"End of Chapter Summary and Improvement" Buoyancy PPT

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"End of Chapter Summary and Improvement" Buoyancy PPT

Part One: Key Breakthroughs

Calculation of buoyancy

For the calculation of buoyancy, there are usually three basic methods:

1. Calculate using Archimedes' principle. If the density of the liquid ρ liquid and the volume V of the liquid displaced by the object are known, it can be calculated according to the Archimedean principle formula F float = G displacement = ρ liquid g V displacement (when the object is immersed in a gas, F float =ρqi gV row), this method is also called principle method or formula method.

2. Calculation using floating or suspended conditions. If an object floats on the liquid surface or is suspended in a liquid (or gas), it can be calculated based on the fact that the buoyancy force on the object is equal to the gravity (that is, F float = G object). This method is also called the equilibrium method.

3. Calculate using the difference between two measurements. If a spring balance is used to weigh the gravity of the same object in the air and in the liquid, and the readings are G and G' respectively, then the buoyancy force on the object in the liquid is F float = G-G'. This method is often called For the weighing method.

Typical example 1 Two identical pieces of plasticine, both with a volume of 10 cm3 and a mass of 12 g, are kneaded into a solid shape and the other into a bowl shape. They are placed in two containers A and B filled with water, as in As shown in the figure. The buoyancy force of water on them is F A =______N, F B =__________N (g is taken as 10 N/kg).

[Analysis] In picture A, the solid plasticine is sinking to the bottom, then the volume of water displaced by the plasticine V = V mud = 10 cm3 = 1×10-5 m3, and the buoyancy force F A on the plasticine = ρ water V Discharge g=1.0×103 kg/m3×1×10-5 m3×10 N/kg=0.1 N; then in the question picture B, the plasticine is in a floating state and is acted upon by a balancing force, that is, the plasticine is subjected to Buoyancy F B = G mud = m mud g = 12 × 10-3 kg × 10 N/kg = 0.12 N.

【Answer】0.1　0.12

Typical example 2: As shown in the figure, a block weighing 20 N is hung on a spring dynamometer and immersed in an overflow cup filled with water. After the block comes to rest, the mass of the water overflowing from the cup is 0.5 kg. ,g is taken as 10 N/kg. beg:

(1) The indication of the spring dynamometer;

(2) Density of the block.

End-of-Chapter Summary and Improvement PPT, Part 2: Experimental Activities

Experiment 1: Explore what factors are related to the size of buoyancy

1. Experimental equipment: As shown in the picture, a spring dynamometer, a block, a cup of salt water, and a cup of clean water.

2. Experimental operation

(1) As shown in Figure a, measure the gravity of the object.

(2) As shown in Figures b, c, and d, read the readings of the spring dynamometer when the block is immersed in water at different depths, and calculate the buoyancy force.

(3) Immerse the object in salt water and measure the buoyancy.

3. Experimental phenomena

From the position shown in Figure b to the position shown in Figure C, the buoyancy force on the block increases; from the position shown in Figure c to the position shown in Figure d, the buoyancy force on the block remains unchanged; from the position shown in Figure d to At the position shown in Figure e, the buoyancy force on the block becomes larger.

4.Experimental conclusion

The buoyancy force an object experiences in a liquid is related to the volume and density of the liquid immersed in it. The greater the volume and density of the liquid immersed in the liquid, the greater the buoyancy force the object experiences.

Typical example 1: Students used experimental equipment such as plastic bottles and fine sand to explore "factors affecting buoyancy" (ρ water = 1.0×103 kg/m3). They put forward the following three conjectures based on life experience and gave corresponding examples:

Conjecture 1: The size of the buoyancy force is related to the density of the object. Example: an iron block sinks in the water, and wood can float on the water;

Conjecture 2: The buoyancy force is related to the density of the liquid. Example: an egg sinks in water but can float in salt water;

Conjecture 3: The buoyancy force is related to the depth of immersion in the liquid. Example: In a swimming pool, the more a person squats down, the greater the force of the water lifting him up.

In order to verify whether the above conjecture is correct, the students chose plastic bottles filled with fine sand and other experimental equipment for investigation. The experimental process is shown in the figure.

(1) According to Figure c and Figure d, it can be verified that the third conjecture is ________ (optional "correct" or "wrong");

(2) To verify whether the conjecture 1 is correct, you can explore it after _________ in a plastic bottle (you need to re-run the experiment in Figure a);

(3) The students discussed that the examples in Conjecture 1 and 2 reflect that the sinking and floating of objects may be related to density. When choosing an object that can float on the salt water in Figure e, the density of the object should be less than _________g/ cm3 (calculate the result to one decimal place).

[Analysis] (1) The buoyancy force on the object is the same when it is at the position shown in the picture c and d in the question, and the volume of the liquid displaced is equal to the volume of the object, but the depth of the object is different, so it can be concluded that the object is immersed in the same type of In liquids, the buoyancy force experienced has nothing to do with the depth of immersion in the liquid. According to the question pictures c and d, it can be verified that the third conjecture is wrong; (2) Verify whether the first conjecture is correct, that is, verify whether the size of the buoyancy force is related to the density of the object. According to the control variable method, the density of the object needs to be changed, that is, when the volume of the object does not change, When the density of the plastic bottle changes, adding some fine sand to the plastic bottle can change the overall density of the plastic bottle; (3) From the question pictures a and c, it can be seen that the buoyancy force F on the object immersed in water is F = 0.34 N. From the question pictures a and e, it can be seen that , the buoyancy force F' buoyancy experienced by an object immersed in salt water is 0.38 N. According to Archimedes' principle, F buoyancy = ρV row g and when immersed in water and salt water, V row is equal. The density of salt water ρ salt water ≈ 1.1 g/ cm3, when choosing an object that can float on the salt water in picture e, the density of the object should be less than 1.1 g/cm3.

【Answer】 (1) Wrong (2) Add some fine sand (3) 1.1

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