How is the buoyancy formula derived?

How is the buoyancy formula derived?

We can start with the definition
The physical meaning of buoyancy is that an object is under the action of water
Now suppose we draw an arbitrary area in the water, then this part of the water is forced by gravity g and other parts of the water
If we change this part of the water into another object, the gravity of the water discharged by it is g, and the force of the other parts of the water on it is f, that is, buoyancy
This proves Archimedes principle: the buoyancy of an object in water is equal to the gravity of draining water

Formula derived from buoyancy calculation formula
As the title, from F floating = ρ liquid GV row derived liquid, object density, volume, mass and so on

From F floating = ρ liquid GV row, we can deduce that:
(1) ρ liquid = f floating / (GV row)
(2) V row = V substance = f float / (ρ liquid g) (V row = V substance, condition: immersion)
(3) When the object floats, f floats = g object
M = g / g
ρ = m / V

Derivation of buoyancy formula
Because f floating = P liquid GV discharge, g = 9.8n/kg, is it possible to infer that liquid density × volume of discharged liquid = weight of discharged liquid?

Buoyancy is equal to the weight of the object draining water, and the formula given by the landlord is: liquid density × volume of the object draining liquid = weight of the discharged liquid, which is obviously wrong. Density x volume should be mass, and it should be multiplied by the acceleration of gravity

Ask a junior high school physics pulley block calculation problem,
When a person's gravity is 500N and a bag of sand is lifted by the movable pulley of two ropes, the mechanical efficiency is 93.75%?

(g stone * h) / (g person 2H) = 93.57%
G = 935.7n
Maximum gravity of sand Shi 935.7n

Xiao Ming used pulley block to lift 200N weight from the first floor to the second floor at a constant speed. The height of the first floor is 2m, the weight of the movable pulley is 50N, and the friction between the pulley and the shaft and the weight of the rope are not taken into account. Then the mechanical efficiency of the pulley block? (3 pieces of rope)

W = GH = 200N * 2m = 400j
W extra = g dynamic H = 50N * 2m = 100J (the work done on the moving pulley is extra work)
W total = w useful + W extra = 400j + 100J = 500J
Mechanical efficiency of pulley block = w useful / W total = 400j / 500J = 80%
Another method: w = GH = 200N * 2m = 400j
Tension f = gtotal / 3 = (200N + 50N) / 3
The distance of pulling force movement is s = 3H = 3 * 2m = 6m
W total work = FS = (200N + 50N) / 3 * 6m = 500J
Mechanical efficiency of pulley block = w useful / W total = 400j / 500J = 80%
Please click adopt, if you don't know, ask

All the junior high school physics formulas

Deformation of remark formula of physical quantity (unit) formula
Gravity g (n) g = mg m: mass G: 9.8n/kg or 10N / kg
Density ρ (kg / m3) ρ = m / V M: mass V: Volume
The direction of resultant force F (n) is the same: F = F1 + F2 name symbol name symbol
Mass m kg m = PV
Temperature T ° C ° C
Velocity v m / S M / S v = s / T
Density p kg / M? Kg / M? P = m / V
Force (gravity) f Newton (cow) n g = mg
Pressure P Pascal PA P = f / S
Work w Joule (joule)

