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Author Topic:   Buoyancy Quiz

ShrimpBurrito posted 03-01-2003 12:59 AM ET (US)
Here are buoyancy questions to think about. I'll post correct answers Sunday or Monday.
Question #1: There is a boat in a pool, and in the boat is a boy and a rock. The boy picks up the rock and tosses it into the pool. The rock sinks to the bottom. No water leaves the pool from the splash made by the rock.
Does the pool's water level a)rise; b) lower; or c) stay the same?
Question #2: A driver is sitting in a car that's not moving. Also in the car is a helium-filled balloon, which is resting up against the car's ceiling somewhere near its middle. The driver hits the gas and the car accelerates forward, and the driver is thrust back into his seat.
What happens to the balloon? a) it floats toward the back of the car; b) it floats toward the front of the car; c) it stays put.

ShrimpBurrito posted 03-01-2003 12:59 AM ET (US)
Here are buoyancy questions to think about. I'll post correct answers Sunday or Monday.
Question #1: There is a boat in a pool, and in the boat is a boy and a rock. The boy picks up the rock and tosses it into the pool. The rock sinks to the bottom. No water leaves the pool from the splash made by the rock.
Does the pool's water level a)rise; b) lower; or c) stay the same?
Question #2: A driver is sitting in a car that's not moving. Also in the car is a helium-filled balloon, which is resting up against the car's ceiling somewhere near its middle. The driver hits the gas and the car accelerates forward, and the driver is thrust back into his seat.
What happens to the balloon? a) it floats toward the back of the car; b) it floats toward the front of the car; c) it stays put.
cjd posted 03-01-2003 09:15 AM ET (US)
c)
forward
I understand the first (i think)
does the second have anything to to with the relationship between inertia and weight?.
Is inertia the opposite of momentum?
I was an english major, so if both answers are wrong, I would like an opportunity to submit a make up paper.
love this stuff
witsendfl posted 03-01-2003 10:12 AM ET (US)
Hope I remember my Physics
1)C The water will remain the same because the weight of the rock is helping the displacment of the water under the hull. When the rock is thrown in the pool the hull displaces less water. Now I am confused ! Lets say that rock was in a Sphere that had a slight amount of negative bouancy. The sphere sinks but it occuplies more space for water displacment. Then what? Does it only matter on the size of the mass?
I hope this isn't the question to win a million bucks. I am out of life lines !!!
2) Surface tension? A Object will tend to stay at rest unless acted upon by a unbalanced force. A object that is in motion will stay in motion unless acted upon by a unbalanced force. MOVE TOWARDS THE BACK OF THE CAR!
Do I win a MILLION Dollars... I hope so
I hope your explanation is good !!!
Now I am at my "WITSEND"
witsendfl JimK
WhalerGoFar posted 03-01-2003 10:14 AM ET (US)
Let me see now, first things first.
The answers to the pop quiz are I believe that Number 1…”A” and Number 2… Stay put .
I could be wrong but what the hell, if I get held back a year for failing this quiz then I will be able to date the younger girls and stay out of the draft.
prj posted 03-01-2003 07:44 PM ET (US)
Thinking about ShrimpBurrito's questions got me through some housecleaning chores.
Answers:
Q 1. b) lower
Q 2. perhaps indeterminant
Question 1 was particularly interesting as it has generated some quick and seemingly obvious answers.
Known: "The rock sinks to the bottom" indicating that it is denser than water.
When the rock is in the boat, it is displacing an amount of water equal to it's WEIGHT. When the rock is tossed into the pool, the rock is displacing an amount of water equal to its VOLUME.
As the rock's density is known to be greater than water's, the rock displaces more water when doing so via weight than volume, therefore when tossed into the water, the boat raises, water level lowers.
Question 2: As the helium balloon is spectacularly light, lighter than air even, one would need to know the coefficient of friction between balloon and headliner to determine whether the miniscule force of inertia would overcome this bond.
doobee posted 03-01-2003 08:18 PM ET (US)
Sounds like you have the question pretty well answered so I'll just throw in this interesting fact. If the foam were allowed to expand freely, it would have enough volume to fill the hull 2 and a half times. The hulls buoyancy could be improved by adding less foam. Less foam means less weight, which means more buoyancy, but then it wouldn't have the strength of the boat we all know and love.
