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

["Anthony Lapinski" <Anthony_Lapinski@pds.org>]

These concepts can be confusing to kids.

I believe that Einstein's happiest thought was that if a person jumps off
a cliff (no air), then she has no sensation of weight. This led to his
general theory of relativity.

True weight is mg, while apparent weight is given by a bathroom scale,
which shows the upward support force. Jump off a cliff, and that scale
reads zero. You have no apparent weight, just like those floating
astronauts in the orbiting shuttle. Plenty of true weight. You can also
hang a wood block with a spring scale. Release the block and it falls due
to its true weight. But the scale reads zero while the block falls.

Scale can also be used with fluids. Suspend a 1-kg mass from a spring
scale. It weighs about 10 N. When completely submerged, the scale reads
about 9 N. The difference is the buoyant force. So the buoyant force is
abut 1 N -- the weight of water displaced (which can be measured).

Kids understand these true weight/apparent weight ideas using these scale
activities and their applications.

Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu> writes:
> On Nov 8, 2010, at 8:40 AM, Scott Orshan wrote:
>
> >  I'm fine with that definition. Now can we get the astronauts to stop 
> > telling everybody that they are weightless, and in Zero G?
>
> But, with that definition they *should* be saying that they are
> weightless.
>
> > > "... the weight of a mass M in a specified frame of
> > > reference is M times the free-fall acceleration in that specified
> > > frame of reference."
>
> John Mallinckrodt
> Cal Poly Pomona
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