THERMO Spoken Here! ~ J. Pohl © TOC     NEXT    ~   24

Dog Greets Owners


Scenario:  Walking back from a shoreline run, a woman saw her husband and their dog, at a distance, walking toward her. The dog, off-leash, walked beside the man, sniffing and digging ... as dogs at the shore do. At a moment, a distance from them (the dog and man) the woman shouted, "Psst! Daisy come!". Daisy loves the beach and she loves to run.

Immediately the dog bolted and ran full speed to the woman, ran around her, then ran back to the man, around him, then ran back to the woman... All running and walking ended with the three together. Laughing, the woman said,"Poor Daisy! She must have run ten times as far as you or me."

♦  Make some reasonable assumptions then calculate the approximate distances involved.

Conditions and Assumptions:  

  • The space of the motion (one-dimension) is along a straight line, we label as the X axis. The event is the sum of discrete increments of motion. Call them Stages.

  • Initial Positions and Speeds:

          Man: Position, Pman = 0 ft I         Constant Speed: Vman4 ft/s I

     Woman: Position, Pwoman = 360 ft I     Constant Speed: Vwoman4 ft/s (-I)
commences when the dog hears, "Daisy come!"upon the is scenario description is reasonably complete but it lacks notations and specifics. Initial conditions are: man (and dog) a distance from the woman moving at constant speed Vwalk, each toward the other. The event commences upon the woman's yell. The man and woman proceed steadily in linear space and time - along a line, toward each other. This is clear, needs no approximation.

The motion of the dog is less easy to prescribe. erratic. The dog accelerates to Vd, moves to the .

at some initial time. The distances travelledApparent questions are: i) One style of academic texts and general thinking about physical system/events is an approach wherein one knows (or had been told) facts about a system/event, those facts thererafter to be used to deduce or calculate some other fact being consitent with approximations aand laws of physics.

the generic idea in education parlance is "given these things known" therafter "solve" (in accord with the laws of physics) to determine something else, the "answer" of its query. It is easier to do this when a complete, through specification of system/event/charistics are known.

=== OLD ======

Virtually all text-problem-statements omit detail then request an "answer" when only an "approximate answer" (as a "minimum" or "maximum") is obtainable. Students are expected to approximate to obtain an approximate answer.

So, once solved it is proper to tell, as a consequence of approximation, is the answer magnitude smaller (or larger) than the truth and why. Consider the following physics text problem:

Statement:  "A dog runs back and forth between its two owners, who are walking toward one another. The dog starts running when the owners are 10 meters apart. If the dog runs with a speed of 3 m/s and the owners each walk with a speed of 1.3 m/s how far will the dog have traveled when the owners meet?"

i)  "A dog runs back and forth between its two owners..."   Students are likely to realize the system/event is three entities: a dog and two persons.

ii)  Some students are likely to approximate each entity (the dog and two persons) each as a BODY, each as a point moving in space. Others will imagine living persons, a crazy dog, all in motion. Better that the Author state something like "... approximate the dog and the persons as a BODIES."

iii)  "The dog starts running when the owners are 10 meters apart..." Students will know this initiates the event. "... when the owners meet." denotes the end of the event.

iv)  "If the dog runs with a speed of 3 m/s and the owners each walk with a speed of 1.3 m/s..."  The authors know it is not possible for a happy buoyant dog to move at 1.3 m/s for the duration of the event. What the author means is, "Assume the dog and persons move at constant speed. Thus the authors ask for an approximate solution.

v)  "... how far will the dog have traveled when the owners meet?"  The author means, "subject to the system models "BODY," and "assuming the turning of the dog requires no time" and "its speed is constant at 3 m/s..." Physics education is poor sometimes.

This Solution:   Let the persons be designated as "p1," "p2" and the dog as "d." The event is described in terms of the relative positions of the dog owners. We write that vector equation.

The position of person "1" plus
the position of "2 relative to 1"
equals the position of person "2."

Equation (1) specifies that the persons are ten meters apart at time equal to zero. The distance between them diminishes with time until it equals zero. Consequently the time the persons walk until they stand in the same space is:

To start the persons are 
10 meters apart. That 
distance diminishes 
2.6 meters each second.

So now we understand that the duration of the approximated event is 3.85 seconds. So what does the dog do during this time period. Knowing dogs, as we all do, we envision craziness... running, dancing, turning. It is difficult to proceed to solve this problem knowing dogs as we do.

The dog of the physics text author turns on a dime an always has constant speed: 3 m/s. The time that the author's dog can run is 3.85 seconds. Obviously the distance the author's (imaginary) dog can travel is:


The message of this problem was the relevance of physics to dogs and owners. The premise is reasonable. The event, numbers and how it unfolds are believable. The author knew how to "find" (the word usually used) the missing fact, the "straight-line" distance traveled by the dog.

Dog Greets Owners

Problems in HS physics texts some­times omit detail then request an "answer" when only an "approximated" answer is obtainable. For example!

A dog runs back and forth between its two owners, who are walking toward one another. Initially the owners are 10 meters apart, the dog runs at 3 m/s and the owners each walk at 1.3 m/s. How far will the dog have traveled when the owners meet?"

Premise presently unwritted!