Re: "acceleration due to gravity" (high school)

[Leigh Palmer <palmer@SFU.CA>]

> I appreciate all the points raised in this thread.  I'll add
> another.  In the typical
> HS. Lab. the corrections mentioned are relevant only as part of the
> "theory."  They are
> undetectable, if my quick calcs. are correct.
>
> The moon adds or subtracts a max. of ~ one part in E6.  My intuition
> suggest the sun's
> is about the same, and the frame force is 0.7 %
>
> Do HS's still do the Behr free fall?  Even if its resolution was
> better than 0.7 % How
> would one measure the frame's contribution to g?

That is exactly the point.

> Are there gravitometers other than mass oscillators (pendulum and
> spring)?   Does a
> satellite separate the effects? (Do compared clocks (time dilation)
> measure only that
> due to mass?)

It happens that I've looked into gravimetry recently. There are two commercial
instruments with sensitivity of 1 microgal. 1 gal is one cm/s^2 or, for the
left-handed, one dyne/gm. In using these instruments for exploration one must
correct for Sun, Moon, tides, etc. By the way, the Sun has a much greater
gravitational influence than the Moon. It has about 26 million times the Moon's
mass, and it's only 400 times as far away. The "frame force" you speak of need
not be corrected for at all. It is important in determining the period of a
pendulum in the high school lab, however. If you were to turn it off somehow it
would make pendulum clocks intolerably inaccurate. There are 86,400 seconds in
a day, and centrifugal force slows a clock at midlatitude by minutes per day.

Leigh