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Re: [Phys-L] gravity, weightlessness, etc.

But your _sensation_ of your weight IS the force UP.  Take that away (falling) and you feel weightless (at least for a short time).  I think this may be why some people (I did when teaching) go for the 'apparent weight' approach.  At least you can maintain a Newtonian view if you always analyze rotating systems from the stationary observer and then try to explain the sensations of someone in that rotating system. Otherwise, as far as I can tell, you lose the the third law since there is no object acting on you outwards to be the agent of the centrifugal force.  When you are working hard to move students from Aristotelian to Newtonian viewpoints and don't want to just deep-six rotational systems, working with the idea that we often experience forces backwards--the floor or chair pushing up as our weight and the lack of such as a 'weightless feeling'--has some value.   The feeling of being pushed back into the seat as the airplane takes off can be interpreted as the seat pushing you forwards while your inertia wants to keep you where you were, but there is no force acting on the front of you, at least not in the Newtonian world.  This preserves the third law pair--chair forwards on you, you backwards on chair but your sensation, like for weight is backwards.   Again, I was stuck for 37 years doing mostly first year physics and none of it for physics majors, so never pushed myself or the classes too far away from Newtonian ideas--which were difficult enough for the students.  [I was taught with and continued to use throughout my teaching years the FRAMES OF REFERENCE you know by bias here.]  JD won't agree with any of this, but I suggest it is not so very wrong as a first approach.  I often would say "if you take more advanced courses you will find that there are different ways to interpret some of the things we've covered, but the changes should make sense to you when you get there and you should understand why we took the approach we did in light of your current math and physics backgrounds."

On 1/19/2018 7:32 PM, John Denker via Phys-l wrote:
On 01/19/2018 05:20 PM, LaMontagne, Bob wrote:

Is it still considered OK to define your weight as the force on the
bottom of your shoes when you attempt to stand upright? That seems to
take into account all the "real" and "apparent" forces involved.
I'd say it's /equal to a good approximation/.
It's a reasonable starting point ... but I
wouldn't promote it to a "definition".

There's a 0.1% correction due to buoyancy in air,
which means the scale-reading is less than the
true weight.

There are larger corrections if you're flailing
your arms as you "attempt" to stand upright ...
although these average out over time.

Also I assume the intent was the downward force
exerted /by/ the bottom of your shoes (not the
upward acting "on" them).
Weight = F = mg
where g is a vector in the /down/ direction, by
definition. (This is not the definition of g,
but rather the definition of "down".)

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Saint Mary's College

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