(1) buoyancy measurement. Measure the gravity g of the object in the air with a spring dynamometer, then immerse the object in the liquid and read the indicator f' of the spring dynamometer. The difference between the two readings is the buoyancy of the object in the liquid, that is, F float = G-F'.
(2) Archimedes principle. The buoyancy immersed in a liquid is equal to the gravity of the liquid it displaces, which is Archimedes' principle.
(3) Calculation of buoyancy. Archimedes principle is expressed by formula: F float =G row or F float = ρ liquid V row G.
The meaning and unit of the symbol in the formula: f float- buoyancy-n; ρ liquid-density of liquid-kg/m3; Line V-Volume of liquid discharged by the object-m3; ;
Gram Newton/kg.
When the density of liquid and the volume of liquid displaced by the object are known, this formula can be used to calculate the buoyancy of the object.
2. The ups and downs of objects and their applications
Floating and sinking conditions of (1) objects
If the gravity of an object immersed in liquid is g and the buoyancy is f, then ① when F > G, the object floats; ② When f floats < g, the object sinks; ③ When f float =G, the object is suspended.
(2) Application of floating and sinking conditions
One application: submarine. The submarine changes its own gravity by filling and draining water into the water tank, thus realizing the ups and downs.
Application 2: Densitometer. Densitometer is an instrument used to measure the density of liquid. When it floats in different liquids, the exposed liquid level volume is different, and the density of the liquid is read according to the scale corresponding to the liquid level.
Application 3: hydrogen balloon, hot air balloon, airship. These planes are all floating in the air by using the buoyancy of the air.
Typical example
Example 1. As the picture shows, there is a long straight flat-bottomed glass tube floating on the water, and there is a small amount of fine sand in it. The bottom area of the glass tube is 5× 10-4m2, and the length of the glass tube immersed in water is 0.1m.. Find:
(1) What is the total weight of glass tube and fine sand?
(2) When some salt is sprinkled into the water and stirred evenly, the glass tube floats 0.01m.. What is the density of the salt solution at this time? (ρ water = 1.0× 103kg/m3, g= 10N/kg)
Analysis:
(1) Because the glass tube floats on the water surface, F float =G exists, so as long as the volume of water displaced by the glass tube is calculated, the buoyancy of the glass tube can be calculated according to Archimedes principle, and finally the total weight of the glass tube and fine sand can be calculated.
(2) according to F-float =G, expand the left F-float = ρ liquid v row g to get the density of brine.
Solution:
( 1)
v row = SH = 5× 10-4×0. 1 m3 = 5× 10-5 m3。
g = F float =ρwater gV row = 1.0× 103× 10×5× 10-5N = 0.5n。
(2)
After sprinkling salt,
V' = sh' = 5×10-4× (0.1-0.01) m3 = 4.5×10-5m3.
Because f floats' = g.
That is, row V of ρ brine' g=G'.
Then.
Example 2. As shown in the figure, ball A and ball B are connected by a thin thread and put into a water cup. The force on the thin line is 0.5N, and the two balls are just suspended in the water. If the thin thread is broken, the following judgment is correct ()
A, ball A sinks, ball B floats, ball A floats, ball B sinks.
C, a and b all sink. D, a and b are still suspended in water.
Solution:
Before the thin thread breaks, the ball A is subjected to three forces: buoyancy, gravity and the downward pull of the rope. Under the action of these three forces, the ball is in a state of balance, and the resultant force of these three forces is zero, that is, F floating nail =G nail +F pull ①.
Similarly, the ball B is also acted by three forces: gravity, buoyancy and upward pulling force of the rope, so F floats B +F' pulling force = G B 2.
And the sizes of F-pull and F' pull are equal, both equal to1n. After the thread breaks, balls A and B are subjected to gravity and buoyancy respectively, and the buoyancy and gravity are the same as before the thread breaks. Compared with ① formula, it is known that the buoyancy of F floating armor > G armor and armored ball is greater than gravity, and the armored ball floats; Comparing with the formula (2), we know that F floats B.
The correct answer is: b
Analysis:
This topic mainly examines students' understanding of Archimedes' principle, the application of object fluctuation conditions, and their ability to analyze and solve problems. The key to solve the problem is to start with the force changes of the two balls before and after the thin line is broken, and compare the movements of the ball A and the ball B before and after the thread is broken.
Example 3. A piece of paraffin will sink in alcohol and float in water. Please determine the density range of this paraffin. How to accurately measure the density of paraffin wax with measuring cylinder and appropriate amount of alcohol and water? (ρ alcohol =0.8× 103kg/m3)
Solution:
ρ wax > gt; ρ alcohol, you can know ρ wax.