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Re: [Phys-L] [SPAM] Re: Nice question on buoyancy and balance



Insightful approach!

I do a related demo. Crush two fresh Alka-Seltzer tablets in a latex
glove, and secure tightly to a small beaker of water on a scale. Get the
total mass. Then allow the tablets to dissolve in the water and predict
what will happen to the scale reading as the glove inflates. Few students
ever get this. Then cut the glove so the gas escapes. Predict the scale
reading compared to the initial value (much easier).

I learned this demo at a poster session at a summer AAPT meeting. It's
good to show that gases exert buoyant forces, not just liquids. You could
also show a helium balloon, but there's nothing really to "predict."

Another related question. If you stand on a bathroom scale and remove all
the air in a room, what would happen to the reading?

Fluids is a rich conceptual topic!


Phys-L@Phys-L.org writes:
This question is interesting because of the way it is posed.
1. Both beakers show the water as having risen the same amount
2. The string attached to the ping pong ball has to pull up on the
bottom,
which introduces an N3L distractor.

Basically Antti's solution is what I would have given if I had gotten in
early. The idea that the ping pong ball adds practically no force on the
left is a nice idea. Then just noting that the decrease in tension of
the R
string has to be the same as the weight of the water displaced,
immediately
gives the solution. The force to counteract the weight of the water
displaced has to be provided by the balance, so it sinks until it hits the
pedistle.

The problem requires more than one consideration. Students at the
concrete
operational level will not be able to consider two variables at the same
time, so they will not be able to get it. Some transitional students will
get it, and mostly the students who get it will be the formal operational
students, provided they understand the physics. Actually some of the
concrete operational students will not believe that the water had risen
the
same amount, so an actual physical demo might be helpful for them.

So if this is set up as a demo perhaps it should first start with the
question as to how much the water will rise, then go on to the "buoyancy"
part of the demo. The only way I can see to do it would be to have 2
heavy
sinkers of the same mass and size. Start by asking what happens when you
put them into the water. Students should be able to predict that the
water
will rise the same amount and that the balance will still be neutral.
Then
after demoing this, fasten a ping-pong ball by string to one of the
sinkers
and drop both of them into the beakers with the balance constrained. Ask
which way the balance will go. A Number of students will predict the
ping-pong ball side will rise. Show them the result and have them come up
with why it fell. Then proceed to the demo with the steel ball on a
string.
But first get them to predict how much the water rises. Again some will
predict it rises more on the steel side. Put the steel ball in the beaker
with the balance constrained. And have them come up with a reason why the
water rises the same amount. Finally have them predict what happens to
the
balance, and then show them the result. Finish with discussion of how it
could be predicted. Do all of this in the style of the ILDs. It may be
wise to try to use a small enough beaker that the rise in water can be
readily seen.

My suggested sequence will be beneficial to all. First it will help
students who think that lighter objects of the same volume that sink
displace less water. This paradigm is a marker of lower than mature
concrete operational thinking. Then the second part establishes that the
total mass is what matters when you add the ping pong ball. The total
gravitational force on L is greater than on R. It also demos that the
internal forces do not matter. The third part is really for the immature
concrete operational students, and is not needed for formal operational
students. Finally the last demo brings in the idea of buoyancy.

I think that just posing the question is overload for beginning students,
but is a good puzzler for advanced 2nd year students as well as faculty.

John M. Clement
Houston, TX


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