Basic Thermodynamics ~ J. Pohl © (1/19/14) | www.thermospokenhere.com (C256a) (C256) |
The following statement, principles and conclusions (S/P/C) was submitted to Physics Help Forum in request of clarification.
S-1: Say a block is being raised at constant velocity by a tension in a cord against the force of gravity. (Q-1) Is there any work done on the block?
C-1: My answer was "no" since Work = Force*Distance. Force = Mass*Acceleration, and if velocity is constant then acceleration is zero and force equals zero. (Q-2) So then work would be 0.
C-2: However work also comprises mgΔH, and if the height is changing then there is an increase in potential energy, so there is work... (Q-3) Can someone please resolve this contradiction?
In Statement, S-1, tension and gravity forces are identified in relation to a block moving upward at constant speed. Newton's 2nd Law applies with the block as system.
Question 1: Is there work? Strictly speaking, No. Work is associated with a start/stop or commence/end type of event. Events for which a force is displaced an increment without regard for the time required. Our event the time-wise nature "continuing." For our event (continuing) "work rate" is germane.
Newton's 2nd Law has no work-related term. However, were the 2nd Law multiplied by the vector velocity, two work-rate terms appear. The case we study is very special. Since vertical speed is constant, left-of-equality is zero, which greatly reduces this discussion. , we avoid
nd , one can incompletely describes a physical scenario and event (a block rising). Next, without application of principles, ad hoc facts are patched to conclude work did occur and then by another patching, work did not occur. A proper approach is to cast the problem, apply Newton's 2nd Law... and so on.
(1) Scenario: By statement, the student identifies physical matter (actual or imagined) that includes Earth, a Block and a supporting/lifting structure (fitted with pulleys, a cord). Figure 1, a sketch of the assumed physical arrangement, is drawn for the purpose of discussion/inquiry. We call this sketch a physical scenario or simply scenario.
(2) Event/System/Surroundings: The scenario is a beginning sketch. In accord with the anticipated event (and the answers sought) part of the scenario is selected to be the system. That matter not selected, the remainder of matter of the scenario collectively, is called surroundings. The next step is to isolate the system and represent it as a free-body-diagram.
Isolation: Selection of the system is the first step of Newton Method. The system is imagined extracted from reality (the scemario). The next step, is called isolation. The system is modelled, notated...
.Body and Boundary Forces: Since the system is imagined a part apart from reality, forces are applied to approximated.
Free Body Diagram: Forces are applied to the system to represent effects the surroundings might have upon the system during an event. , are applied to the system as its FBD is drawn. force are model, time, coordinates/characteristics and Body and Boundary forces.
placed imaginatively, is a The sketch, "physical scenario," depicts the basic material components and configurations in the location of, relevant to the parts of the system, surroundings relative to the event. The scenario is a sketch of what there is, the material associated with some event, ongoing or to happen.
The scenario is physical reality PRIOR to selection of a system. Before and until a system is selected, there are no forces. no forces act. It is with isolation of system from surroundings that the represents the the, oOnce the system is selected, is is identified matter is "extracted," isolated from all else for the purposes of analysis.Isolation is a focusing upon properties and characteristics of the system. Buit this information will be too little to predict or explain behavior of the event. The system, though it is mass separated form all other mass, will still be influenced (in its event) by contiguous matter, the surroundings. Newton (and others) invented the concept, force, as the mechanism whereby surroundings effect influence events of the system. of of cause. The system will change with its event. The system, as matter set apart, separated from surroundings, needs a manner to recognize that "surroundings being there" might influence the event. System selection, isolation, selection of forces with notation and a new sketch... these steps yield a free-body-diagram.
Existance is an event, of course. For other events change is more pronounced. A physical scenario is real; it is what we see when we LOOK at reality. Reality is real. A physical scenario is complicated; difficult to specify. The physical scenario is an observation, vision or idea.
Once analysis begins, reality, its physical scenario is reduced to less, is reduced to a model, an approximation of the physicality seen. must be transformed. The detail of reality cannot be drawn; is too complicated. The physical scenario must be reduced to a sketch. It must be sketched; cast more simply but retaining its salient features. A physicist or engineer must translate the physical reality seen (or imagined) into a first reduction, to a Physical Sketch.
Each study of physical reality has its own reality. Analysis hopes to study real matter in space and time. Matter exists in coexistence (with other matter), analysis addresses event-of-matter (the subject matter: system) in time, due respect being paid to all other matter. Analysis hopes to tell more about the present or predict a part of the future.
(3) Analysis (Newton's Method): Directed to our S/Q... Committment to matter of interest happens upon selection of "system." System is half of a separation of reality. System is the half of interest; surroundings are the other half. The free-body-diagram (FBD) models the system; appropriate forces of surroundings on the system approximate conformance of the real surroundings.