Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

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



From "Holt Physics" by Serway & Faughn
i.e. the high-school version of Serway & Faughn

In the chapter that covers work, kinetic energy, the "work/KE theorem",
energy, conservation of energy, and the "equivalence" of mass and
energy ... at the end of the chapter, review question #1 asks:

1. Can the speed of an object change if the net work
done on it is zero?

That's it. It seems like a simple, qualitative, conceptual question.

One thing I like about this question is that it asks about "speed"
rather than asking directly about kinetic energy. Therefore it is
at least one step removed from the worst sort of rote regurgitation
exercise. The student must put a couple of ideas together to answer
the question, joining them at a point (KE) that is not explicitly
mentioned in the question. This doesn't require a super-ginormous
amount of thinking, but it requires some.

Since I am not in the mood for trolling, I will not ask for a show
of hands. I will not ask y'all how you would have answered the
question. Let me blurt out my answer:

It's a multi-part answer:
a) The expected answer is "no". That is, if you apply the lessons
of this chapter without thinking too much, the answer is "no".
b) On the other hand, if you think about the actual physics for
a femtosecond, you realize that the correct answer is "yes".
c) You could repair the question by asking about a structureless
point particle, rather than an "object". Thereupon, the
answer would be "no".

The wrong physics affects the meaning of the chapter, not just this
one question. The /object/ the book uses to illustrate the work/KE
theorem in fact perfect for illustrating the exact opposite ...
because their object is a *car*. I kid you not. Presumably every
kid in the class knows that if you are driving along in a car and
apply the brakes, the KE of the car decreases, and the speed
decreases, even though no work was done on the car.

I give these authors some credit for /trying/ to move in the
right direction. Physics that applies to the real world (such
as cars) is vastly preferable to physics that applies only to
structureless point particles ... but that's only if you get
the physics correct. Bogus real-world physics is not an
improvement over ivory-tower physics or baby-play physics.

The thing that cracks me up is that even though the kids know the
answer is "yes" in real life, they blithely answer "no" on the
homework. Why are people content to learn things that cannot
possibly be true?

It seems they are playing a game, without knowing the rules of
the game, or even knowing the goal. IMHO getting a good grade
on the homework is not the goal. The goal is to learn stuff
that will be useful in real life. If the book is wrong and
the homework is wrong and the guy on TV is wrong, that is no
excuse for believing things that cannot possibly be true.
Deal with it. Dealing with it does not mean accepting it
at face value. Dealing with it is your responsibility.