ArtW

hi,i am back

YES!! I KNOW IT NOW.

I am wrong. I found the mistake. Wikipedia gives me the best answer. Thank you all. From Wikipedia: Compressible objects As a floating object rises or falls, the forces external to it change and, as all objects are compressible to some extent or another, so does the object's volume. Buoyancy depends on volume and so an object's buoyancy reduces if it is compressed and increases if it expands. If an object at equilibrium has a compressibility less than that of the surrounding fluid, the object's equilibrium is stable and it remains at rest. If, however, its compressibility is greater, its equilibrium is then unstable, and it rises and expands on the slightest upward perturbation, or falls and compresses on the slightest downward perturbation. Submarines rise and dive by filling large tanks with seawater. To dive, the tanks are opened to allow air to exhaust out the top of the tanks, while the water flows in from the bottom. Once the weight has been balanced so the overall density of the submarine is equal to the water around it, it has neutral buoyancy and will remain at that depth. The height of a balloon tends to be stable. As a balloon rises it tends to increase in volume with reducing atmospheric pressure, but the balloon's cargo does not expand. The average density of the balloon decreases less, therefore, than that of the surrounding air. The balloon's buoyancy decreases because the weight of the displaced air is reduced. A rising balloon tends to stop rising. Similarly, a sinking balloon tends to stop sinking.

When you push it down, and do not release, resisting force from air inside the balloon = pressure from water + elastic force from the skin of the balloon + pressure from your hand it is bigger than ...?

We are fighting! Hahahaha We are fighting at http://www.guokr.com/post/56593/ Come on! Poeple in the world, go to China to fight, but you have to understand Chinese at first. Please consider, if you pull the balloon up and release, what will happen?

Just at this moment, the resisting force from air inside baloon is bigger the pressure from water. The balloon wants to be bigger.

中国的果壳网也在直播讨论 Guokr in China discussing... http://www.guokr.com/post/56593/ NO DOUBT, SMALLER! but do you still remember that your hand is pulling the balloon! Your hand makes it smaller, even if indirectly.

This is a simple question, but it is saying a principle for "SYSTEM", BALANCE. This SYSTEM is originally at a balanced point, even though to get this point is difficult. Now, nothing is changed but the downwards pushing force, and then the force disappears. The system will return into BALANCE, which means the balloon will go to the original position.

The increasing speed of the force from the air inside the balloon, is faster, than the increasing speed of the force of compression from the water, as the depth becomes greater and greater. The increasing speed of the volume, is faster, than the reducing speed of the volume, as the depth becomes greater and greater. Air is cooler and denser near the floor, yes! BUT, water is cooler and denser near the bottom, too. You know, consider the case in sea. We can assume the container is deep enough, so a stable balanced point can exist.

Air in the balloon will resist the compression. The balloon suspends somewhere means all forces are balanced. The bigger the compression force from water is, the bigger the resisting force from air is. The bigger resisting force makes the balloon biger, and then the floating force becomes bigger, and the balloon goes up, renturns to the old place.

Usually, the two obvious stable positions, at the surface and on the bottom, exist. Here, this question means that the system, including the balloon and air inside, the stone and water, is at a STABLE and BALANCED point. This STABLE and BALANCED point may exist, even if it is difficult to tune. At the ideal point, all forces get to the balanced values, the balloon with the stone will suspends at this point. I give you an example alike. The story happened the last winter. My daughter’s balloon quietly suspended in the air inside my house, did not touch the ceiling and kept the distance of 1 meter, and stayed there. I remembered this question I just put here today, and she met this question in her test. I asked her how the balloon would go if I pulled it down, and she gave me the answer as the most people, YES, just SINK! To the floor! She believes these two questions are the same, I also believe. What happened later? I pulled it down by the string under the balloon, and held it for several feet, and for several seconds, then release it. IT RETURNED TO THE ORIGINAL POSITION! YES! This is a real story! OK, now I make a little bit change about the question. The new one is: ================================================================================ A balloon with a string, stably suspending in the air inside a house, gently pull it down, then release, it will: •A. suspend in the position where it is released; •B. go up and suspend at the original height; •C. sink to the floor; •D. go up and stop at the ceiling. The right answer is __ . ================================================================================ Now, I have told the REAL answer, tell me why please. Thank you a lot!

I mean, the balloon is below the surface, and beyond the bottom, and stops somewhere.

A small balloon with a stone, stably suspending in the water, gently push it down, then release, it will: A. suspend in the position where it is released; B. go up and suspend at the original height; C. sink to the bottom of the container; D. float to the surface of water. The right answer is __ . I think the correct answer is B, but the most Chinese people believe the answer is C. My reason is that the balloon will go back to the balanced point. I put the question on many forums in China, and I am not persuaded. This question is famous because it often appears in the important exams, for example, the BIG exam for university entrance. Unfortunately, answer C is often chosen as the correct answer even though it is wrong. It is difficult to explain, especially hard to explain for the people who know something about physics. I want to know how the people in the other countries think. This is a question about System Science, so I hope true scientists in this area give us the right answer, and show it in a test. This is a hot question now in China, so take a part in it and have fun. Thank you. If you understand Chinese, you could go to: http://www.tianya.cn...1/1893401.shtml http://www.tianya.cn...09.shtml#Bottom http://bbs.cntv.cn/t...810381-1-1.html