The explanation and exercises of physical buoyancy in the second volume of the second day of junior high school should be difficult and have answers.

When analyzing the ups and downs of objects, we should pay attention to three balanced States and two unbalanced processes.

As shown in the figure, an object immersed in liquid is subjected to two forces: one is vertical downward gravity; One is vertical buoyancy. Whether an object floats or sinks in a liquid depends on the gravity and buoyancy it receives.

(1) float —— (unbalanced process) When buoyancy is greater than gravity, that is, F float >; G, the object floats; At this time, the resultant force direction of the object is vertical upward, and the object moves upward in the liquid.

(2) Sinking —— (unbalanced process) When buoyancy is less than gravity, F floats < G and the object sinks; At this time, the resultant force direction of the object is vertical downward, and the object moves downward in the liquid.

(3) Suspension-(equilibrium state) At this time, when buoyancy is equal to gravity, that is, F float =G, the resultant force is zero, and the object is suspended in the liquid, which can be anywhere in the liquid.

(4) Float-(equilibrium state) At this time, the buoyancy of the object is equal to gravity, that is, when F float =G, the resultant force is zero and the object floats in the liquid.

⑤ Sinking —— (equilibrium state) At this time, the buoyancy is less than gravity, that is, F floating +N=G(N is the supporting force of the object), the resultant force is zero, and the object sinks to the bottom of the container.

★ Understand:

(1), so the ups and downs of objects can also be written as:

When f floats >: g, ρ liquid >; When ρ object, the object floats;

When F float =G and ρ liquid = ρ object, the object is suspended in the liquid;

When f floats.

② Suspension and floating

Suspension: The object is completely immersed in liquid. If buoyancy equals gravity, the two forces are balanced and the resultant force is zero. An object is in equilibrium, and it can be at rest anywhere in the liquid. We say that an object is "suspended" in a liquid;

Floating: the object is completely immersed in the liquid. If the buoyancy is greater than gravity, the resultant force of the two forces is upward and the object moves upward. As the object continues to rise, part of its volume will begin to surface and its buoyancy will decrease. Until it is equal to gravity, the resultant force of buoyancy and gravity on the object is zero, so it no longer floats, and the floating movement of the object ends, that is, it floats on the liquid surface.

Same-all objects are in equilibrium, and the gravity g and buoyancy of objects are a pair of balancing forces.

The difference lies in their different positions in the liquid. Suspended objects completely enter the liquid and can be at rest anywhere in the liquid, and their volume is equal to the volume of the liquid displaced by the object; Floating means that the object is still on the surface of the liquid, and its volume is greater than the liquid volume displaced by the object;

Second, the application of buoyancy

(1) Ship: It is a hollow object made of solid steel. The solid steel increases the volume of the liquid it displaces and makes it float on the water.

(2) Submarine: The buoyancy of the submarine immersed in liquid remains unchanged, but there are two water tanks in the boat to fill or drain water at any time, which changes the gravity of the submarine and enables it to float, sink or suspend.

(3) Hot air balloon: A hot air balloon can be launched by heating the air inside the balloon to make its density less than that of the outside air and its buoyancy greater than its gravity.

(4) Airship: If the airbag of the airship is filled with gas with a density less than air, the airship can be launched like a hot air balloon.

Case analysis

Example 1: As shown in the figure, the ordinate represents the mass of the object and the abscissa represents the volume of the object. Images show the relationship between the mass and volume of objects A and B, respectively. The following statement is correct.

A. Put the object A into the water, and it will float on the water.

B. Put the object B into the water, and it will surely sink to the bottom.

C. Tie objects A and B of equal volume together and put them into the water, which will inevitably sink to the bottom.

D. Bind objects A and B with equal volume together and put them into the water to float on the water surface.

Analysis: The density of two objects can be obtained from the image: It can be seen that if objects A and B are put into the water, A will sink to the bottom of the water and B will be suspended in the water. As we can see from the figure, if two objects ab with the same volume are tied together, their density is:

So it will sink to the bottom of the water.

Correct answer: C.

By comparing the gravity and buoyancy of objects, we can judge the ups and downs of objects in liquid, and we can also judge the ups and downs of objects in liquid by the density of objects and liquids.