Now its time to pull the plug . . .
whale on!
timbaho posted 03-01-2003 08:45 PM ET (US)
ShrimpBurrito,
1.C-stays the same level
2.C-stays the same place
Kev
newt posted 03-01-2003 10:42 PM ET (US)
I will try the quiz.
Question 1: PRJ is right on...the water level will go down. He explained it nicely.
Question 2: Not sure, but would guess that the balloon moves forward. When the car accelerates everything with mass that is not constrained will move back - including the air. In effect, the air in the back becomes even more dense and will Squeeze the least dense object (in this case the balloon) forward.
ShrimpBurrito posted 03-01-2003 11:06 PM ET (US)
Some of you are reading too much into Question #2. This question has to do specifically with buoyancy, so keep thinking about that.
The coefficient of friction between the balloon and the headliner is irrelevant. If it makes it easier, assume there is only enough helium in the balloon so that it is suspended an inch or two from the ceiling.
CJD --- The momentum of an object = (its mass) X (its velocity)
Thus, the momentum of an object at rest is zero.
Inertia, on the other hand, is the tendency of an object to resist any attempt to put it in motion, or if it is moving, to change its speed or direction. Inertia is a passive property, and thus, doesn't do anything except oppose forces. It can be calculated, but it's a little more complicated and is something I have not touched for years.
But you don't need to know any of that to correctly answer the question. It's a tough one.....

Jerry Townsend posted 03-02-2003 12:03 AM ET (US)
In responce to ShrimpBurrito's questions:
The answer to question 1 is lower as properly explained by PRJ. The answer to question 2 is really loaded - that is, in the first millisecond or so, the balloon will move toward the rear of the car simply because of inertia effects, however with that relative motion, forces will act on the balloon to tend to move the balloon forward. That is, air is compressible and therefore, the air pressure acting on the rear of the balloon will be higher than the pressure acting on the front of the balloon which will tend to move the balloon forward. Further, the relative motion frictional force between the balloon and the roof of the car will tend to move the balloon forward. But again, the car must be moving forward relative to the balloon for these forces to be realized. ---- Jerry/Idaho
jimh posted 03-02-2003 10:50 AM ET (US)
This thread was originally part of another. The mixing to the two questions/topics made both of them harder to follow, so I separated these responses from the other thread.
David Jenkins posted 03-02-2003 10:58 AM ET (US)
In regards to question number 2, the ballon most definitely moves forward. I cannot explain why (doesn't make sense to me), but it is an experiment that my kids have done for several years now. We put the Harris Teeter (local grocery store) ballons in the back of the station wagon (with all windows up), hit the gas, and the ballons move forward. The harder I punch the gas petal, the faster the ballons move to the front of the car.
kingfish posted 03-02-2003 01:34 PM ET (US)
1. I'm with prj for all the reasons he articulated.
2. I don't have a clue about the balloon. I believe David Jenkins, but I know even less about why the balloon would act that way than he does, and it makes my head hurt to think about it.
kf
Tom W Clark posted 03-02-2003 01:50 PM ET (US)
The explanation for the balloon's behavior is simple. Both air and helium weigh something (so does the balloon itself). The combination of the helium and the balloon itself still weight less than the same volume of air, right?
So when the car accelerates, the inertia of the air and the balloon in the car cause it all to want to squish to the back of the car. Because the air is denser than the helium filled balloon, the balloon will "float" to the front of the car being squeezed by the denser air that is trying to make its way to the back of the car.
Remember this is a question about buoyancy. The helium filled balloon is behaving just as it does in a static state as it tries to float up through air and hits the roof of the car. During acceleration, the direction it floats is more horizontal.
kingfish posted 03-02-2003 02:09 PM ET (US)
OK, so I laid down on my side on the floor and then tried to think about it, and that didn't make my head hurt quite so much, so while I was down there I took a little nap...
kf
JimCha posted 03-02-2003 08:08 PM ET (US)
1. c) based on Archimedes principle.
2. a) based on inertia of balloon, will drift to back.
love these physics questions even though off topic. JimC
RWM posted 03-02-2003 10:33 PM ET (US)
If my pool wasn't frozen over and it wasn't so late I'd for sure have the answer to 1. And if it wasn't Sunday night and all the stores closed where I can get a helium ballon I'd for sure have the answer to 2.
