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Re: [Phys-L] treating force as a vector ... consistently

The student can only pull you if you hang on. If you let go, it's much
easier for the other student to pull the rope.

A stationary rope can't magically pull you. However, if a rope hangs from a
ceiling (as in a gym), why must you pull downward in order to move upward?

On Sun, Aug 28, 2016 at 2:40 PM, LaMontagne, Bob <RLAMONT@providence.edu>
wrote:

As a student, I would be a little confused by this. It is one thing if you
sit in a chair holding one end of a rope and another student pulls you. It
is another (and more confusing) thing for a stationary rope to somehow
magically pull you along. You now have to consider the motion of you
muscles and all kinds of complicated "action-reaction" pairs.

Bob at PC
________________________________________
From: Phys-l <phys-l-bounces@www.phys-l.org> on behalf of Jeff Bigler <
jcb@alum.mit.edu>
Sent: Sunday, August 28, 2016 1:58 PM
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] treating force as a vector ... consistently

I agree with pretty much all of this, but it begs the question of how you
would describe the N3 force pair in the example of the book on the table.

One of the demos I do for N3 is to put a student in an office chair with
wheels, tie a rope to a lab table, and ask the student to apply a force to
the rope (and say which direction the applied force is in), and then ask
the student to explain, referencing the forces involved, why they moved in
the direction they did.

Jeff Bigler
Lynn English HS; Lynn, MA

Sent from my phone, probably while multitasking and possibly using
voice-to-text. Please pardon any typos, speakos, autocorrect errors and
other vagaries, and ask about anything that doesn't make sense.

On Aug 28, 2016, at 1:24 PM, Scott Orshan <sdorshan@aol.com> wrote:

I thought I'd drop my two cents in, and describe how I teach Newton's
3rd.

As a preface, I'm not big on using momentum as a beginner concept. I
can't see or feel momentum, and it changes with the frame of reference.

I *can* feel forces and their effects, so I prefer presenting physics
from the force model with beginners.

The first part of the approach is to get rid of the words "action" and
"reaction". They are filled with varied and contradictory meanings from all
walks of life. "I hit Jimmy, so he hit me back." Not a N3 force pair. "I
pull on one end of a rope, and you pull on the other end." Not a N3 force
pair. "The stuff shoots out of the rocket so the rocket moves in the
opposite direction." Not a N3 force pair. The N3 force pair(s) in a rocket
are too complex to contemplate, and result from the pressure gradient in
the combustion chamber and hollow areas of the rocket. The rocket formula,
on the other hand, is a simple conservation of momentum formula.

The question/answer pair "Q: How does a rocket work? A: Newton's 3rd
Law." says nothing about either rockets or Newton's 3rd Law.

The second part is to define the nature of N3 force pairs.

They happen between the same objects.

They are equal in magnitude.

They are opposite in direction.

They are the same type of force (gravity and contact [EM] are of most
concern in the beginner classroom).

This next one is not one I've seen mentioned, but I think it's an axiom
as well:

They start and end together. Same start time, same duration, or the
slightly more complicated parallel for variable forces. (In the real world,
reactions tend to follow actions.)

That last axiom means that the weight of a book can not be a force pair
to the table's normal force. The book was interacting with the Earth long
before it was placed on the table.

Two vectors pointing in the same direction with the same magnitude may
be mathematically equivalent, but in Physics, the vector has units and
other meanings.

In math, 3=3, but in Physics, 3 only equals 3 if the units are the same.

Real world forces exist as pressures (not applied at a point), and real
world objects are deformable and/or elastic. That's what differentiates an
engineer from a scientist. Two elephants pulling on you from opposite sides
is very different from two elephants pushing on you from opposite sides.
However, the net force is zero in both cases.

Scott Orshan


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Forum for Physics Educators
Phys-l@www.phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l
_______________________________________________
Forum for Physics Educators
Phys-l@www.phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l