Physics 8 Lesson 12: Popularity
1. Theoretical summary
1.1. Conditions for objects to float, objects to sink
– An object of mass P immersed in a fluid is acted upon by an Archimedes repulsion force F._{A}:
+ The object moves on the surface of the liquid when F_{A} > P.
+ The object moves downward when F_{A} < P.
+ The object is suspended (completely immersed) in the liquid when F_{A} = P.
Eg: The specific gravity of the ship is less than the specific gravity of the water, so the ship can float on the water.
1.2. The magnitude of the Archimedes repulsion when the object is floating on the surface of the liquid
– When the object floats on the surface of the liquid, the Archimedes repulsion force:
+ Formula: \(F_{A}\) = dV
In there:
 V: The volume of the part submerged in the liquid (not the volume of the object),
 d: Specific gravity of the liquid
Note:
– When a solid object is submerged in a liquid vessel, three cases occur: The object sinks; The object is suspended in the liquid; The object floats on the surface of the liquid.

The cases where the object is sinking, suspended in a liquid and rising, are relatively easy cases to analyze and students often make mistakes. However, the case where the object is at rest at the bottom of the jar and especially the case where the object is at rest on the surface of the liquid, is the case that students are easily confused.

In case the object is at rest at the bottom of the jar, students usually only understand in this case P > \(F_{A}\) without noticing that when the object is at rest at the bottom of the jar, the forces acting on the object must be balanced. together
2. Illustrated exercise
Question 1: Help Binh answer An in the introduction, knowing that the ship is not a solid block of steel but has many voids.
Solution guide
The ball made of steel has a specific gravity greater than the specific gravity of water, so it sinks.
The ship is made of steel, but the space is designed so that the specific gravity of the whole ship is less than the specific gravity of the water, so the ship can float on the water.
Verse 2: Dropping a steel ball into mercury will it float or sink? Why?
Solution guide
Drop a steel ball into mercury and the steel ball will float because the specific gravity of the steel is less than the specific gravity of the mercury.
3. Practice
3.1. Essay exercises
Question 1: What happens if a steel ball is dropped into mercury? Knowing that steel has a specific gravity of 78500 N/m^{3}, mercury has a specific gravity of 136000 N/m^{3}.
Verse 2: A swimming buoy with a volume of 25 dm^{3} and 5 kg weight. What is the lift force acting on the float when it is submerged in water? The specific gravity of water is 10000 N/m3.
Question 3: An object has a specific gravity of 26000 N/m^{3}. If the object is hung on a dynamometer and then submerged in water, the dynamometer will read 150 N. If the object is hung in the air, what will the dynamometer show? Given that the specific gravity of water is 10000 N/m .^{3}.
Question 4: Why does a piece of wood float in water?
3.2. Multiple choice exercises
Question 1: A barge in the shape of a box is 4m long and 2m wide. Know that the barge is submerged in water 0.5 m deep. The specific gravity of water is 10000 N/m .^{3}. What is the weight of the barge?
A. P = 40000 N B. P = 45000 N
C. P = 50000 N D. Another result
Verse 2: An object is in a liquid. Which of the following statements is most correct regarding forces acting on an object?
A. An object in a liquid is acted upon by a single force, gravity.
B. An object lying in a liquid is acted upon by a single force, the ASimeter thrust.
C. An object lying in a liquid is under the influence of gravity and an Archimede force in the vertical and opposite directions.
D. An object in a liquid is under the influence of gravity and an Archimede force that is vertical and in the same direction.
Question 3: When the object is floating on the liquid, the ASimeter repulsion has the magnitude:
A. Less than the weight of the object.
B. Greater than the weight of the object.
C. Equal to the weight of the object.
D. Less than or equal to the weight of the object.
Question 4: The ASimeter thrust is less than the weight then:
A. Objects sink
B. Floating object
C. Objects suspended in liquid
D. The object sinks to the bottom of the liquid
4. Conclusion
Through this lesson, you will be acquainted with the knowledge related to Floating along with related exercises at many levels from easy to difficult…, you need to understand:
 Understand the conditions for objects to float, objects to sink
 Know how to calculate the magnitude of the Archimedes repulsion when the object is floating on the surface of the liquid
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