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



Seems to me what you said is that we ignore the string altogether such that the forces on mass 1 are Weight 1 and Weight 2. We DON'T do that in intro courses (teach one sometime). The forces on mass 1 are Weight 1 and the tension in the string. That the tension in the string is the same magnitude as Weight 2 is a consequence of choosing the mass of the string to be at or near zero, but that is not the same as saying the force on mass 1 is weight 2. This approach leaves it open to adding the mass of the string later without any conceptual changes.

rwt

On 12/13/2013 10:48 AM, John Denker wrote:
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?
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Richard Tarara
Professor of Physics
Saint Mary's College

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