Know how weight and volume calculate buoyancy

1. (1) (2)
if the object is completely submerged, f floating = P liquid GV matter
(3) if the floating and sinking conditions are not known, assume that it is completely submerged, calculate the buoyancy with F floating = P liquid GV matter, and then calculate the gravity of the object G matter = mg. Third, compare the size relationship between F floating and G object. If f floating is larger than g object, f floating = g object; If f floats less than g, then f floats = P liquid GV. If equal to both OK!

2. If it is determined to be an iron ball, the normal steps are:
first use f = ρ The water GV row calculates the buoyancy, the iron will sink, V row = V ball
then, f pull = G-F float, calculate the indication of the dynamometer.

3. Know the weight and volume, calculate the density of the object, and compare it with the density of water. If it is smaller than water, it will float, and if it is larger, it will sink

4. 1. Buoyancy of an object in liquid = weight of liquid displaced by the object
2. Liquid weight = liquid density * liquid volume (i.e. the volume of the object in the liquid)
3. It can be concluded that: buoyancy = density * Volume

that is, volume = buoyancy / density

so it is impossible to calculate volume without knowing density.

5. Buoyancy is the buoyancy under what conditions, the water surface is still or completely submerged in the water? Is liquid density known
when the water surface is still, buoyancy is equal to gravity, which means that only gravity can't be used to calculate
total immersion, and the ratio of buoyancy to gravity is equal to the ratio of liquid density to object density. When the liquid density is known, the object density can be obtained, the mass can be obtained with gravity, and the volume can be obtained with mass ratio density

6. Weight is gravity. What does gravity look at specifically? About buoyancy, I hope I can help you to do a good job in physics buoyancy. Whether you are learning buoyancy for the first time or reviewing the entrance examination, you will feel that the more you learn this part of the content, the more problems you have, the more difficult it is to do. Often when the entrance examination is approaching, many good students take some "buoyancy" exercises from various channels to ask me to solve them. I will explain this phenomenon from two aspects
first of all, the buoyancy problem is introced after the study of force, density and pressure. As we all know, these three contents themselves play an important role in the knowledge points of senior high school entrance examination. Therefore, it is not strange that it is difficult to learn buoyancy. On the other hand, because students do not know the requirements of high school entrance examination, so they feel difficult to learn and try their best to solve problems. In particular, some teachers also think that the more difficult the questions are, the more effective they will be, and the greater their grasp of the senior high school entrance examination. They do not hesitate to drill a large number of super outline questions and competition questions for students, which makes both teachers and students overburdened. This kind of learning method with high cost (refers to unnecessary energy and time) is not advisable
I think it is better to learn "buoyancy" from the following aspects
first, the key to learn buoyancy well is to understand the knowledge of force, balance of two forces, density and pressure thoroughly
because buoyancy is actually an application of the above knowledge. Or on the basis of mastering the above knowledge, to analyze and solve some simple problems in life and proction. It is necessary to have more difficulty and types of questions
example: as shown in the figure, the same object is immersed in two different liquids, and the buoyancy is F1 and F2 respectively; The pressure on the bottom of the object is F1 'and F2', the pressure is P1 and P2, and the liquid density is 1 ρ 1 and ρ 2, then: F1F2, f1'f2 ', p1p2, ρ one ρ It is not necessary and very difficult to calculate the buoyancy immediately. However, if we calmly see that the object is in the state of two force equilibrium in a and B containers (the special case of floating and suspension), we can immediately know that F1 = g object, F2 = G2 object, so F1 = F2; Secondly, we think that buoyancy is the force that the liquid holds the object upward, which is essentially buoyancy, that is, F1 and F1 ', F2 and F2' are the same force, of course F1 '= F2'. According to the pressure relationship, P = FS, because the bottom area of the same object is the same, the pressure is the same, so P1 = P2. Finally, according to F1 = F2 ρ 1gV1＝ ρ 2 g V 2 because v 1 ＜ V 2 ρ 1＞ ρ 2
Second, pay attention to the imbalance of two forces and the balance of three forces in buoyancy problems
generally speaking, there are few problems about the balance of three forces in junior high school physics. At present, the problem of spring scale formally appears in the syllabus of textbook examination. For this kind of problem, we must learn simple force analysis, but we can't expand it arbitrarily
example: the volume is 1 × When a 10-3 m3 object is immersed in water, the buoyancy of the object is () n. if the resultant force of buoyancy and gravity of the object is 20n, the gravity of the object is () n. For the first space, it is easy to use Archimedes principle f = ρ The buoyancy of liquid GV row is 9.8N, but in the second space, the gravity g is just opposite to the buoyancy direction. Only when G > F floats, can G-F float = 20n, so g = f float + 20n = 29.8n. For example, when a submarine dives into the Yangtze River from the East China Sea, does its buoyancy change? Some students think that the submarine is in a suspended state, so the buoyancy remains unchanged. This is obviously wrong. It should be noted that the V row remains unchanged, and the liquid density becomes smaller, so the buoyancy becomes smaller
secondly, pay attention to put the object into the container with or full of a certain liquid, and be very careful about the buoyancy problem. Because Shengyou must consider the two situations of fullness and under fullness
for example: if something weighs 10 N, put it gently into a container with water, overflow 2 n of water, and find out the buoyancy of the object
if we easily think that f floating = g drainage, we will get f floating = 2 n immediately, but this is only the answer when it is full of water. If we carefully analyze Shengyou, we should consider that when it is not full, the water level should rise after the object is placed, and the buoyancy should be greater than 2 n if it is full first and then overflows. If the object floats or floats at last, the buoyancy is the largest, f = g = 10 N, so the answer is 10 N ≥ f ≥ 2 n. 4、 The problems in real life and proction and the floating body problems are mainly clear here: 1. The application of ships (such as the displacement of ships and the waterline of ships); 2. Submarines (knowing that submarines float and dive by changing their own gravity); 3. Balloons and airships (knowing that Archimedes principle is also applicable to air buoyancy, In a word, don't do questions blindly, especially don't do a lot of difficult problems, because buoyancy is not a higher knowledge point in the high school entrance examination. Analysis of common problems in buoyancy part: 1. Hang the object on the hook of the spring dynamometer, record the indication of the spring Dynamometer when the object is in the air and immersed in the water,
for G and f respectively, apply the knowledge of buoyancy to calculate the volume and density of the object immersed in the water
F floating = g matter-f... ①
F floating= ρ Water V row g... ②
from these two formulas, V row = f float/ ρ Water g
= (G-F)/ ρ When water g
is immersed, V row = V object
so the volume of object v object = (g object-f)/ ρ Water g
and density ρ Matter = m matter / V matter = g matter / GV matter ρ Object = g object ρ Water / (g matter-f)
2. Apply buoyancy knowledge to calculate the density of floating objects
F floating= ρ Water V discharges g... ①
F floats = g matter= ρ Matter V matter g... ②
buoyancy is equal to gravity, ① = ②< br /> ρ Substance V substance G= ρ Water V discharge g
ρ Things V things= ρ Water V row
if 3 / 5 volume of floating object is immersed in water, i.e. V row = (3 / 5) V object,
then the density of object is 3 / 5 of the density of water, i.e ρ Material = (3 / 5) ρ Water
3. Calculate the density of liquid according to the knowledge of buoyancy
(1) hang the object on the hook of the spring dynamometer
record the indication of the spring dynamometer in air, water and another liquid. They are g, F1 and f respectively
f-float = g-f1... ①
f-float= ρ Water V row g... ②
① = ② V row = (g-f1)/ ρ Water g
when the same object is immersed in water and another liquid, the volume of the separated liquid is equal, but the buoyancy is not equal
buoyancy in another liquid F-1 = G-F... ③
F-1= ρ Liquid V discharge g... ④
③ = ④ ρ Liquid = (G-F) / V discharge g
= (G-F) ρ Water / (g-f1)
(2) the same object floats on the water surface and another surface successively. The buoyancy is equal
when an object floats on the water surface, 3 / 5 of the volume of the object is immersed in the water, and V row 1 = (3 / 5) V object
when an object floats on another liquid surface, 2 / 3 of the volume of the object is immersed in the liquid, and V row = (2 / 3) V object
F floating= ρ Water V row 1g... ①
F float= ρ Liquid V discharges g... ②
F floats = g substance= ρ Object v object g... ③
the density of object can be obtained from ① and ③
the density of another liquid can be calculated from ② and ③ (or ① and ②)
4. According to the floating and sinking conditions of objects, the density of objects or liquid can be compared
(1) the same object is immersed in three different liquids, which are floating, floating and sinking
when floating, ρ Object ＜ ρ When liquid 1
is suspended, ρ Things= ρ When liquid 2
sinks, ρ It's not the same thing ρ Liquid 3
was obtained ρ Liquid 1 ＞ ρ Liquid 2 ＞ ρ Liquid 3
(2) three different objects immersed in the same liquid are in floating, suspending and sinking states respectively
according to the above method, the density of three objects can be compared.

