increase in the
magnitude.

r = (5, 5 + 10t, 0) and v = (0, 10, 0)

so the cross product r x v is the "determinant":

| xhat yhat zhat |
r x v = | 5 5+10t 0 |
| 0 10 0 |

which turns out to be (0, 0, 50) -- constant.

- Craig

On Mon, 21 Jul 2014 18:38:02 +0000
Paul Lulai <plulai@stanthony.k12.mn.us> wrote:

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. Paul.

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

-------- Original message --------
From: treborsci@verizon.net
Date:07/21/2014 1:05 PM (GMT-06:00)
To: Phys-L@Phys-L.org
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)
treborsci@verizon.net
www.sciamanda.com<http://www.sciamanda.com>
-----Original Message-----
From: Paul Lulai
Sent: Monday, July 21, 2014 1:36 PM
To: Phys-L@Phys-L.org
Subject: [Phys-L] angular momentum

Hello.
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
Paul.
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