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Cork in Bucket

Science brain teasers require understanding of the physical or biological world and the laws that govern it.


Puzzle ID:#9854
Fun:*** (2.96)
Difficulty:*** (2.25)
Submitted By:electronjohn*us****




A cork, say from a wine bottle, is held by a robotic clamp at the bottom of a bucket of water and released the moment the bucket is dropped off the side of a tall building. What will happen to the cork during the fall? Will it float to the top of the bucket normally, slower than normal, faster than normal, or not at all?

Assume that there is no air resistance to slow the bucket down.


The cork will stay at the bottom of the bucket and not float up at all during the fall, since the cork, the bucket and the water all fall at the same acceleration g (neglecting air resistance as all good physics questions do). The buoyant force of the water does not push the cork upward in this case since in free fall the buoyant force is zero. In other words, a cork only floats up when the water around it is heavier and pushes it up. In free fall the water is weightless, as is the cork and bucket, so there is no force to push the cork to the top of the bucket.

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Jan 23, 2003

Excellent, I didn't get it.
Jan 25, 2003

good one, i get it, and i would have gotten it right if i had been thinking about the buoyant force changing to zero
Apr 03, 2003

You are a Scientist
Apr 03, 2003

I got it because, unfortunately, I am in physics right now...bleah. lol. Good one though
Apr 04, 2003

I got it!!!! I'm so proud!!! I am turning 12 in a few days, happy b-day to me!
Apr 12, 2004

That one was nice. Made you have to go back to Physical Science/Physics.
Apr 12, 2004

Fortunately, I have never taken Physics yet so I did not understand a word you said!lol
Apr 12, 2004

Sorry, E-John, but you're dead wrong! The Bucket is NOT in free fall - an object is in free fall only if it is out of any gravitational field - the bucket, having been dropped off a building, is being accelerated at 32 ft/sec/sec, as is the water and the cork. Normal gravity is acting on all of them just as if they were still sitting on the roof - and the cork will bob to the top as soon as the robot clamp turns it loose - back to the drawing board, Bob
Apr 12, 2004

Unklemyke, if the bucket is not in free fall when is anything in free fall?! In fact it is in free fall by definition, it is falling freely. How do you think they filmed those zero g shots for Apollo13? Ever hear of the Vomit Comet? It is a big plane that goes up and down with the down part being a free fall and the people inside float around like they are in space. It is used to train astronauts. Anyway it is the same thing.
If you still think I a wrong please go to a college and tell a physics professor the question and when he agrees with me bet him $100 he is wrong.
Apr 12, 2004

well...i think this is a weird teaser because who the heck has a robot, a cork and a bucket of water? and if u dropped the bucket, wouldn't the water spill? and what the heck happens AFTER the bucket hits the ground...there is ground isn't there?
(user deleted)
Apr 13, 2004

Interesting! As soon as my clamp robot is finished, I'm going to check this out.
Jul 25, 2004

I dropped the ball on this one...err...cork, whatever. Kool!
Dec 15, 2005

unklemyke, look up freefall in the dictionary before you make fun of someone for, um, being correct.

Apply your vast knowledge of using smilies to physics next time. Excellent teaser - with a correct answer.
Jan 19, 2006

actually, it is true, and the answer is absolutely correct, though I didn't think of it right away, so also challenging. Good teaser.
Feb 03, 2006

best teaser in the science category that I've read here
Oct 01, 2006

This is absolutly correct if you think about it.
The bucket is in freefall, just as a the space shuttle is when orbiting the earth (it doesn't hit the ground like the bucket as it is moving so fast the earth curves under it quicker than it falls).