I'm going to have to rest on all those hours I spent getting those engineering degrees and say...
The water will drop in height.
The ballon will move backward (if you left the window open).
cjd posted 03-03-2003 08:58 AM ET (US)
i can live with the fact that I am wrong on both counts. i cannot live another day without sleep.
Please shrimproll, lay it on us ;)
ok one more try. maybe the ballon will move forward, as the acceleration will open the angle of the roof and the balloon will "crawl" up to the front of that car. is this anything like the speed of light from a train headlamp (train in motion) vs the speed of light on a train headlamp of a parked train?
ShrimpBurrito posted 03-03-2003 10:30 AM ET (US)
Ok, ok. The questions have been correctly answered above with correct explanations.
The answer to Question #1 is (b) -- the water level will lower. PRJ correctly answered first.
Reason: As PRJ explained, when the rock is in the boat, it displaces its total weight. If it weighs 10 lbs, for example, then it's making the boat 10 lbs heavier.
Because the rock is 3-5 times as dense as water, it sinks in water. When the rock is sitting at the bottom of the pool, it displaces its volume, where it likely displaces only 2-3 lbs of water. Thus, the boat rises and the water level lowers.
The answer to Question #2 is again (b) -- the balloon "floats" towards the front of the car. Newt correctly answered first.
Reason: As Newt and Tom Clark explained, "when the car accelerates, the inertia of the air and the balloon in the car cause it all to want to squish to the back of the car. Because the air is denser than the helium filled balloon, the balloon will "float" to the front of the car being squeezed by the denser air that is trying to make its way to the back of the car."
Along the same lines, if you drove off a bridge with the same helium balloon, the balloon would sink to the floor....until you reached the inevitable constant velocity.
ShrimpBurrito posted 03-03-2003 10:38 AM ET (US)
Don't feel bad if you didn't get these questions right. Two famous physicists apparently answered Question #1 incorrectly.
David Jenkins posted 03-03-2003 01:57 PM ET (US)
Fantastic! Many thanks for Newt and Tom Clark for explaining the balloon phenomenum. For those of you that have not witnessed it, I urge you to try the experiment (especially if you have children). If you punch the accelerator hard enough, the balloon does not "crawl" to the front of the car; it flies up there!

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Author	Topic: Buoyancy Quiz
ShrimpBurrito	posted 03-01-2003 12:59 AM ET (US) Here are buoyancy questions to think about. I'll post correct answers Sunday or Monday. Question #1: There is a boat in a pool, and in the boat is a boy and a rock. The boy picks up the rock and tosses it into the pool. The rock sinks to the bottom. No water leaves the pool from the splash made by the rock. Does the pool's water level a)rise; b) lower; or c) stay the same? Question #2: A driver is sitting in a car that's not moving. Also in the car is a helium-filled balloon, which is resting up against the car's ceiling somewhere near its middle. The driver hits the gas and the car accelerates forward, and the driver is thrust back into his seat. What happens to the balloon? a) it floats toward the back of the car; b) it floats toward the front of the car; c) it stays put.
ShrimpBurrito	posted 03-01-2003 12:59 AM ET (US) Here are buoyancy questions to think about. I'll post correct answers Sunday or Monday. Question #1: There is a boat in a pool, and in the boat is a boy and a rock. The boy picks up the rock and tosses it into the pool. The rock sinks to the bottom. No water leaves the pool from the splash made by the rock. Does the pool's water level a)rise; b) lower; or c) stay the same? Question #2: A driver is sitting in a car that's not moving. Also in the car is a helium-filled balloon, which is resting up against the car's ceiling somewhere near its middle. The driver hits the gas and the car accelerates forward, and the driver is thrust back into his seat. What happens to the balloon? a) it floats toward the back of the car; b) it floats toward the front of the car; c) it stays put.
