When stone a falls freely from the top of the tower from static S1, stone B falls freely from static S2 from the top of the tower. If the two stones fall to the ground at the same time, the tower height is: () a S1 + S2 B (S1 + S2) ²/ 4S1 C S1 ²/ 4（S1+S2) D (S1+S2) ²/ S1-S2

When stone a falls freely from the top of the tower from static S1, stone B falls freely from static S2 from the top of the tower. If the two stones fall to the ground at the same time, the tower height is: () a S1 + S2 B (S1 + S2) ²/ 4S1 C S1 ²/ 4（S1+S2) D (S1+S2) ²/ S1-S2

B
Assuming that the tower height is h, the time taken for a to fall S1 from the tower top is T1, the time taken from the tower top to the ground is t, and the time taken for B to fall is T2
Then t-t1 = T2 is satisfied
And H = 1 / 2GT ^ 2, S1 = 1 / 2 * g * T1 ^ 2, h-s2 = 1 / 2 * g * T2 ^ 2
h=(s1+s2)^2/(4*s1)

Please explain the two physics problems in grade 2 If a wooden box placed on the horizontal ground is lifted up by force, but it is not lifted up, the following statement is correct: (a) it is always stationary, and the supporting force of the ground remains unchanged. The greater the upward pulling force of B, the greater the supporting force of the ground. C has a pulling force, and the wooden box is no longer in a balanced state. D. the wooden box is stationary, and the sum of the pulling force and the supporting force of the ground remains unchanged. Please explain these four options! thank you!

Tensile force plus supporting force is equal to gravity, and the supporting force changes with the change of tensile force. Specifically, the supporting force decreases with the increase of tensile force
The equilibrium state is the state of uniform motion or static state, and the wooden box is static, so it is always in the equilibrium state
The object is in equilibrium, and the combined external force in any direction is equal to zero, so D is correct

1. Fill more than half of the water in a plastic bag and measure it as 9N, then immerse it into the cup containing water. The spring dynamometer shows 6N, and the water surface in the bag is compared with that in the cup (higher than that in the cup). 2. Place a fixed pulley at one corner of the table, put the wood block on the table, hang it with a fine thread and pass through the fixed pulley, pull the fine thread with a force of 0.5N, do not pull it, 0.6N, move at a uniform speed, and use a force of 0.7n, When the force is stopped, the wood block continues to move, and the friction force of the wood block is (0.6N) The wood block is 4N

Buoyancy is equal to the gravity of the discharged water. The discharged water weighs 3N, so 9N water is not completely submerged
At 0.5N, the wood block does not move and is subject to static friction; At 0.6N, the wood block moves in a straight line at a uniform speed, and the sliding friction is equal to gravity; When the tensile force is greater than the sliding friction force, the friction force is the tensile force that moves in a straight line at a uniform speed

1. (multiple choice) to increase the speed of a DC motor, the method you can take is () A. Change the position of the magnetic pole B. Increase the current in the coil C. Increase the supply voltage of the motor D. Increase the strength of the magnetic field 2. As for the conditions for generating induced current, the following statement is correct A. Any conductor moving in a magnetic field produces an induced current B. When a part of the conductor in a closed circuit moves in a magnetic field, an induced current must be generated C. When a part of the conductor in the closed circuit moves along the magnetic induction line in the magnetic field, the induced current can be generated D. When a part of the conductor of the closed circuit cuts the magnetic induction line in the magnetic field, the induced current can be generated

Are you a junior high school or a senior high school? The speed of DC motor is related to many factors. In general, EI = t Ω, EI is the input voltage and current, t is the load torque, Ω is the rotational angular speed and speed, which are directly related. The above four cases can only be analyzed assuming that other conditions remain unchanged. A. change the position of magnetic pole, turn the motor over

Two physics questions about electricity, 1、 Two electroscopes of the same kind have the same amount of negative charge. If their metal balls contact, the foil opening angle of the two electroscopes will not change. Why? 2、 A.b.c3 is the same electroscope. A is positively charged, B is twice as negatively charged as a, and C is not charged. First contact the metal balls of B.C, separate them, and then contact the two metal balls of A.B. and then separate them. At this time, A.B is not charged, and C is negatively charged Why?

1. Because the type and quantity of charge are equal, the charge will not move. Charge movement will occur only when the type or quantity of charge is different
2. This is based on the principle of average charge distribution. First, let the positive charge of band a be + Q and the negative charge of band B be - 2q
After BC contact, according to the principle, they all have points with - 2q + 0 / 2 = - Q
After AB contacts, the charges cancel each other out, so only C is negatively charged and AB has no point

As shown in the figure, a long enough board with a mass of M = 10kg and a smooth upper surface moves to the right along the horizontal ground at an initial speed of V0 = 5m / s under the horizontal tension of F = 50N. There are enough small iron blocks with a mass of M = 1kg. Put an iron block at the rightmost end of the board without an initial speed. When the board moves L = 1m, And put the second iron block on the rightmost end of the board without initial speed. As long as the board moves L, put an iron block on the rightmost end of the board without initial speed. (take g = 10m / S2) ask: (1) After the first iron block is put on, when the board moves L, what is the speed of the board? (2) How many pieces of iron are there on the final board? (3) How far is the last piece of iron from the right end of the board?

(1) The board initially moves at a uniform speed by F= μ Mg solution, μ＝ F
Mg＝50N
100N＝0.5
After the 1st iron block is put on, the wooden board shall make uniform deceleration movement, that is:
μ (M + m) G-F = MA1 is substituted into the data to obtain: A1 = -0.5m/s2;
According to the formula of velocity displacement relationship, there are: v
two
one
−v
two
0
= 2a1l, V1 = 2
6m/s
(2) Let n pieces of iron finally rest on the board. Then the acceleration of the board motion is: an= μ nmg
M
first   After the iron block is put on: 2a1l = v02-v12
second   After the iron block is put on: 2a2l = v12-v22
After the nth iron block is placed: 2anl = vn-12-vn2
From above: (1 + 2 + 3 +... + n) × 2（ μ nmg
M）L=v02-vn2
When the board stops, VN = 0, n = 6.6;
(3) From placing the first iron block to just placing the seventh iron block   In the process of iron block, the expression in (2) can be obtained:
six × (6+1)
two × 2（ μ nmg
M）L=v02-v62
From the top 7   When the movement from the iron block to the board stops, set the displacement of the board as D, then:
two × seven μ mg
Md=v62-0     
Simultaneous solution: D = 4
7m
A: (1) after the first iron block is put on, when the board moves L, the speed of the board is 2
6m/s．
(2) Finally, seven pieces of iron were put on the board
(3) The distance between the last iron block and the right end of the board is 4
7m．