jar question
in [Physics]
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From: Bell Foster on 2 Apr 2006 05:23 Here is a test question that I didn't get and don't understand: "An empty jar is pushed open-side downward into water so that air trapped in it cannot get out. As the jar is pushed deeper, the buoyant force on the jar...decreases." I don't understand the answer of "decreases" (as opposed to say "stays the same" or "increases") because I understood buoyant force to be related to the weight of the displaced fluid, and pushing the jar further down displaces more fluid, which weighs more. So it seems to me it should be "increases"...maybe someone can explain to me why/how I'm wrong.
From: Martin Hogbin on 2 Apr 2006 06:04 "Bell Foster" <bell3774(a)gmail.com> wrote in message news:1143969800.842841.263490(a)u72g2000cwu.googlegroups.com... > Here is a test question that I didn't get and don't understand: > "An empty jar is pushed open-side downward into water so that air > trapped in it cannot get out. As the jar is pushed deeper, the buoyant > force on the jar...decreases." > > I don't understand the answer of "decreases" (as opposed to say "stays > the same" or "increases") because I understood buoyant force to be > related to the weight of the displaced fluid, and pushing the jar > further down displaces more fluid, which weighs more. So it seems to me > it should be "increases"...maybe someone can explain to me why/how I'm > wrong. As the jar is pushed further down the water pressure increases. This compresses the air in the jar, reducing its volume and thus reducing the volume of water displaced. This reduces the buoyancy force on the jar. Martin Hogbin
From: Bell Foster on 2 Apr 2006 05:37 > As the jar is pushed further down the water pressure increases. > This compresses the air in the jar, reducing its volume and thus > reducing the volume of water displaced. This reduces the > buoyancy force on the jar. How does what you describe reduce the buoyance in the jar though? For that to happen, the volume of the jar would have to decrease, and it stays the same...
From: CWatters on 2 Apr 2006 06:00 "Bell Foster" <bell3774(a)gmail.com> wrote in message news:1143970648.085926.297260(a)j33g2000cwa.googlegroups.com... > > > As the jar is pushed further down the water pressure increases. > > This compresses the air in the jar, reducing its volume and thus > > reducing the volume of water displaced. This reduces the > > buoyancy force on the jar. > > How does what you describe reduce the buoyance in the jar though? For > that to happen, the volume of the jar would have to decrease, and it > stays the same... No it doesn't stay the same. You said the neck of the jar is open. Therefore the air inside is compressed and some water goes in the neck of the jar reducing it's displaced volume.
From: Martin Hogbin on 2 Apr 2006 10:50 "CWatters" <colin.watters(a)pandoraBOX.be> wrote in message news:2hNXf.350880$hH7.10535002(a)phobos.telenet-ops.be... > > "Bell Foster" <bell3774(a)gmail.com> wrote in message > news:1143970648.085926.297260(a)j33g2000cwa.googlegroups.com... > > > > > As the jar is pushed further down the water pressure increases. > > > This compresses the air in the jar, reducing its volume and thus > > > reducing the volume of water displaced. This reduces the > > > buoyancy force on the jar. > > > > How does what you describe reduce the buoyance in the jar though? For > > that to happen, the volume of the jar would have to decrease, and it > > stays the same... > > No it doesn't stay the same. You said the neck of the jar is open. Therefore > the air inside is compressed and some water goes in the neck of the jar > reducing it's displaced volume. Which was just about what I was going to say. Martin Hogbin
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