cjd	posted 03-01-2003 09:15 AM ET (US) c) forward I understand the first (i think) does the second have anything to to with the relationship between inertia and weight?. Is inertia the opposite of momentum? I was an english major, so if both answers are wrong, I would like an opportunity to submit a make up paper. love this stuff
witsendfl	posted 03-01-2003 10:12 AM ET (US) Hope I remember my Physics 1)C The water will remain the same because the weight of the rock is helping the displacment of the water under the hull. When the rock is thrown in the pool the hull displaces less water. Now I am confused ! Lets say that rock was in a Sphere that had a slight amount of negative bouancy. The sphere sinks but it occuplies more space for water displacment. Then what? Does it only matter on the size of the mass? I hope this isn't the question to win a million bucks. I am out of life lines !!! 2) Surface tension? A Object will tend to stay at rest unless acted upon by a unbalanced force. A object that is in motion will stay in motion unless acted upon by a unbalanced force. MOVE TOWARDS THE BACK OF THE CAR! Do I win a MILLION Dollars... I hope so I hope your explanation is good !!! Now I am at my "WITSEND" witsendfl JimK
WhalerGoFar	posted 03-01-2003 10:14 AM ET (US) Let me see now, first things first. The answers to the pop quiz are I believe that Number 1…”A” and Number 2… Stay put . I could be wrong but what the hell, if I get held back a year for failing this quiz then I will be able to date the younger girls and stay out of the draft.
prj	posted 03-01-2003 07:44 PM ET (US) Thinking about ShrimpBurrito's questions got me through some housecleaning chores. Answers: Q 1. b) lower Q 2. perhaps indeterminant Question 1 was particularly interesting as it has generated some quick and seemingly obvious answers. Known: "The rock sinks to the bottom" indicating that it is denser than water. When the rock is in the boat, it is displacing an amount of water equal to it's WEIGHT. When the rock is tossed into the pool, the rock is displacing an amount of water equal to its VOLUME. As the rock's density is known to be greater than water's, the rock displaces more water when doing so via weight than volume, therefore when tossed into the water, the boat raises, water level lowers. Question 2: As the helium balloon is spectacularly light, lighter than air even, one would need to know the coefficient of friction between balloon and headliner to determine whether the miniscule force of inertia would overcome this bond.
doobee	posted 03-01-2003 08:18 PM ET (US) Sounds like you have the question pretty well answered so I'll just throw in this interesting fact. If the foam were allowed to expand freely, it would have enough volume to fill the hull 2 and a half times. The hulls buoyancy could be improved by adding less foam. Less foam means less weight, which means more buoyancy, but then it wouldn't have the strength of the boat we all know and love. Now its time to pull the plug . . . whale on!
timbaho	posted 03-01-2003 08:45 PM ET (US) ShrimpBurrito, 1.C-stays the same level 2.C-stays the same place Kev
newt	posted 03-01-2003 10:42 PM ET (US) I will try the quiz. Question 1: PRJ is right on...the water level will go down. He explained it nicely. Question 2: Not sure, but would guess that the balloon moves forward. When the car accelerates everything with mass that is not constrained will move back - including the air. In effect, the air in the back becomes even more dense and will Squeeze the least dense object (in this case the balloon) forward.
ShrimpBurrito	posted 03-01-2003 11:06 PM ET (US) Some of you are reading too much into Question #2. This question has to do specifically with buoyancy, so keep thinking about that. The coefficient of friction between the balloon and the headliner is irrelevant. If it makes it easier, assume there is only enough helium in the balloon so that it is suspended an inch or two from the ceiling. CJD --- The momentum of an object = (its mass) X (its velocity) Thus, the momentum of an object at rest is zero. Inertia, on the other hand, is the tendency of an object to resist any attempt to put it in motion, or if it is moving, to change its speed or direction. Inertia is a passive property, and thus, doesn't do anything except oppose forces. It can be calculated, but it's a little more complicated and is something I have not touched for years. But you don't need to know any of that to correctly answer the question. It's a tough one.....
Jerry Townsend	posted 03-02-2003 12:03 AM ET (US) In responce to ShrimpBurrito's questions: The answer to question 1 is lower as properly explained by PRJ. The answer to question 2 is really loaded - that is, in the first millisecond or so, the balloon will move toward the rear of the car simply because of inertia effects, however with that relative motion, forces will act on the balloon to tend to move the balloon forward. That is, air is compressible and therefore, the air pressure acting on the rear of the balloon will be higher than the pressure acting on the front of the balloon which will tend to move the balloon forward. Further, the relative motion frictional force between the balloon and the roof of the car will tend to move the balloon forward. But again, the car must be moving forward relative to the balloon for these forces to be realized. ---- Jerry/Idaho
jimh	posted 03-02-2003 10:50 AM ET (US) This thread was originally part of another. The mixing to the two questions/topics made both of them harder to follow, so I separated these responses from the other thread.
