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Re: [Phys-l] g...

My students think a bathroom scale shows their downward weight. It
actually shows the upward support force, which can change in a accelerated
frame (e.g., elevator). So the scale shows their apparent weight. That's
all. You can call it whatever you want, but there is no consensus among
teachers/textbooks. You need to call it something when you're trying to
correct students' misconceptions about what a scale reads, why the
astronauts float, etc. Students think that because a scale reads zero that
there is no "g" up there!

Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu> writes:
On 11/19/2006 10:42 AM, Anthony Lapinski wrote:
I mean that a scale does not always show your "true weight (w = mg).

That's for sure. That we can agree on!

Scales show apparent weight -- how heavy you THINK you feel. So when an
elevator accelerates upward, you feel heavier. You feel more pressed to
the ground, and a scale would register a higher reading.

Calling it "apparent weight" still seems like a step in the wrong
direction.

I would say the scale (in particular a spring scale) shows the force
impressed on the scale. Similar your proprioception tells you the
force impressed on you by the scale or elevator or whatever. These
forces may or may not have anything to do with mg i.e. weight. The
fact that these forces are /sometimes/ equal to weight doesn't mean
they are always equal to weight. In equilibrium, weight is balanced
by other forces. In non-equilibrium, weight i.e. mg gives rise to
accelerations. Weight is just like any other force! The force is
the same, no matter whether the object is in equilibrium or not.
Force is force. Weight is weight. Acceleration of the object does
not change the force.

(Acceleration of the reference frame is another matter entirely,
but that's not what we're discussing here.)
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