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Re: [Phys-l] a conservation equation



Agreed! You are beating a straw man.

Noone in this thread is denegrating the usefulness of the momentum conservation theorem for closed systems.
We were speculating regarding the possible usefulness of defining a "potential momentum function" and turning the impulse/momentum theorem into a conservation principle encompassing external forces. This would be analagous to defining a potential energy function and turning the work/energy theorem into a conservation principle encompassing conservative external forces (which we routinely, and usefully do)..

Bob Sciamanda
Physics, Edinboro Univ of PA (Em)
treborsci@verizon.net
http://mysite.verizon.net/res12merh/

--------------------------------------------------
From: "Herbert Schulz" <herbs@wideopenwest.com>
Sent: Wednesday, March 17, 2010 12:45 PM
To: "Bob Sciamanda" <treborsci@verizon.net>; "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Subject: Re: [Phys-l] a conservation equation


On Mar 17, 2010, at 11:02 AM, Bob Sciamanda wrote:

The conservation of energy when only conservative forces are acting is
really only of significant usefulness in problems involving more than one
dimension - giving a really useful meaning to a force/space integral being
independent of path. Since there is only one dimension of time, the
usefulness of an analogaous approach for time dependent forces does not add
any great advatage (or difference) over the direct integration of N2 in
time.

Bob Sciamanda
Physics, Edinboro Univ of PA (Em)
treborsci@verizon.net
http://mysite.verizon.net/res12merh/

Howdy,

Conservation of Energy (a scalar conservation law) and Conservation of Momentum (a vector conservation law) are true under different conditions. It is possible for either, both or neither to be true depending upon the conditions. Even though Conservation of Momentum is a vector law it certainly is useful when the system's motion is one-dimensional. E.g., find the final velocities on a head on one-dimensional elastic collision (i.e., Energy is conserved) between a mass M and 2M where M is initially moving in the +x direction at a speed v and the and 2M is stationary when there are no external forces on the system.

Good Luck,

Herb Schulz
(herbs at wideopenwest dot com)



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