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



You can do the experiment with a couple of wireless force sensors. PASCO has recently released one that is fairly inexpensive. Their SparkVue app for smartphones will read them either live, or as data loggers. Give each student a sensor, connect them by a string, and have one pull the other.

-> -----Original Message-----
-> From: Phys-l [mailto:phys-l-bounces@www.phys-l.org] On Behalf Of
-> LaMontagne, Bob
-> Sent: Sunday, August 28, 2016 1:41 PM
-> To: Phys-L@Phys-L.org
-> Subject: Re: [Phys-L] treating force as a vector ... consistently
->
-> 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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->
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