Just like in space there is really no up and no down. So the cork just stays where it is. Once the bucket hits the ground it acts just like any other cork in a bucket would
Jan 24, 2009

hmm i thought it was obvious..
Jan 24, 2009

Ok, think about this: The bucket is heavier than the water so will fall faster and the water is heavier than the cork so will fall faster than the cork. Forget about the free fall. If you drop a bucket of water off a building, the water will fly out because the bucket is heavier....common sense!
Jan 24, 2009

The teaser is awesome. the comments are hilarious. Who taught you that a heavier object falls faster than a lighter object, enjaytee?
Jan 24, 2009

im sorry but if the gravity is zero what makes the bucket fall? the presure of the water which is being pulled to earth by gravity is the same weather falling or suspended. the only way to reduce the g-force is to propel it toward earth which would reduce the gravity of the water. the cork would rise if the bucket was dropped by natueral force as it would if the bucket was suspended.
(user deleted)
Jan 24, 2009

The cork will float to the top like it would normally do, if it was not falling. neglecting air resistance all things fall to earth at the same rate regardless of weight. So if the three items; the water, bucket and cork are all falling at the same rate then the cork will stay where it is. However the boyancy of the water which is due to the cork being less dense than the water, is pushing the cork up. SOOO we have the force of gravity pushing down on the cork and boyancy pushing up, since gravity is pushing down at the same rate regardless if the object is in motion the cork will rise to the top like it usually does.
Jan 24, 2009

I thought the cork would rise to the top, but with no scientific backing be hind me I was only guessing.
Jan 24, 2009

I agree that the cork would stay where it is, since objects of different size and weight do fall at the same speed. But, I thought maybe the different density of the objects might make a difference against each other - and not against air, as is the bucket.
Now, I don't know!
Jan 24, 2009

Oh, I forgot - there is no air pressure in this teaser...
Jan 24, 2009

Thank you for this great thinking excercise.
Jan 24, 2009

That was awesome!
Jan 24, 2009

nice one dude!xD
Jan 24, 2009

Jan 24, 2009

From Wikipedia ( with my comments interjected:
m = mass
g = force of gravity
p = pressure of fluid on object
V = volume displaced
F = net force on object

The net force on the object is the sum of the buoyant force and the object's weight. F = mg - pVg

If the buoyancy of an (unrestrained and unpowered) object (i.e., the cork) exceeds its weight, it tends to rise. An object whose weight exceeds its buoyancy tends to sink.

Commonly, the object in question is floating in equilibrium and the sum of the forces on the object is zero, therefore;

mg = pVg

and therefore;

m = pV

showing that the depth to which a floating object will sink (its "buoyancy") is independent of the variation of the gravitational acceleration at various locations on the surface of the Earth.

Ergo, the cork will rise at the same speed relative to the bucket as if the bucket was standing still.
Jan 25, 2009

What happened to the bottle of wine?!?!
Jan 25, 2009

nothing's wrong with the answer.
in free fall, the cork does stay in the bottom of the barrel. remember, no water pressure is exerted because relative to the three objects, there is no gravitational force active.
Jan 25, 2009

I'll take your word for it. Thanks for posting.
Jan 26, 2009

Thanks Plokolplok, you are of course correct. I am glad so many of you like this teaser. For those of you that think the answer is wrong I am glad that you sent the time you did thinking about it, that is always a good thing, but I think you should talk to a physics teacher about this one. It is important that you understand why you came up with the wrong answer so you can learn from your mistake. I don't want to say anymore for fear of being hit with more negative karma.
Jan 26, 2009

Just because the bucket is falling, it is not in freefall . The density of the water and the cork do not change. If we are dropping it off a very tall building, i assume we are on a planet, hence gravity is still in force. Since all forces are acting at the same time, both the bucket and the cork start falling at the same rate, but the bouyancy of the cork would still force it to the top
Jan 27, 2009

Not sure what planet you are on but that is not how things work on Earth.

I couldn't resist that one. It is worth any negative Karma you can throw at me. How can people know so little about physics and still think they know more than me, my college physic professors, the judges that allowed this teaser to go up and all the other members that got it right!
Jan 25, 2012

Nice question
Jan 25, 2012

@ttam - You can't use an equation that depends on statics in a dynamic situation.

Buoyant force depends on the pressure created by "stacking" water up in the bucket to create a difference in pressure that is highest at the bottom of the bucket and lowest at the top.

Once the bucket is released, it no longer pushes on the water (and the top of the water no longer pushes on the bit below it, etc) and the pressure gradient disappears.