7. F= ρ VG
know buoyancy F, liquid density ρ， If G is known (9.8 or 10 as required)
the volume V can be calculated

8. There is a case that can be calculated, that is, the object is completely immersed in the liquid, density = gravity / volume (volume is calculated according to buoyancy and liquid density, namely volume = buoyancy / density)
another situation that can not be calculated is that part of the object is immersed in the liquid. At this time, we also know how much the part immersed in the liquid accounts for the whole object or how much the part exposed in the liquid accounts for the object. Otherwise, we can not calculate
please refer to, thank you!

9. Mass = density × Volume
gravity = mass × G
indication of spring dynamometer = gravity buoyancy

10.

Buoyancy = the weight of the object in the liquid = the weight of the object in the air - the weight of the object in the liquid = the volume of the liquid the object displaces × Liquid density × According to the law of buoyancy, we can get the relationship between buoyancy and density:

when the object floats up, the buoyancy is greater than the gravity of the liquid; When an object floats or levitates, buoyancy is equal to the gravity of the object expelling the fluid; When an object sinks, the buoyancy is less than the gravity of the object to displace the fluid

Then, when the fluid density is greater than the object density, the object floats up; When the fluid density is equal to the object density, the object is suspended; When the fluid density is less than the object density, the object sinks

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extended data:

floating and sinking of objects:

I. summarize the relationship between floating and sinking of objects and density: F = g = M × G = P flow × Row V × G when the object is submerged, its V object = V row, so p flow × V object × G = P flow × Row V × g. When V is equal, we can get ρ Objects= ρ Fluid

When the weight of the object is less than the buoyancy of the fluid, and the object is not on the surface of the liquid, it is called floating up; W object, ρ Things & lt; ρ Liquid

When the weight of the object is less than the buoyancy of the fluid and the object is floating on the surface of the liquid, then f floating = w object, ρ Things & lt; ρ Liquid

When the weight of the object equals to the buoyancy of the fluid, the state is called levitation, where f floating = w object, ρ Things= ρ Liquid

When the weight of the object is greater than the buoyancy of the fluid, the state is called sinking, and f floats & lt; W object, ρ Things & gt; ρ Liquid

When the object is in close contact with the bottom of the fluid, the bottom of the object is free from any buoyancy