"The Floating and Sinking of Objects" Buoyancy PPT Courseware 4

Description

"The Floating and Sinking of Objects" Buoyancy PPT Courseware 4

Sorting out new knowledge

1. Floating and sinking conditions of objects

An object floats when buoyancy _________ gravity;

When buoyancy ________ gravity, an object sinks;

An object is suspended or floating when buoyancy _________ gravity.

2. Application of floating and sinking conditions

1. density meter

A density meter is an instrument used to measure the density of liquids. The buoyancy force experienced by the density meter in different liquids is the same, which is equal to its own gravity (the working principle of the density meter). Therefore, the volume of liquid displaced in a liquid with high density is small, so the scale of the density meter is small at the top and large at the bottom.

2. Salt water seed selection

Shrunken, insect-eaten seeds experience a buoyant force greater than gravity, so they float; plump seeds experience a buoyant force less than gravity, so they sink.

3． submarine

Submarines float and sink by changing their own gravity. When the submarine is filled with water, the gravity increases, and when the gravity is greater than the buoyancy, it sinks; when the submarine is drained, the gravity decreases, and when the gravity is less than the buoyancy, it floats. When not out of the water, the buoyancy force on the submarine remains unchanged.

4. hot air balloon

The nozzle of the hot air balloon heats the air in the air bag. The air temperature increases and the density decreases. When the density decreases to a certain level, the hot air balloon rises. When the nozzle stops heating or reduces the level of heating, the balloon can land.

Interactive exploration

Type 1: Floating and sinking conditions of objects

Example 1 In order to extend the storage time of fruits, fruit farmers immerse the fruits in their own preservative liquid and wrap them in preservative bags to store them for a relatively long time. If the same apple is placed in containers A and B, In the preservative liquid, the floating and sinking conditions of the apples after resting are shown in Figure 9-3-2. Comparing the densities of the preservative liquids in containers A and B, the correct one is ()

A. Big among the best

B． Big among B

C. same size

D． Insufficient conditions for comparison

Example 2 An object weighing 5 N and having a volume of 6×10-4 m3 is put into water. If the resistance of the water is not considered, when the object is stationary, the following statement is correct ()

A. The object floats, F float = 6 N

B. The object is suspended, F float = 5 N

C. The object floats, F float = 5 N

D. The object sinks to the bottom of the water, F floats = 6 N

[ Analysis , the object floats up and finally floats on the water; when floating, the buoyancy force is equal to G, that is, the buoyancy force is 5 N.

Type 2: Application of floating and sinking conditions

Example 3 Selecting excellent seeds is the basic guarantee for a good agricultural harvest. Salt water seed selection is a seed selection method commonly used by farmers in my country: after placing the seeds to be selected in salt water with an appropriate concentration, the good seeds will sink to the bottom of the water, while the secondary seeds will float on the water surface. , the seed selection work can be completed quickly, as shown in Figure 9-3-3. In this method, the key is to grasp the concentration of brine. If the concentration of brine is too low, it may ()

A. All seeds float on the water

B． All seeds are not buoyant

C. Only a few secondary species float

D． The better seeds float and the inferior seeds sink.

[ Analysis The full seeds have greater gravity, so the gravity is greater than the buoyancy force, so the seeds will sink. If the concentration of salt water is too low, because the volume of salt water displaced by the seeds remains unchanged, the buoyancy of the seeds by the salt water is reduced, so only a small number of secondary seeds float, so choose C.

On-site testing

Knowledge point 1: Floating and sinking conditions of objects

1. As shown in Figure 9-3-5, put an apple into concentrated salt water and the apple will float. If the part above the water surface is cut off, the remaining part ()

A. sink to the bottom of the water b. still floating

C. Just suspended D. Unable to judge

[ Analysis Still floating.

2. Li Ming and Wang Cong conducted the following experiment: They immersed a wooden block and an iron block in water at the same time. After letting go, they found that the wooden block floated up and the iron block sank. They discussed the above phenomenon. Which of their statements do you think is correct? ()

A. The buoyant force on the wooden block is greater than the buoyant force on the iron block

B． The buoyant force on the iron block is greater than the buoyant force on the wooden block

C. The buoyant force on the wood block is greater than the weight of the wood block, and the buoyant force on the iron block is less than the weight of the iron block.

D． Objects with higher density sink and objects with lower density float.

Knowledge point 2: Application of floating and sinking conditions of objects

3． As shown in Figure 9-3-6, a small ball floats on the water, half of it above the water. When a little salt is added to the water and stirred, when the ball comes to rest, the buoyant force it experiences will be ()

A. become smaller b． get bigger

C. unchanged D. Unable to judge

[Analysis] The ball floats on the water, and the buoyancy force is equal to gravity. When a little salt is added to the water and stirred, the density of the water increases, and the volume of the salt water displaced by the ball decreases, and it still floats. Since the gravity of the ball does not change, the buoyancy does not change.

4. Regarding the floating and sinking of submarines, which of the following statements is correct ()

A. Submarines float and sink by changing their own gravity

B． Submarines rise and sink by changing their buoyancy

C. The deeper the submarine sinks, the greater the buoyancy force it experiences.

D． When the submarine surfaces, the buoyancy force it experiences remains unchanged.

[ Analysis When the submarine is immersed in water, the volume of water displaced does not change. According to Archimedes' principle, the buoyancy force it receives also does not change, so option B is incorrect. The buoyancy force on the submarine has nothing to do with the depth of submersion, so C is wrong. As the submarine surfaces, the volume of water displaced becomes smaller and smaller, and the buoyancy force it receives becomes smaller and smaller, so D is wrong.

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Update Time: 2024-10-24

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