Leonard G. Caillouet

09-05-1995, 02:54 PM

Peter Davidson made two points that were close to being correct but are

in fact examples of his own mistaken description of the problem of

describing work. He states that "work is a form of energy..." In fact

work is the transfer of energy. When describing the work done by an

object, or the work done on an object one must define the question of

interest carefully. As Peter stated, the system must be defined. His

mistake can be illustrated most simply by the example of the object

thrown straight up which falls to rest in exactly the same location. The

problem is that the object doing the work we are interested in measuring

was not defined carefully. There was indeed no net work done on the

object thrown BY ITS ENVIRONMENT, but there certainly was work done by

the thrower on the object. The thrower displaced the object against the

force due to gravity and changed its mechanical potential by the distance

above its resting position it was lifted. The fact that gravity caused

the object to be displaced in the other direction does not make the work

done by the thrower zero.

Similarly, the fact that the kinetic energy transfered to the

particles of water is ultimately dissipated to the environment does not

make the work done on the water BY THE SWIMMER zero. This work would

seem to be the transfer of energy that is of interest in this

experimental condition.

To quickly summarize, the discussion of work must include the

definition of the bodies of interest because work is not energy but the

transfer of energy. If the bodies of interest are not described in the

statement of the problem we can expand the system infinitely to the level

of the universe and the problem becomes one of "understanding entropy."

Careful use of terms such as work and energy is essential.

Considering them to be equal would be like using heat and thermal energy

to mean the same thing. A final note to Peter on this point: You

suggested that there may be confusion on the list concerning "effort",

"work", and "energy". I suspect that few of us are confusing effort and

energy, but the latter terms may be used carelessly. The more important

lesson is to carefully define the objects of study between which energy

is being transfered. It is one of the first lessons in mechanics that

Newton would have taught, I'm sure. It's certainly one that most of us

who studied physics or mechanics had to learn.

Leon.

Leonard G. Caillouet

Louisiana State University

MS Candidate, Dept. of Kinesiology

in fact examples of his own mistaken description of the problem of

describing work. He states that "work is a form of energy..." In fact

work is the transfer of energy. When describing the work done by an

object, or the work done on an object one must define the question of

interest carefully. As Peter stated, the system must be defined. His

mistake can be illustrated most simply by the example of the object

thrown straight up which falls to rest in exactly the same location. The

problem is that the object doing the work we are interested in measuring

was not defined carefully. There was indeed no net work done on the

object thrown BY ITS ENVIRONMENT, but there certainly was work done by

the thrower on the object. The thrower displaced the object against the

force due to gravity and changed its mechanical potential by the distance

above its resting position it was lifted. The fact that gravity caused

the object to be displaced in the other direction does not make the work

done by the thrower zero.

Similarly, the fact that the kinetic energy transfered to the

particles of water is ultimately dissipated to the environment does not

make the work done on the water BY THE SWIMMER zero. This work would

seem to be the transfer of energy that is of interest in this

experimental condition.

To quickly summarize, the discussion of work must include the

definition of the bodies of interest because work is not energy but the

transfer of energy. If the bodies of interest are not described in the

statement of the problem we can expand the system infinitely to the level

of the universe and the problem becomes one of "understanding entropy."

Careful use of terms such as work and energy is essential.

Considering them to be equal would be like using heat and thermal energy

to mean the same thing. A final note to Peter on this point: You

suggested that there may be confusion on the list concerning "effort",

"work", and "energy". I suspect that few of us are confusing effort and

energy, but the latter terms may be used carelessly. The more important

lesson is to carefully define the objects of study between which energy

is being transfered. It is one of the first lessons in mechanics that

Newton would have taught, I'm sure. It's certainly one that most of us

who studied physics or mechanics had to learn.

Leon.

Leonard G. Caillouet

Louisiana State University

MS Candidate, Dept. of Kinesiology