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Re: [Phys-L] feeler-dealer, third law, et cetera



On 12/13/2013 07:10 AM, Richard Tarara wrote:
Solving these type of situations through free-body diagrams
emphasizes Bruce's point--there is never a weight 2 acting on weight
1 force vector unless the objects are in contact (one above the
other).

That has nothing to do with the point I was making.

My point is that it is exceedingly common in introductory
classes to neglect the weight of the string, so that the
tension at one end of the string is equal to the tension
at the other end. To say the same thing in other words:
no momentum accumulates in the string. It is not necessary
to do a molecule-by-molecule accounting for the dynamics
of the string ... or indeed /any/ accounting for the
dynamics of the string.

Why cannot we make the same kind of approximation with
respect to the electrostatic field ... where it is
actually a much /better/ approximation?

I really didn't expect to get pushback for applying
Coulomb's law.

Is Newton's law of universal gravitation also off-limits?

What approximations *are* we allowed to make?