[Phys-L] Re: A Third law question

[s.goelzer at COMCAST.NET (Scott Goelzer)]

Even though it seems trivial to us, the following sequence is
surprising to some students:

Two students each hold bathroom scales vertically by the rim of the
scales then face each other and push. Another set of students call
out the forces.
1) Have one student push 'harder'.
2) Have one student be much more massive than the other (be nice so
the callers can read the forces).
3) Place one student on a moving cart kneeling (spotters needed)
4) Place both on carts opposing and push away from each other
(already suggested).

FBD's for everyone!


I had a similar situation with the student not believing the force
went away after contact ceased.
This worked for her: Push a dynamics cart with a large soft block of
foam taped to it with another block that she held. Both deform, both
spring back when contact was lost. She could not push one without
deforming the other.

Scott






*******************************************
Scott Goelzer
Physics Teacher
Coe-Brown Northwood Academy
Northwood NH 03261
sgoelzer@coebrownacademy.com
*******************************************


On Oct 11, 2005, at 2:18 PM, Rick Tarara 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
>
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> Professor of Physics
> Saint Mary's College
> Notre Dame, Indiana
> rtarara@saintmarys.edu
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