Poll

12 votes (27.27%)
8 votes (18.18%)
24 votes (54.54%)

44 members have voted

Wizard
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Wizard
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June 4th, 2012 at 1:54:55 PM permalink
First put a beaker on a scale. Then glue a cork to the bottom of the inside of a beaker. Wait for glue to dry. Then fill it with water.

The glue is water-soluable. Eventually it will loosen and the cork will rise to the top.

At the moment the cork breaks free from the bottom of the beeker, will the scale register more, less, or equal weight?

Please put your answer in "spoiler" tags if you're confident you got the answer right (Doc), and don't want to spoil it for others, unless they choose to click on the spoiler button. This suggestion applies to any kind of math/physics/trivia question.

Have a nice day.
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aceofspades
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June 4th, 2012 at 2:08:07 PM permalink
Quote: Wizard

First put a beaker on a scale. Then glue a cork to the bottom of the inside of a beaker. Wait for glue to dry. Then fill it with water.

The glue is water-soluable. Eventually it will loosen and the cork will rise to the top.

At the moment the cork breaks free from the bottom of the beeker, will the scale register more, less, or equal weight?

Please put your answer in "spoiler" tags if you're confident you got the answer right (Doc), and don't want to spoil it for others, unless they choose to click on the spoiler button. This suggestion applies to any kind of math/physics/trivia question.

Have a nice day.





My answer:

Per my limited knowledge of chemistry/physics, the law of conservation of matter should apply - basically, The mass of an isolated system cannot be changed as a result of processes acting inside the system. Therefore, will remain "SAME".
Mosca
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June 4th, 2012 at 2:14:12 PM permalink
equal.
NO KILL I
FinsRule
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June 4th, 2012 at 2:22:47 PM permalink
Less. I go against the crowd.
DJTeddyBear
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June 4th, 2012 at 2:32:32 PM permalink
Very interesting and intriguing. It's got my gears turning. And here's what I came up with:

"Logic" dictates that at that moment, things change causing the beaker's weight to change. However, conventional logic is wrong.

The weight of the contents remains constant. When the cork breaks free, water displaces it, forcing it to float, causing people to assume the total weight has changed.

The other "conventional" logic is that the mere disturbance of the cork breaking free causes motion, which causes the scale to register changes.

However, according to Newton's law of actions having an equal and opposite reaction, the scale registers no change.


Bottom line: Assuming the bond is strong enough to hold the cork long enough for the scale to 'settle' before breaking free, the scale will remain unchanged.
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Face
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June 4th, 2012 at 2:41:38 PM permalink
If asked last year, I'd probably gnaw on this question for quite awhile, and I'm sure my brain would have split into opposing sides and waged war. But a year ago I asked a similar question about fish in a cooler, and Doc and Mosca set me straight. This question is close enough to that one for me to say
it'll weigh the same
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Doc
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June 4th, 2012 at 3:10:08 PM permalink
Quote: Wizard

Please put your answer in "spoiler" tags if you're confident you got the answer right (Doc), and don't want to spoil it for others, unless they choose to click on the spoiler button.



Well, if you're going to call me out by name on such a thing, I guess I'll have to toss in a bit of a twist. Now I just know that everyone is clicking on the "Show Spoiler" buttons right off the bat, but why not think for a sec about what possible twist I could come up with?

O.K., in an equilibrium state, with the cork glued to the bottom or floating on the surface, the scale will (should anyway) read the same thing.

On the other hand (Twist #1), the Wizard asked "At the moment the cork breaks free...." This may be interpreted as the transient problem as the cork accelerates upward and water accelerates downward below the cork, followed by the cork and water slowing down toward the final equilibrium state. During that transient, the scale reading should remain, on average, the same as before, but it could vary up and down erratically. Visualize this as if you are standing on the scale and shaking all over, up and down. You don't expect the scale reading to remain steady do you?

Taking this to an extreme, it is possible that the cork will pop out of the water and be free-flying in the air for a very brief period (Twist #2). It might even splash some water droplets into the air with it. If the scale is responsive enough to be able to record that instant, it should display a value less than the original one, followed by a higher-than-original value as the cork splashes back down. Analogy: you're standing on the scale and jump into the air.

Yep, one of those "less" votes is mine, just to be contrary. But I assure you that if you are considering equilibrium states, then "equal" is the right answer, and the average value during the transient period should be the same, too.
EvenBob
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June 4th, 2012 at 3:36:50 PM permalink
Didn't they do a similar thing on Mythbusters where everybody
in an elevator had to jump up at the same time and the myth
said their weight in the elevator would disappear. As I recall,
it still weighed the same when they were all suspended in the
air because their mass was still in the elevator. Maybe the cork
thing is different, I get all my knowledge from TV and cereal
boxes.
"It's not enough to succeed, your friends must fail." Gore Vidal
Wizard
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Wizard
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June 4th, 2012 at 3:41:45 PM permalink
In the interests of discussion, let's allow a few days to submit answers before discussing the solution openly.

Based on some hidden comments, let me emphasize that I'm asking about the effect on the scale the exact moment the cork breaks free. Your instruments are delicate enough to measure this. Let me also rule out the issue of the cork rising up the air and possibily splashing water out of the beaker, because even if these events happened they would be after the cork broke free of the bottom. However, if you wish to discuss the whole timeline in your answer, assume the cork is never airborne and no water splashes out.
It's not whether you win or lose; it's whether or not you had a good bet.
bigfoot66
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June 4th, 2012 at 3:54:42 PM permalink
Less. As the cork is moving upwards, it is providing an additional support force to the water above it. The amount of force that the scale is pushing upwards on the beaker to support is decreased by the amount of force that the cork exerts on the water above it as flies upward through the water. Ignore the fact that the cork will splash some water above what was the surface of the water. The total amount of upward force exerted by both the cork and the scale's support force (read: weight) have an inverse relationship. It is the same principal as tying a balloon to a brick.
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