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Re: [Phys-l] check your work (and kinetic energy)



On 01/08/2012 06:42 PM, Paul Lulai wrote:

The level at which I teach, most work done is typically done to
something outside of a system.

Yup, that's a problem.

In the end case; some energy has left as heat, some energy did work
to the person's hand catching the ball, maybe some sound etc...

And that's the solution to said problem: Divide the system
into a bunch of subsystems.

To say the same thing in more detail: The textbook probably
talks about "the" system and "the" environment ... but that
is strictly amateur hour. Instead, think about something
complicated, like fluid dynamics: the only sensible approach
is to divide the fluid into some huuuge number of parcels.
Each parcel is a subsystem unto itself, and it also serves
as part of the environment for neighboring subsystems. You
get to ask how much energy, momentum, entropy, et cetera
flow across the boundary of *each* parcel.

I include all objects within my system.

You're allowed to do that, but you're not required to, and it's
usually not worth the trouble, unless "the" system too simple
to have any interesting internal structure. Usually you can
get much more useful answers by formulating the question(s)
differently, i.e. by asking about the various subsystems
individually.

====

If that's not a helpful answer, please refocus the question.