The method of calculation about buoyancy

Part of the junior high school physical buoyancy buoyancy calculated both focus is difficult to account for a certain proportion of the calendar year entrance examination scores. Many candidates solving ideas are felt helpless, this part of the kinds of questions in the exam easier to lose points correctly answer buoyancy calculation, help students abstract thinking ability, creative ability, hands-on mental and physical ability and the ability of the comprehensive application of knowledge.

A proper understanding of the concept in the buoyancy

On the basic concept of the buoyancy content, it is possible to correctly understand, Solutions buoyancy calculation key question, such as the definition of the buoyancy of the causes of the direction and the buoyancy of the buoyancy, the objects floating in the liquid, sinking, suspended or floating on the liquid surface and so on.

2 master drifting objects in the liquid condition

When F floating> G material (ρ matter <ρ liquid), objects floating, stationary objects floating in the liquid surface, then F float = G thing.

When F floating = G was (ρ matter = ρ fluid, the object was suspended.
F the floating <G物(ρ物> ρ liquid objects sinking, stationary objects sink to the bottom, when F floating = the the G objects-N (supportive.
When the object is suspended in a liquid, or floating on the liquid surface, the two forces balance.
Correct understanding of Archimedes' principle.
Correct understanding of Archimedes is key to calculating the buoyancy.
In the formula "F floating = G row = m row g = ρ fluid gV row" in the G row represents the object row of gravity of the liquid, the m row represents the quality of the object row liquid, ρ the liquid density of the liquid is indicated, V row indicates the object displaces liquid volume.

When the object is fully immersed in the liquid, V row = V was when the object portion is immersed in the liquid, V row <V matter.
The size of the buoyancy only with the density of the liquid, scheduled to open the volume of the liquid, and has nothing to do with other factors. Archimedes' principle applies to all liquids and gases.

4 a common method of buoyancy calculation

(1 spring balance method (F float = G thing-F read

This method applies only known as the objects gravity by the buoyancy in the liquid with a spring balance.
Example 1 a piece of metal in the air, said hanging on the spring dynamometer reading of 27 N, it is submerged in water, said dynamometer reading of 17 N, this piece of metal by the buoyancy is much? (G = 10 N / kg) Links to free papers Download Center http://eng.hi138.com
analysis piece of metal submerged in water, said, showing the number of spring dynamometer shows the number smaller than in the air, said this is because the piece of metal by the role of the upward buoyancy of the water, floating F = G-F = 27 N-17 N = 10 N

(2 balance method (F float = G objects

This method only applies to the object part is immersed in the liquid in a floating state, or completely submerged in liquid in a suspended state, according to the object of the "F floating = G" and buoyancy.

Example 2 will be a mass of 800 g of copper block (volume 2 dm3 into the water still suffered buoyancy is how much?

The analysis of this question, there is no clear-cut copper block is solid or hollow, so can not determine the state of the copper block into the water, but we can seek the density of the copper block:
ρ copper = m / V = ​​0.8 kg / 2 × 10-3 m3 = 0.4 × 103 kg/m3
Because ρ the copper <ρ water, so the copper block placed in water in a floating state, i.e. F floating = G copper

(3 Archimedes' principle method (F float = G row = m row g = ρ fluid gV row

This method is suitable for all liquid or gas.
Example 3 a mass of 2.7 kg of a solid aluminum block aluminum (ρ = 2.7 × 103 kg/m3 placed in sufficient water, the buoyancy force is much?
Analysis because ρ the aluminum> ρ water, so the aluminum block on sufficient water must sink, i.e.

V the platoon = V material = 2.7 kg/2.7 × 103 kg/m3 = 10-3 m3
Then can demand according to Archimedes' principle of buoyancy. Posted in the free papers Download Center http://eng.hi138.com

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