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Re: [Phys-L] Systems



Two extreme kinds of things that are called systems are what the engineers call a control mass and a control volume. The control mass system consists of a fixed set of material such as a rock, a beaker full of water, the gas inside a cylinder that is capped off with a piston. The control volume is a closed region of space. Energy, momentum, entropy, and material can flow into and out of the control volume. Energy, momentum, and entropy can flow into and out of the control mass--but material doesn't flow into or out of the control mass. An example of a control mass might be two rocks. They might undergo a collision and break up into several rocks. The system is still well behaved and various conservation laws can be applied to say something about the system during and after the process based on information about the system before the process. The boundary of the system is not necessarily relevant. It can be complicated and it can be changing in a complicated fashion but there is no need to think about the boundary of the system.

One can think of hybrid systems, a rocket comes to mind, but the extremes are important. Both are useful. Recent discussions have brought a couple of questions about systems to mind. I will address both of them to you, John Denker, as in both cases the questions resulted from something you said.

When we were talking about a beaker of water on a pan balance, with a steel ball having been lowered intoit, I considered the beaker of water as the system, hence my system was what the engineers call a control mass. Because the steel ball was in contact with the system, momentum was flowing from the steel ball into the system. You insisted on considering a control volume to be the system. To my mind, by doing so, you complicated the problem in that you then had to concern yourself with momentum flow in the string that was supporting the steel ball. Why the insistence on a control volume in this case? To me, your introduction of the equation dE=PdV+TdS indicates that you are comfortable with using a control mass as your system.

The other question can be exemplified by a rotating bullet. It seems reasonable to consider the bullet to be the system. And it seems reasonable to create a fixed imaginary boundary about a region in space that is at all times occupied by the bullet and call that region the system. But I am not familiar with the idea of calling the spin of the bullet a subsystem and what is not the spin of the bullet another subsystem. It seems that each subsystem should be a system itself. I can see considering the bullet to consist of two parts, a nose part and a tail part for instance, and each of those parts can be a subsystem, but I'm not familiar with the way you used the term subsystem as a subset of the degrees of freedom. Is that a common use of the expression?