Example 2: Wood block and iron block, solid cube, side length 2cm, wood block density 0.6× 103kg/m3. Put it in water, and calculate the buoyancy (g= 10N/kg) of the wood block and the iron block respectively when it is at rest.

Analysis: When the wood block is put into water, it will float on the water surface, and the volume of liquid it displaces is smaller than that of the wood block. When it is in water, it will be affected by gravity and buoyancy, and its buoyancy can be calculated by the balance of the two forces. Iron will sink to the bottom of the water and be subjected to three forces in the water. The volume of liquid it displaces is equal to the volume of iron, and the buoyancy can be solved by Archimedes principle.

Solution: ∵ ρ wood

F driftwood =G=mg=ρ wood GV = 0.6×103kg/m3×10n/kg× 8×10-6m3 = 0.048n.

∵ ρ iron > ρ water, ∴ iron sinks;

F floating iron = ρ water gV row =1.0×103kg/m3×10n/kg× 8×10-6m3 = 0.08n.

When an object is put into a liquid, we must first judge the ups and downs of the object in the liquid, and then calculate the buoyancy according to the specific situation.

Example 3: short answer: the ship will float a little after entering the sea from the river. Why? As we all know, the density of sea water is greater than that of river water.

Answer: No matter in sea water or river, ships are floating on the water, and their buoyancy is equal to the gravity of the ship, that is, the buoyancy of the ship remains unchanged. According to the condition of force balance, it can be seen that the volume of liquid displaced by the ship is inversely proportional to the density of liquid. Because the density of seawater is greater than that of river water, the volume of seawater discharged by the ship is smaller than that of river water discharged by the ship, so the ship will float a little after entering the sea from the river.

This is the concrete application of the condition of object floating and sinking in real life, and it is necessary to master the principle of ship floating on the water.

Example 4: As shown in the picture, it is a water storage tank designed by Xiao Ming to prevent water from being cut off at home. When the water depth in the water tank reaches 1.2m, the float A just blocks the water inlet pipe and releases water into the water tank. At this time, the float A has a volume exposed to the water (the float A can only move vertically at the position shown in the figure). If the pressure of the water inlet is 1.2× 105Pa, the cross-sectional area of the nozzle is 2.5㎝2, the bottom area of the water storage tank is 0.8m2, and the weight of the float A is 10N. Q:

(1) How much water can the water storage tank hold?

(2) What is the volume of float A?

Analysis: The water storage capacity of the water tank can be calculated by the water inflow volume of the water tank, and its volume can be calculated according to the force balance condition and Archimedes principle.

Solution: (1) v = sh = 0.8m2×1.2m = 0.96m3.

m =ρV = 1.0× 103?/m3×0.96 m3 = 960?

(2) When the float just blocks the water outlet pipe,

F floating =GA+F pressure

F pressure = PS =1.2×105pa× 2.5×10-4m2 = 30n.

ρ water Vg= GA+F pressure = 10N+30N.

V=6× 10-3m3 .

This is also a practical problem, in which not only buoyancy, but also the concepts of density and pressure are investigated, which is comprehensive, so we should pay attention to combining several parts of knowledge well when solving problems.

Example 5: A transport ship with a mass of 2.0× 106kg sank to the bottom of the sea in a shipwreck accident, and the distance from the sunken ship to the sea surface was about1000 m measured by the ultrasonic wave of the salvage ship; Divers dive into the bottom of the sea and find the hatch cover of a sunken ship, with an area of about 0.5m2 After opening it, they enter the cabin to make a salvage plan. One of the schemes to salvage a sunken ship is to tie many buoyancy bags to the hull, each with a volume of about 10m3, and use the buoyancy received by these buoyancy bags to make the ship float (the density of seawater is 1.03x 103kg).

Analysis: According to the formula of liquid pressure, the pressure acting on the hatch cover can be calculated, and the buoyancy of the bag, the gravity of the ship and the pressure acting on the hatch cover can be calculated by using Archimedes principle. . . . .

Solution: (1) hatch cover pressure:

(2) Weight of carrier:

Pressure of seawater on hatch cover;

Every floating bag is floated by seawater.

The second question of this problem is very open, and there are many physical quantities that can be solved, which also gives students more space.

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