The calculation formula of physics practical problems in junior middle school

[mechanics part]
1. Speed: v = s / T
2. Gravity: g = mg
3. Density: ρ = m / V
4. Pressure: P = f / S
5. Liquid pressure: P = ρ GH
6. Buoyancy:
(1) F float = f '- f (pressure difference)
(2) F = G-F (apparent gravity)
(3) F floating = g (floating, floating)
(4) Archimedes principle: F floating = g row = ρ liquid GV row
7. Lever balance condition: F1 L1 = F2 L2
8. Ideal slope: F / g = H / L
9. Ideal pulley: F = g / n
10. Actual pulley: F = (G + G dynamic) / N (vertical direction)
11. Work: w = FS = GH
12. Power: P = w / T = FV
13. Principle of work: W hand = w machine
14. Actual machine: wtotal = w + W extra
15. Mechanical efficiency: η = w / W
16. Pulley block efficiency:
(1) η = g / NF (vertical direction)
(2) η = g / (G + G) (vertical direction without friction)
(3) η = f / NF (horizontal direction)
[thermal part]
1. Endothermic: q = cm (t-t0) = cm Δ t
2. Exothermic: q = cm (t0-t) = cm Δ t
3. Calorific value: q = q / M
4. Efficiency of furnace and heat engine: η = w / Q fuel
5. Heat balance equation: Q discharge = q suction
6. Thermodynamic temperature: T = t + 273k
[electrical part]
1. Current intensity: I = q electric quantity / T
2. Resistance: r = ρ L / S
3. Ohm's Law: I = u / R
4. Joule's Law:
(1) Q = I & # 710; 2rt universal formula)
(2) Q = uit = Pt = UQ, electric quantity = u & # 710; 2T / R (pure resistance formula)
5. Series circuit:
（1）I＝I1＝I2
（2）U＝U1＋U2
（3）R＝R1＋R2
(4) U1 / U2 = R1 / R2 (partial pressure formula)
（5）P1/P2＝R1/R2
6. Parallel circuit:
（1）I＝I1＋I2
（2）U＝U1＝U2
（3）1/R＝1/R1＋1/R2 [ R＝R1R2/(R1＋R2)]
(4) I1 / I2 = R2 / R1 (split flow formula)
（5）P1/P2＝R2/R1
7 constant resistance:
（1）I1/I2＝U1/U2
（2）P1/P2＝I12/I22
（3）P1/P2＝U12/U22
8 electric power:
(1) W = uit = Pt = UQ (universal formula)
(2) W = I & # 710; 2rt = u & # 710; 2T / R (pure resistance formula)
9 electric power:
(1) P = w / T = UI (universal formula)
(2) P = I & # 710; 2R = u & # 710; 2 / R (pure resistance formula)
10. Electromagnetic wave: C = λ f

How to remember junior high school physics formula_ What are the Physics Formulas in junior high school

How to remember junior high school physics formula is the most troublesome thing when I learn junior high school physics. There are many junior high school physics formulas. It's not easy to memorize them by rote, and if I don't remember them for a long time, I will lose them if I pass the exam
However, I later summed up a little trick - remember the theorem by experiment
Because when every formula theorem appears in the textbook, there must be a small experiment to explain the principle
I use "VCM simulation experiment" to do experiments desperately. I don't need to memorize the principles when I do experiments. When I think of the experiments I have done, the corresponding formula principles come out naturally in my mind. Maybe this is why the so-called three-dimensional knowledge is more acceptable than the plane knowledge
By the way, we provide a complete collection of junior high school physics formulas
Unit formula of physical quantity
Name symbol name symbol
Mass m kg m = PV
Temperature T ° C ° C
Velocity v m / S M / S v = s / T
Density p kg / M & sup3; kg / M & sup3; P = m / V
Force (gravity) f Newton (cow) n g = mg
Pressure P Pascal PA P = f / S
Work w Joule (joule) J W = FS
Power P watt W P = w / T
Current I a (a) a i = u / R
Voltage U V U = IR
Ohm resistance (r = u / R)
Electric work w Joule (joule) J W = uit
Electric power P watt (watt) W P = w / T = UI
Heat Q Joule J Q = cm (T-T °)
Specific heat C coke / (kg ° C) J / (kg ° C)
The speed of light in vacuum is 3 × 108 M / s
G 9.8 N / kg
Sound speed 340m / s in air at 15 ° C
The safety voltage is not higher than 36 v

Junior high school physics pulley block rope moving distance how to seek

When a fixed pulley, the distance of the rope is equal to the distance of the object; when a moving pulley, the distance of the rope is equal to twice the distance of the object

Pulley block, how to judge the number of segments of that rope?
I don't know who can help me

How to determine the number of strands of a rope depends on how many strands are connected to the movable pulley (including the hook on it)
You know f = 1 / N * g. if there are several strands of rope that bear the weight of the movable pulley, then n is equal to several strands