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Re: Electrostatics problem



Leigh is of course correct. Explicitly:
Define
A = angle of elevation of the line joining the (equal masses);
T1, T2 = angles from the vertical of the two strings.
F(el) = the electrical coulomb force magnitude.

Then
T1*sinT1 = T2*sinT2 (cuz each side = F(el) *cosA.)
T1*cosT1 = T2*cosT2 (cuz each side = mg)

Dividing:
tanT1 = tanT2; QED

This only fails if the masses are unequal.

Bob

Bob Sciamanda (W3NLV)
Physics, Edinboro Univ of PA (em)
trebor@velocity.net
http://www.velocity.net/~trebor

----- Original Message -----
From: Leigh Palmer <palmer@SFU.CA>
To: <PHYS-L@lists.nau.edu>
Sent: Friday, September 10, 1999 4:45 PM
Subject: Re: Electrostatics problem


Leigh: Newton's 3rd law is part of translational equilibrium in this
case. This alone doesn't make the angles the same.

The gravitational force is entirely vertical. The horizontal components
of the tension forces must, therefore, be equal to the horizontal
components of the electrostatic forces and oppositely directed (they
equilibrate the horizontal components of the electrostatic forces). The
strings have the same lengths, so the only way the horizontal
components of forces which must lie along the strings can be made equal
is by making the strings make equal angles with respect to the vertical.

Leigh