A bookshelf is divided into upper, middle and lower layers, with a total of 384 books. If you take as many books from the upper layer as from the middle layer, put them in the middle layer, and then take them out from the middle layer as from the lower layer Put the same number of books in the lower layer, and finally take out the same number of books from the lower layer and put them in the upper layer. At this time, the number of books in the three-layer bookshelf is equal. How many books were there in the middle layer of the bookshelf? Don't use the hypothesis AB or ZY

A bookshelf is divided into upper, middle and lower layers, with a total of 384 books. If you take as many books from the upper layer as from the middle layer, put them in the middle layer, and then take them out from the middle layer as from the lower layer Put the same number of books in the lower layer, and finally take out the same number of books from the lower layer and put them in the upper layer. At this time, the number of books in the three-layer bookshelf is equal. How many books were there in the middle layer of the bookshelf? Don't use the hypothesis AB or ZY

It's very simple to use the method of setting unknowns, but because you don't need to assume, you can use the method of backward deduction, that is, backward deduction
First of all, for the convenience of expression, we stipulate that there are three times in this operation. The first time, we take books from the upper layer to the middle layer, the second time, we take books from the middle layer to the lower layer, and the third time, we take books from the lower layer to the upper layer
Then, there are 384 copies in each layer, so after all the steps, there should be 384 / 3 = 128 copies in each layer
The derivation begins as follows:
After the third operation, the number of books in the upper layer is twice that after the second operation. Then, before the third operation, there should be 128 / 2 = 64 books in the upper layer. Because these books are taken out from the lower layer, there are 128 + 64 = 192 books in the lower layer
In the second operation, after the second operation, that is, before the third operation, the number of books in each layer is: upper 64, middle 128, lower 192
In the same way as above, it is easy to get that before the second operation, there should be 192 / 2 = 96 in the lower level and 128 + 96 = 224 in the middle level
At this time, the number of books in each layer is: 64 in the upper, 224 in the middle and 96 in the lower
Continue to push, before the first operation, there should be 224 / 2 = 112 in the middle level and 64 + 112 = 176 in the upper level
Before the first operation, the original state is: upper 176, middle 112, lower 96
Now you can answer this question. There are 112 books in the middle class
Finally, we give a process of book number change: (up, middle, down) - initial state (176112,96) - after the first operation (64224,96) - after the second operation (63128192) - after the third operation (128128)
P. S. the writing is a little long. If there is any unclear expression, please ask

A bookshelf is divided into upper, middle and lower layers, with a total of 384 books. If you take out as many books as the middle layer from the upper layer and put them in the middle layer, then take out as many books as the lower layer from the middle layer and put them in the lower layer, and finally take out as many books as the upper layer from the lower layer and put them in the upper layer. At this time, the number of books in the three layers is equal. How many books were there in the middle layer of the bookshelf?

At present, there are 384 △ 3 = 128 copies in the upper, middle and lower layers, 128 △ 2 = 64 copies in the upper layer, 128 + 64 = 192 copies in the lower layer, 128 copies in the middle layer, 192 △ 2 = 96 copies in the lower layer, 128 + 96 = 224 copies in the middle layer, and 64 copies in the upper layer when the lower layer is not given to the upper layer, so there are 224 △ 2 = 112 copies in the middle layer when the upper layer is not given to the middle layer

When an object moves in a straight line with uniform acceleration, the initial velocity is 0.5m/s, and the displacement in the 7th second is 4m more than that in the 5th second;
Xm-xn = (m-n) at & # 178; how can this formula judge t = 1

m=7
n=5
T = 1 (because 7S and 5S are, of course, one second)
So: 4 = (7-5) a * 1 ^ 2
The solution is a = 2m / S ^ 2

