Re: Internal or external?

[Leigh Palmer <palmer@SFU.CA>]

Well, there was quite a flurry while I was entertaining 18
guests (including my three-year-old granddaughter) at a day-
early Canadian Thanksgiving dinner. One thing I want to point
out about my definition that I didn't emphasize: It is simpler
than Hecht's. Since it is correct (in that no cases are left
untreated or incorrectly treated), why is it not better than
Hecht's? Is not simpler better?

With my definition of weight the water is of no significance.
It constitutes a source of force in contact with the mass.
An object floating freely in water (no elevator to complicate
matters) is not weightless*. Are you intending to ask me the
question "What is the reading on the bathroom scale?"?

With my definition the weight in the elevator not accelerating
(not "at rest"; there is an important difference, as you will
recognize if you think carefully about it) is 1000 N. The
weight in the elevator accelerating upward is 1500 N, and the
weight in the elevator accelerating downward is 500 N.

Now let me answer the questions you probably meant to ask. The
scale will read the difference between the mass's weight and
the bouyant force. In case 1 the bouyant force is the weight
of the displaced water, or 200 N. In case 2 it is 300 N, and
in case 3 it is 100 N. Thust the scale readings will be:

1. 1000 N - 200 N = 800 N

2. 1500 N - 300 N = 1200 N

3. 500 N - 100 N = 400 N

You should be very careful in forming questions. The two
ambiguities in this one make it difficult to answer without a
lot of explanation, but I hope I've made it clearer now.

John Mallinckrodt seems to be using exactly my definition in
his posts. Cliff Parker is using Hecht's convention where g
refers only to earth's gravitational pull. Hecht is not wrong;
he certainly understands the physics as well as the rest of us.
It is my claim that his convention does not help the student
understand; in particular he could understand better if his
introduction were along the lines I propose.

Leigh

* There is a strong difference in sensation between floating
and falling freely.

> Leigh,
>
> To clarify your definition of weight in my mind, please tell me the weight in
> the following cases.  I have a 100 kg mass with a density of 5000 kg/m^3.
> The mass is placed on a bathroom scale in a tub that can be filled with
> water.  The tub is placed in an elevator that may be accelerated and is at a
> location where g is approximately 10 N/kg to simplify calculations.  What is
> the weight of the mass in the following cases, first with no water in the
> tub, and second, with enough water to completely submerge the mass?
>
> 1.  The elevator is at rest.
>
> 2.  The elevator accelerates up with a = 5 m/s/s.
>
> 3.  The elevator accelerates down with a = 5 m/s/s.
>
> Thanks,
>
> Bob Carlson