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Re: [Phys-L] The Make-Believe World of Real-World Physics



On 07/18/2013 02:21 PM, Anthony Lapinski wrote:

For the ball problem, Earth's acceleration is constant.

I am aware of the earth-fixed reference frame. Really I am.
I know the gravitational acceleration g@earth in this frame.
Explaining it to me 27 more times is not going to help.

The fact remains, however, that this is only one leg of the
elephant. The other parts of the elephant are important.
Describing your favorite leg correctly ... in minute detail
... again and again ... will not make the rest of the elephant
go away.

A master of the subject should be able to see things in more
than one way. A big part of teaching depends on being able
to see things from the student's point of view.

Sure, you are free to choose your favorite leg of the elephant.
Sure, you can probably eventually "teach" students to always
choose some arbitrary leg to the exclusion of all else ... but
why would you want to? The thing about arbitrary choices is
that they can be learned by rote and not otherwise. Arbitrary
choices are never intuitive. Arbitrary choices are outside
the scope of reasoning, let alone critical reasoning.

Critical reasoning means checking every new idea to see how
it connects to previously-known ideas. It is the opposite
of critical reasoning to insist that some new ivory-tower
definition of "acceleration" is right and all other definitions
are simply wrong ... even though they are still in constant
use by thoughtful experts, and always will be.
http://www.av8n.com/physics/acceleration.htm

For the ISS question, this topic has come up before

Yes, the question has come up before.

with true weight (mg)
and apparent weight (scale reading).

No, that is not the right answer. That's not even the right
way to formulate the question.
http://www.av8n.com/physics/weight.htm

If you jump off a cliff, the bathroom
scale reads zero. No apparent weight, but gravity (mg) is still acting and
makes you accelerate.

mg is nonzero in one frame but not in another ... specifically,
mg is ZERO in the frame that is natural, intuitive, convenient,
and conventional for the ISS question as originally asked.

What part of "frame-dependent" do you not understand?

Easy to show this in class.

Letting the class see one leg of the elephant does not solve
the overall problem.

Makes sense to students.

No, it does not. It fails the most elementary critical-thinking
test, because it cannot be reconciled with what every modern
child knows about weightlessness.

So the orbiting satellite is similar, or seems to be similar.

What part of "ambiguous" do you not understand?

Dogmatic attachment to the terrestrial reference frame went out
of fashion in the year 1543 or thereabouts.

Einstein
said a person in free-fall has no sensation of weight, but what makes her
fall?

I very much doubt that Einstein said any such thing.

A much better question would be, why does a desk sitting in the
lab /not/ follow the natural, freely-falling trajectory? Why is
the desk accelerating relative to any natural, inertial, freely-
falling reference frame? These are among the questions that
Einstein asked, and answered. The answers can be explained at
the high-school level:
http://www.av8n.com/physics/geodesics.htm#fig-darts

Motion is simple when analyzed relative to a freely-falling "elevator"
frame. Free fall does not require explanation. Anything else does.

This is the first law of motion writ large: Straight-line motion
through spacetime does not require explanation. Anything else does.