Re: Effect of Moon on balance?

[John Clement <clement@HAL-PC.ORG>]

An equal arm balance should show no changes because both the test mass and
the balance mass are equally affected.  However a very accurate spring
"balance" should show changes.  As John pointed out the motion of the Earth
is determined by the average force on the center of mass.  However when the
moon is overhead the mass on the spring balance will weigh slightly less
than when the Moon is on the other side of the Earth.  Notice that the base
of the balance is resting on the Earth.  The mass however is free to go up
or down the same way that water can be pulled by the Moon.  Such balances do
exist.  I saw a video that showed a spring balance and mass that could
detect the gravitational force due to a massive concrete beam.

Usually one calls a spring balance a spring scale, but the word balance is
appropriate, but misleading to students.  The spring scale is actually
reading the balance between the spring force and the applied force.  For
conversation with students I would favor the term "spring scale", and
reserve the word balance for the chemical balance which balances weights.
Similarly the electronic chemical scale should also not be called a balance.

John M. Clement
Houston, TX

> I wrote:
>
> > The gravitational pull of the Moon cannot be measured on the
> Earth  using a
> balance; precision of the balance has nothing to do with it.
> > The Earth is in free fall toward the common center of mass in
> the Earth-Moon
> system.
>
> John Denker wrote:
>
> > That might be true if the earth were pointlike. But it's not.
> > The "center" of the earth is in free-fall toward the center of
> mass, but your
> laboratory is not.
> > Tides are easily detectible.
> http://www.monmouth.com/~jsd/physics/tides.htm
>
> Yes, I can see that (excellent web page!). But my question was:
> is it possible
> to measure changes of gravitation force due to Moon
> when you have:
>
> -- a laboratory somewhere on the Earth
> -- constant conditions (e.g. density of air which may affect
> buoyance etc.) in
> the laboratory
> -- very precise balance
> -- object of known mass on the balance
>
> So all other things being equal: would the *balance reading* in
> the laboratory
> vary in time due to change in relative position of the Earth and Moon?
> My answer is no. Should I change my opinion?

> Regards,
>
> Antti
>
> Antti Savinainen
> Senior Lecturer in Physics and Mathematics
> Kuopion Lyseo High School
> Puijonkatu 18
> 70110 Kuopio, FINLAND
> E-mail: antti.savinainen@kuopio.fi
> Personal web page: http://kotisivu.mtv3.fi/oma/physics/