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



On 08/28/2016 10:24 AM, Scott Orshan wrote:
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).

That's helpful in many ways, especially for the sorts of forces
that one encounters in the introductory course.

However, there is one small fly in the ointment: the "same type"
criterion gets a little weird when applied to Feynman's charged
disk experiment. The field pushes on the particle, and the particle
pushes on the field, but the "sameness" of these two interactions is
not obvious at first glance (or even second glance).

Reference: The statement of the question, including a diagram of the
apparatus, is presented in
Feynman volume II chapter 17 "The Laws of Induction"
section 17-4 "a paradox"
http://www.feynmanlectures.caltech.edu/II_17.html#Ch17-S4

The answer is spelled out at
Feynman volume II chapter 27 "Field Energy and Field Momentum"
section 27–6 "Field momentum"
http://www.feynmanlectures.caltech.edu/II_27.html#Ch27-S6