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[Phys-L] Re: A Third law question



At 01:18 PM 10/11/2005, Rick, you wrote:

Way back to basics here. During a Socratic Dialog lab on Newton's laws, one
student became hung up on N3 in the part of the exercise where students are
asked to hold a massive disk in their hand and then accelerate it upwards.
She could understand that to do so, she had to increase the force of her
hand on the disk to be greater than the force of the earth on the disk.
What took the next 1/2 hour (and I think without ultimate success) was to
try and understand how the disk was able to increase its force back on the
hand so that N3 would hold. I talked to her about inertia--about the
resistance of the mass to changes in motion. I had her holding a 5 kg mass
and then trying to accelerate it horizontally (to take out the gravitational
factor). I dropped that mass into her hands to have her experience the
increased force (of her hand and on her hand) necessary to produce the
needed acceleration to stop the mass. She declined (wisely) to try and
catch the falling mass with her hand in contact with the top of the table.
We talked about moving things in space and I brought out an air track so we
could look at a 'frictionless' situation. In the end though, she was still
having trouble. She could 'understand' how she increased the force of her
hand on the disk but couldn't really accept the inertia arguments about how
the disk increased its force back on the hand. 'Where does the disk get the
extra force when I push up with more force than its weight?' was her
repeated question.

Any suggestions here? How would you try to deal with this question?

Rick

Why wouldn't a student find weight and inertia puzzling?
It IS puzzling. The following is an effort to describe this
view-point from the perspective of one who is fresh to the Universe.

"There is a mysterious effect which in most parts of the universe
provides a resistance if one attempts to move a mass that is resting
in the observer's frame. Moreover, if the mass is already moving,
the resistance also shows up if one tries to move it in a different
direction than the line in which it is already moving.
Both of these changes are called an acceleration, in the standard
physics scheme of things.

"We are more used to observing things near the Earth surface.
Here, the situation is even more mysterious.
A mass does not stay in place here, it begins to drop as soon as
that is allowed. It is easy to see that this change in position is
also an acceleration, a constant acceleration directed towards
the Earth.

"Strangely enough, the mysterious effect becomes apparent if one
attempts to restrain this acceleration, and hold the mass stationary
in the observer's frame. This force of restraint is called the
mass's weight.

"It is as though one were accelerating the same mass in the 'upwards'
direction in deep space. In experiments, it is found that the force
in question is proportional to the object's mass, and to the
object's acceleration if "deep space"-like conditions are set up.
Near Earth, the force on the stationary mass is proportional
simply to its mass.

"You can see a strange parallelism between effects due to
acceleration in deep space, and stillness in the Earth's vicinity.
The former is labelled an inertia effect, the latter, a
gravitational effect."


Brian Whatcott Altus OK Eureka!