Take the unit with you
There must be units and staff
That letter means magic

Ohm's law
I=U/R
Current: I ampere (a)
Voltage: uvolt (V)
Resistance: R ohm (Ω)
Formula of specific heat capacity
Q=mc*t
Heat: Q Joule (J)
Specific heat capacity: CJ / (kg * ℃)
Time: T seconds (s)
Mass: m kg (kg)
Combustion energy formula (calorific value formula)
Q=mq
Heat: Q Joule (J)
Mass: m kg (kg)
Calorific value: Q coke / kg (J / kg)
Work formula:
W = FS or w = GH
Work: W Joule (J)
Force: F Newton (n) gravity: G Newton (n)
Moving distance along the force: S M rising height: H M
Power formula:
P=W/t
Power: P watt (W)
Work: W Joule (J)
Time: T seconds (s)
Electric power formula
1.P=UI
Power: P watt (W)
Voltage: uvolt (V)
Current: I ampere (a)
2.P=I^2R
Power: P watt (W)
Current: I ampere (a)
Resistance: R ohm (Ω)
3.P=U^2/R
Power: P watt (W)
Resistance: R ohm (Ω)
Voltage: uvolt (V)
Electric work formula
W=Pt
Electric energy: W kWh (kW * h)
Power: P watt (W)
Time: hours (H)
Note: 1. [P] in the above formula can be replaced by electric power 1 ~ 3
2. Electric power unit (KWH) can be converted to Joule, 1kW * H = 3.6 * 10 ^ 6J
Pressure formula:
Solid pressure
P=F/S
Pressure: P Pascal (PA)
Pressure: fnon (n)
Area: s M2
Liquid pressure
P=ρgh
Pressure: P Pascal (PA)
Liquid density: ρ kg / m3 (kg / M & sup3;)
Gravity formula: G newton per kilogram (n / kg)
Depth: HM (m)
Buoyancy formula
1.F=G
Buoyancy: F Newton (n)
Weight of drained water: G Newton (n)
2.F=ρgV
Liquid density: ρ kg / m3 (kg / M & sup3;)
Gravity formula: G newton per kilogram (n / kg)
Volume of drained liquid: vm3 (M & sup3;)
Buoyancy: F Newton (n)
3.F=G1-G2
Buoyancy: F Newton (n)
Weight: G1 Newton (n)
Gravity after the object sinks into water: G2 Newton (n)
Density formula
ρ=m/V
Density: ρ kg / m3 (kg / M & sup3;)
Mass: m kg (kg)
Volume: vm3 (M & sup3;)
Gravity formula:
G=gm
Gravity: G Newton (n)
Mass: m kg (kg)
Gravity mass relation: G newton per kilogram (n / kg)
Speed formula:
v=S/t
Speed: V meters per second (M / s)
Distance: S M
Time: T seconds (s)
The above is the formula of junior high school physics. I played word by word for half an hour. It's very hard

One end of the light spring is fixed to the vertical wall, and the other end is close to a wooden board with mass m = 2.0kg (the spring is not connected with the module). The friction coefficient between the wooden block and the horizontal ground is 0.5. Under the action of external force, the module compresses the spring for a certain distance and then stops at point A. as shown in the figure, the external force is removed, the wooden block moves to the right, after leaving the spring, it continues to slide and finally stops at point B, The distance between AB is x = 1.0m
(1) Calculate the friction force of the block in the process of moving to the right;
(2) Calculate the elastic potential energy of the spring when the block is at point a;
(3) Please qualitatively explain the change of acceleration and velocity of block from a to B

(1）?=μmg=0.5*2*10=10N
(2) According to the kinetic energy theorem, 1 / 2mvt ^ 2-1 / 2mv0 ^ 2 = - μ MGX + W projectile, 0-0 = - 10 + W projectile, w projectile = 10J, e projectile = w projectile = 10J
(3) There are three processes in the process of the block moving to the right. The first process is: the block is not separated from the spring, and the block is subjected to the right elastic force and the left friction force. In this process, the elastic force decreases all the time, and the friction force does not change,
Acceleration a = (f bullet - μ mg) / m, direction to the right, has been decreasing, until it becomes 0, in this process, the speed has been increasing. Next, the second process, the wood block is still connected with the spring, the acceleration direction of the wood block to the left, the size of a = (μ Mg-F bullet) / m, has been increasing, the speed is decreasing. Finally, to the third process, the acceleration direction to the left, the size is constant μ G, The speed keeps decreasing until it reaches 0

What is 400 microamperes

One ampere equals 1000 Ma, and one million microamperes
So 400 microampere equals 4 * 10 ^ - 4 a

Newton's second law experiment
In the experiment of Newton's second law, 1. Why is the gravity of the block much less than that of the car? 2. Why does it not need to be balanced again after balancing the friction for the first time

1. In fact, the pull on the car is not equal to the gravity of the block. Imagine that the block and the car are accelerating together, and the block needs acceleration, so the pull on the rope to the block is actually less than the gravity of the block. The pull on the car is equal to the force on the rope. This force is less than the gravity of the block. The lighter the block, the more it will add

The three sides of a triangle are 5cm, 8cm and (x + 1) cm respectively. Then what is its circumference

8-5

A wooden box with mass m is on the horizontal plane. One end of a light spring is suspended above the wooden box. The other end is hung with objects a and B connected by thin wires. After cutting the thin wires between a and B, a makes a simple movement. When a reaches the highest point, the pressure of the wooden box on the floor is
A. The mass of B is m

When a reaches the highest point, the wooden box is pulled upward, which is called AB gravity,
So the pressure of the forest box on the floor is the gravity of the wooden box minus the tension
（M-m-m）g=（M-2m）g

Sometimes it can be used in the mixed operation of four items of fraction______ It makes the calculation simple

In the mixed operation of four items of fraction, sometimes the operation law can be applied to simplify the calculation