No pressure difference, no water wanting to displace the cork and no buoyant force.
Jan 25, 2012

I have only one question, who has to sit in the bucket while it falls to verify the answer?
Jan 25, 2012

This was a great, well thought out teaser. The answer was fairly obvious to anyone with any science knowledge, and even though it has been over 20 years since I've taken a physics class, the answer jumped right out at me, guess there are some things you don't forget.
I must say though, I think i enjoyed the comments as much as the teaser.
Jan 25, 2012

I don't agree or disagree with the answer, because I wasn't there to observe the dropping of the bucket.
Jan 25, 2012

LOL I agree with Habs about loving the comments! Physics isn't at the top of my "I'm really good at this" list, but I appreciate the scientific process. Thanks for a good teaser that really got some interesting reactions!
Jan 25, 2012

Wow! The teaser was too hard for me, but I loved all the comments.
Jan 25, 2012

Wow! The teaser was too hard for me, but I loved all the comments.
Jan 25, 2012

I thought this was a trick question. You never know with teasers. I can't say much about the comments because I barely read them.
Jan 25, 2012

Seems like a job for the Mythbusters to put this question to bed with a demonstration.

Have to admit my "seat of the pants" guess was opposite of the answer and I'm not even going to try to puzzle it out properly. I will say however that the age-old assertion that something IS so because my mummy/daddy/teacher told me so and here are some "sciencey-sounding" words so you know I can copy and paste from a wiki site just like anyone else to say the opposite - doesn't actually hold a lot of water (Sorry).

Things like this either ARE or ARE NOT true based on rigorous principles and repeatable experiments and not because of anecdotal references to unnamed experts.

I guessed wrong but I admit to not actually knowing the answer. The back and forth comments above seem like just second-hand anecdotal not actual presentations of rigorous science.

Believing something because a beloved expert "said so" sounds like faith to me not science. I'm not saying that the answer is wrong because I don't know. I'm just calling out some of the back and forth ...and adding to it I suppose.

No bad karma intended to anyone.
Jan 25, 2015

And what am I supposed to do with all this knowledge, and now that I have it, why do I care?? Just curious. Loved all the comments and bickering FUN!
Jan 25, 2015

Way too much information for me. *staggers off for more coffee and an aspirin*
Jan 25, 2015

lol Babe, it is just one of the many things they taught us in school that was apparently vitally important that we know, yet as very little practical use in "real life" outside certain fields. I have never really need to know what the capital of Ethiopia is, what year Haiti gained independence from France or how to put an uncooked egg in a wine bottle, but we learned it all.
Jan 25, 2015

The cork would float to the top just as it would if it were not falling. This is a closed and independent 'system' and the fall would not have anything to do with what is happening to the cork.
Jan 26, 2015

I would use a practical demonstration to solve the problem, but I am afraid that I would get arrested trying to get to the top of a tall building carrying a bucket containing a "suspicious" substance. How could I explain the presence of the cork and who would believe that anyone would attempt such a thing anyway?? I guess I'll just sit and home and read everyone else's comments. Thanks for the "thinker" question.
Jan 26, 2015

When the bucket is at rest at the top of the building the weight of the water exerts a force on the bottom of the bucket. When the bucket begins to fall the water can no longer exert a force on the bottom of the bucket. Which means that the water does not have the ability to exert a force on the cork. Once the bucket comes to rest the water can once again exert force on the bucket.

During free fall, all the objects that are falling, are experiencing an apparent state of weightlessness. (They still have the same mass.) During this state of weightlessness there is no up or down. In essence, the water and the cork do not know which way is up. The water could push the cork horizontally and think it is pushing the cork vertically upward. (Not that water actually thinks.)

And to address the definition of free fall having to be in the absence of gravity::
1.) Where in the universe can you go without gravity?
2.) Without gravity there can be no falling.
3.) "Free Falling" is motion driven only by the force of gravity.

Some think that free falling only happens while in orbit. The difference between free falling near the Earth's surface and in orbit is that in orbit you are considered to be in a state of constant free fall. That is, you are falling toward the Earth, but you also have a tangential velocity that is high enough to move you away from the Earth at a rate that keeps you from getting closer to the Earth. If those velocities are balanced just right you will obtain a geostationary orbit.

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