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[Phys-L] Re: A Third law question



I would give her a kg mass and a blackboard eraser. Have her lift
each one up quickly. She will see that the force she has to apply
is much greater for the kg than for the eraser. The reason why
the reaction force from the kg is greater is not because the kg
somehow knows to push back harder. It's also not directly because
it is more massive. It's greater because SHE had to push harder
to accelerate it upward. Likewise, the eraser pushes back less
because SHE pushes less to get the same acceleration.

What the object does is to determine how much force her hand will
end up pushing up against the object as it accelerates upward.
The lighter object essentially gives way more easily.

In other words, the mass is determining HER force through a
feedback loop involving her eye and hand. (Essentially - your
inertia argument.)

Bob at PC

-----Original Message-----
From: Forum for Physics Educators [mailto:PHYS-
L@list1.ucc.nau.edu] On Behalf Of Rick Tarara
Sent: Tuesday, October 11, 2005 2:18 PM
To: PHYS-L@LISTS.NAU.EDU
Subject: A Third law question

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

*********************************************************
Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, Indiana
rtarara@saintmarys.edu
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