Re: [Phys-l] definitions ... purely operational, or not

[John Denker <jsd@av8n.com>]

On 11/10/2010 07:30 AM, carmelo@pacific.net.sg wrote:
> Suppose we want to know the weight of a hot-air balloon...
> When we release this balloon, Oops, it has a "free-ascend" instead of  
> "free fall". Does this mean that the weight of a hot-air balloon is  
> negative?
>
> Is not this definition of weight using "free fall acceleration" still  
> problematic? :-)

The term "free fall" is a highly technical term in this
context.  There is nothing problematic about it.

Examples and counterexamples include:

 -- Buoyancy:  A buoyant object is *not* in free fall.

 -- Drag: A object falling at terminal velocity is *not* in free fall.

 -- Lift: A glider is *not* in free fall.

 -- Thrust:  A rocket is *not* in free fall.

 -- et cetera.

 ++ An object thrown upward *is* in free fall, if we conduct 
  the experiment in a vacuum or otherwise take care that air 
  resistance is negligible.  In this example, free fall may
  diverge from the vernacular notion of "fall" since the latter
  sometimes connotes falling _downwards_ only.

> Is not this definition of weight using "free fall acceleration" still  
> problematic? :-)

It is not problematic.  It is however one of a dozen reasons
why people who are serious about weighing things correct the
scale reading to remove any contribution from buoyancy.  This 
is particularly obvious when the scale is calibrated in units 
of mass.  Essentially everybody thinks that mass should be 
invariant, not a function of the local air density (in contrast 
to buoyancy, which does depend on air density).