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5 Discussions on Fluid statics: Weight and Upthrust

Physics 213 class discussions

Please answer each question since our teacher did a poor job of not only explaining each situation(1-5), but did not give pictures on the chalk board of forces, etc or to see what is visually going on. Please add anything to help me learn please!

Choose the correct answer or state not enough information if the answer could depend on circumstances not specified. Justify your answer in each case. If there isn't enough information, explain that as well. In all cases the objects are touching the walls of the beakers, and they touch the bottom only if they sink.

1) Two objects of mass 250g each are placed in two identical beakers with identical amounts of water. The objects are different: one floats and the other sinks. Which beaker shows a larger rise in level, the one with the floater, the one with the sinker, or both the same?
2) Now the floater is placed in yet another identical beaker, filled to the same level, but with oil(oil floats in water). It still floats. Does the level rise more, less or the same as when the same floater was placed in water?
3) Now the floater is placed in yet another identical beaker, filled to the same level, but with alcohol(alcohol floats in water). Now it sinks. Does the level rise more, less or the same as when the same floater was placed in oil?
4) Now the sinker is attached to a string to a half inflated (with air) balloon. The whole assembly now has a mass of 251g. The system is placed in the same beaker it was before(with the same water) and it floats with the balloon almost entirely under water. Does the level rise more, less or the same as when the same sinker was alone in the beaker ?
5) Now the balloon is inflated with air with the sinker still attached, and the system is again placed in the same beaker (the beaker is big enough for the balloon not to touch he lips.) The system floats with a small fraction of the balloon under water. Does the level rise (at least slightly) more, (at least slightly) less or (exactly) the same as it did with the half inflated balloon?

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Physics 213 class discussions

Please answer each question since our teacher did a poor job of not only explaining each situation(1-5), but did not give pictures on the chalk board of forces, etc or to see what is visually going on. Please add anything to help me learn please!

Choose the correct answer or state not enough information if the answer could depend on circumstances not specified. Justify your answer in each case. If there isn't enough information, explain that as well. In all cases the objects are touching the walls of the beakers, and they touch the bottom only if they sink.

A Reading.
Buoyancy
In physics, buoyancy is an upward force on an object immersed in a fluid (i.e. a liquid or a gas), enabling it to float or at least to appear to become lighter. If the buoyancy exceeds the weight, then the object floats; if the weight exceeds the buoyancy, the object sinks. If the buoyancy equals the weight, the body has neutral buoyancy and may remain at its level. If its compressibility is less than that of the surrounding fluid, it is in stable equilibrium and will, indeed, remain at rest, but if its compressibility is greater, its equilibrium is unstable, and it will rise and expand on the slightest upward perturbation, but fall and compress on the slightest downward perturbation. It was the ancient Greek, Archimedes of Syracuse, who first discovered the law of buoyancy, sometimes called Archimedes' principle:

The buoyant force is equal to the weight of the displaced fluid.

Typically, the weight of the displaced fluid is directly proportional to the volume of their displaced fluid ...

Solution Summary

5 questions about flotation of a body in fluid are discussed and explained.

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