David Jenkins	posted 03-02-2003 10:58 AM ET (US) In regards to question number 2, the ballon most definitely moves forward. I cannot explain why (doesn't make sense to me), but it is an experiment that my kids have done for several years now. We put the Harris Teeter (local grocery store) ballons in the back of the station wagon (with all windows up), hit the gas, and the ballons move forward. The harder I punch the gas petal, the faster the ballons move to the front of the car.
kingfish	posted 03-02-2003 01:34 PM ET (US) 1. I'm with prj for all the reasons he articulated. 2. I don't have a clue about the balloon. I believe David Jenkins, but I know even less about why the balloon would act that way than he does, and it makes my head hurt to think about it. kf
Tom W Clark	posted 03-02-2003 01:50 PM ET (US) The explanation for the balloon's behavior is simple. Both air and helium weigh something (so does the balloon itself). The combination of the helium and the balloon itself still weight less than the same volume of air, right? So when the car accelerates, the inertia of the air and the balloon in the car cause it all to want to squish to the back of the car. Because the air is denser than the helium filled balloon, the balloon will "float" to the front of the car being squeezed by the denser air that is trying to make its way to the back of the car. Remember this is a question about buoyancy. The helium filled balloon is behaving just as it does in a static state as it tries to float up through air and hits the roof of the car. During acceleration, the direction it floats is more horizontal.
kingfish	posted 03-02-2003 02:09 PM ET (US) OK, so I laid down on my side on the floor and then tried to think about it, and that didn't make my head hurt quite so much, so while I was down there I took a little nap... kf
JimCha	posted 03-02-2003 08:08 PM ET (US) 1. c) based on Archimedes principle. 2. a) based on inertia of balloon, will drift to back. love these physics questions even though off topic. JimC
RWM	posted 03-02-2003 10:33 PM ET (US) If my pool wasn't frozen over and it wasn't so late I'd for sure have the answer to 1. And if it wasn't Sunday night and all the stores closed where I can get a helium ballon I'd for sure have the answer to 2. I'm going to have to rest on all those hours I spent getting those engineering degrees and say... The water will drop in height. The ballon will move backward (if you left the window open).
cjd	posted 03-03-2003 08:58 AM ET (US) i can live with the fact that I am wrong on both counts. i cannot live another day without sleep. Please shrimproll, lay it on us ;) ok one more try. maybe the ballon will move forward, as the acceleration will open the angle of the roof and the balloon will "crawl" up to the front of that car. is this anything like the speed of light from a train headlamp (train in motion) vs the speed of light on a train headlamp of a parked train?
ShrimpBurrito	posted 03-03-2003 10:30 AM ET (US) Ok, ok. The questions have been correctly answered above with correct explanations. The answer to Question #1 is (b) -- the water level will lower. PRJ correctly answered first. Reason: As PRJ explained, when the rock is in the boat, it displaces its total weight. If it weighs 10 lbs, for example, then it's making the boat 10 lbs heavier. Because the rock is 3-5 times as dense as water, it sinks in water. When the rock is sitting at the bottom of the pool, it displaces its volume, where it likely displaces only 2-3 lbs of water. Thus, the boat rises and the water level lowers. The answer to Question #2 is again (b) -- the balloon "floats" towards the front of the car. Newt correctly answered first. Reason: As Newt and Tom Clark explained, "when the car accelerates, the inertia of the air and the balloon in the car cause it all to want to squish to the back of the car. Because the air is denser than the helium filled balloon, the balloon will "float" to the front of the car being squeezed by the denser air that is trying to make its way to the back of the car." Along the same lines, if you drove off a bridge with the same helium balloon, the balloon would sink to the floor....until you reached the inevitable constant velocity.
ShrimpBurrito	posted 03-03-2003 10:38 AM ET (US) Don't feel bad if you didn't get these questions right. Two famous physicists apparently answered Question #1 incorrectly.
David Jenkins	posted 03-03-2003 01:57 PM ET (US) Fantastic! Many thanks for Newt and Tom Clark for explaining the balloon phenomenum. For those of you that have not witnessed it, I urge you to try the experiment (especially if you have children). If you punch the accelerator hard enough, the balloon does not "crawl" to the front of the car; it flies up there!