When the physics papers came out, the floats were cut by half? It was felt that once a column had been requested, it had been lost, and it was not clear which formula should be used, and it was even more unclear what the object was floating or sinking and what the v platoon was. This panic is experienced by almost all junior high school students。
It's a lot worse than you think. All its changes are tied to the same ropeArchimedesI don't know. Today's article will help you to tear this principle apart and crush it and give you a three-step solution that anyone can use. After reading it, you'll find that the floating problem is just the calculation of the manicure。
Why does the floating question always make you lose
Most of the classmates were placing their heads on floats, not because they lacked iqs, but because the knowledge in their heads was a pile of fragments: knowing that f-floating = old liquid gv rows, and knowing that when objects floated, they were gravitational, but the subject matter was completely messed up by ropes and fingers。
The underlying reason is that you don't really understand the source of floating power. Floating power is not a force that comes out of space, but a difference in the pressure of liquids on objects up and down. And we don't have to do it again, because akimid gave us a straight conclusion more than 2,000 years ago。
Archimedes: the absolute core of buoyancy itself
An object immersed in liquids is subject to upward buoyancy, the size of which is equal to the gravity of the liquid it discharges。
That is the foundation of the floating building of the entire junior high school. Whether the object is floating, hanging or sank, even half of it is in the water and half of it is in the air, this principle is valid without exception。
The formula that must be carved into your head
F-f-g-g-g-v
Each letter here is a drop trap
- old liquid is the density of liquids, not objects. The title is 1. 0 x 103 kg/m3 for water, and 0. 8 x 103 kg/m3 for alcohol, so don't take objects in density。
- platoon v is the volume of liquid discharged from the object, which is, in white, the volume of the object leached into the liquid. When the object is fully immersed, the v platoon is equivalent to v; half of the exposed is above the water surface, and only half of the v platoon is v. This distinction is being examined almost every question。
- platoon g is not the gravity of the object. It is the gravity of the liquid。
If you really break these three symbols, your floating base is steady。
Three-step solution: from reading to full score
A lot of students get it when they're done. What is missing is a clear operating process. The next three-step approach, you do three things, and you have a different feeling。
Step one: state-of-the-art
Don't rush the formula. The first thing to do is read out the state of the object with a pen: floating, floating, sinking or being pulled up or down by a rope。
The condition of the object is clear. When floating and suspended, the object is subject to only gravity and buoyancy, a double balance, and f float = g. If the ropes hold, then a pull, triple balance, f = g ± f, (gravity plus gravity minus reverse). This step is almost unnecessary and depends on an understanding of the concept of balance。
Step 2: marks are known and unknown
All known quantities are listed with a physical symbol on the paper: liquid density, gravitational force of the object, rope pull, v, etc., and the required unknown amount is created in letters, e. G. F float and v rows in volume. It looks like a clumsy step, but it turns the topic into a clear data table, and it gives you a glimpse of the formula that should be used to reach known and unknown。
Step three: set the principles, step by step
Once all the data are ready, the akimid principle is formally available: f = gv platoon. If you want to float, you're going straight into the old fluid and v platoon. If v is required, the formula is transformed into v = f float/(g) and then an f float is sought with the first step of the stress relationship。
There is only one core logic: to find buoyancy with stress, to connect v platoons with principles. No matter how the subject is, this logic never turns。
Typical case handler to break down
To look at the complex question that has made millions of heads: a 5-n piece of wood is in the water, and one third of the volume is out of the water, asking what the total volume of the piece is. How much pressure do you need to push down if it's gonna happen
Step 1: set the stage. A third of the wood is floating, indicating that the v platoon equals two thirds of the v object. Floating state, f floating = g-wood = 5n。
Step 2: marks are known to be unknown. G-wood = 5n, old water = 1. 0 x 103kg/m3, g takes 10n/kg. Sets v as unknown。
Step three: sets of principles. F = old water gv platoon, replacing f = 5n, v = 2/3 v substance, received 5n = 1. 0 x 103 kg/m3 x 10n/kgx 2/3 v substance. This can directly decipher v = 7. 5 x 10-4 m3。
The second problem: to be fully immersed, the floating f = old water gv = 1. 0 x 103 x 10 x 7. 5 x 10-4 = 7. 5 n. At this point, the wood block is still subject to gravity 5n, and the downward pressure f plus gravity must be equal to the new float 7. 5n, so the pressure f = 2. 5n. All the way down, there's no truth。
The three pits that are the easiest to get in
Even if it's the right way, a lot of kids turn over the details. You probably stepped on the following three pits:
Pit 1: unit is not uniform. Volume is common to 3 cm, but the volume must be converted to cubic metres when the old water is used in kg/m3. 1m3 = 106cm3, never forget six zeroes。
Pit ii: mistakes that v platoons are v when floating. Floating means that only part of the object is immersed in liquids and that the v platoon is necessarily smaller than the v substance, which is equal only when suspended or deposited and fully immersed。
Pipe iii: mixing the density of objects and liquids. It cannot be overemphasized a hundred times that buoyancy can be judged in terms of density, but that it is necessary to use liquid in calculating buoyancy。
Floating is not magic. It's logic
Looking back, the so-called floating problem is nothing but a coat of force analysis, wrapped in the inner core of the akimid doctrine. And when you stop being blinded by the “shows” above the water, you stare at the v platoon below the water, at the density of the liquid, and at the stress of the object, all the topics become clear。
That's the charm of physics: it's never deep, it's just the simplest way to explain everything. The chapter on buoyancy, what you're missing is not smart, but a chance to figure it out。
Which pit do you most often fall into when you're doing floats? Was it the stress analysis pulled by the rope, or did you forget to convert units? You're welcome to talk about your real experience in the comment area, and maybe next time you'll be the one who'll die。
