Re: [Phys-L] apparent weight

[John Denker <jsd@av8n.com>]

On 12/10/2014 06:04 AM, Anthony Lapinski wrote:
> If you stand on a sensitive scale, will you be slightly lighter at noon
> (with the Sun above you and pulling opposite to Earth's gravity), and
> slightly heavier at midnight (with the Sun below you and pulling in the
> same direction as Earth's gravity)? Ignore any lunar effects.
>
> This came up the other day, and nobody could really agree. One side said
> no because the Earth is in freefall around the Sun, and it is the Earth
> that pulls us to it. The other side said yes 

Neither side is correct.

> due to tidal effects as seen in the oceans.

That's a step in the right direction.  If you pursue that, 
it will tell you what really happens:  At this level of
detail, you are lighter twice a day, both at noon and at 
midnight.  You are heavier twice a day, at 6:00 AM and PM.

I reckon the real question is how best to explain it.
Here's my approach:
  https://www.av8n.com/physics/tides.htm

I find it really helps to build the hands-on model of
the potential.  Locally it is saddle-shaped.

=================

If you do the actual experiment, you quickly discover
a number of complications:

 -- The moon must also be considered.
 -- Neither the moon nor the sun passes directly overhead,
  except on a set of measure zero.  There is a nontrivial
  north/south component to the tidal stress, in addition
  to the more obvious up/down and east/west components.
 -- With a sufficiently sensitive measurement you can
  detect the mass of air above you, which changes with
  the weather.
 -- I assume the "you" in the original question is 
  metaphorical.  Obviously the experiment should be done
  using a test object with constant mass, not an actual
  person.
 -- et cetera.