Re: [Phys-L] Cavendish
Cavendish used the beat ( = half periodic time) of a torsion balance set up
in an experimental hut. The periodic time varied from about 20 minutes
depending on the position of heavy lead balls compared with light test balls
fixed to the arms of the balance. He apparently found this more convenient than
waiting until the motion of the balance subsided. At least one modern teaching
version uses fluid damping to greatly decrease decay time. in order to provide
direct measurement of beam angle change as the fixed masses are swapped in
order to reverse the gravitational torsional bias on the balance beam. . A
secondary feature allows the balance beam angle to be measured by bouncing a
laser beam off the balance to a fixed scale. Recent measurements provide no
better than plus/minus 25 parts per million for the value of big G. This is
considered less precise than the values of other physical constants of
interest. Air drafts and temperature variation are confounding effects to
measuring this weak force On Friday, February 16, 2024 at 06:00:51 PM CST,
Matthew Heaney via Phys-l <phys-l@mail.phys-l.org> wrote:
Cavendish measured the torque, right? Once you know the torque, you can
compute the force from tau = F * r.
On Fri, Feb 16, 2024 at 6:21 PM Anthony Lapinski via Phys-l <
phys-l@mail.phys-l.org> wrote:
> So I teach about universal gravitation and the Cavendish experiment.
> Problem is -- how is the force actually measured? The fiber twists a
> certain amount when the heavy masses are brought close to the hanging bar.
> A torque makes it twist a certain amount, but how does this translate to
> fiorce so that G could be measured?
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