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pbbock
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While reviewing the Archimedes Principle with my AP students using ConcepTest questions from the Wilson, Buffa, Lou text I came across an answer that I believe is incorrect. The question was the second question in a series of questions about a helium filled balloon.
The first question asked what would happen to a helium filled balloon in a jar of helium. The answer was that the balloon would sink to the bottom of the jar. OK, no problem. Then the next question says, "Now the jar is lifted off the table, but the jar remains inverted to keep the helium gas in the jar. What will happen to the balloon?" (From the choices I am assuming the lid is off the jar.)
These were the choices:
A) it floats at the top of the jar
B) it floats at the bottom of the jar, but still fully inside the jar
C) it floats below the bottom of the jar, sticking halfway out the bottom
D) it sinks down to the surface of the table
All of my students deduced that the helium filled balloon would still sink to the "bottom" (now the open end of the jar.) They disagreed, however, about where it would rest. I was very surprised when the answer provided was choice B. This was the explanation, "The balloon sinks in the helium gas (fluid #1), until it hits the surface of the air (fluid #2). Since the balloon floats in air, it will float on the surface of the air and therefore remain inside the jar, but at the bottom. "
This seems very wrong to me. It will only float on the surface of the air if there is a buoyant force. How can there be a buoyant force without some displacement of air? It doesn't seem philosophically different from an air-filled balloon floating on the surface of a liquid.
Can someone help me out here?
The first question asked what would happen to a helium filled balloon in a jar of helium. The answer was that the balloon would sink to the bottom of the jar. OK, no problem. Then the next question says, "Now the jar is lifted off the table, but the jar remains inverted to keep the helium gas in the jar. What will happen to the balloon?" (From the choices I am assuming the lid is off the jar.)
These were the choices:
A) it floats at the top of the jar
B) it floats at the bottom of the jar, but still fully inside the jar
C) it floats below the bottom of the jar, sticking halfway out the bottom
D) it sinks down to the surface of the table
All of my students deduced that the helium filled balloon would still sink to the "bottom" (now the open end of the jar.) They disagreed, however, about where it would rest. I was very surprised when the answer provided was choice B. This was the explanation, "The balloon sinks in the helium gas (fluid #1), until it hits the surface of the air (fluid #2). Since the balloon floats in air, it will float on the surface of the air and therefore remain inside the jar, but at the bottom. "
This seems very wrong to me. It will only float on the surface of the air if there is a buoyant force. How can there be a buoyant force without some displacement of air? It doesn't seem philosophically different from an air-filled balloon floating on the surface of a liquid.
Can someone help me out here?