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



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