One physics problem is written and two physics questions are analyzed

It has a diameter of d and has 20 turns.

The total length is: the grid is 1cm, so the two squares are.

So the diameter is:

The diameter of the enameled wire is very small, if it is measured directly with a scale, or it cannot be measured or the error is too large, as shown in the figure, the enameled wire is tightly arranged on the pencil and wound n turns, and the length of the coil is measured L, then the diameter of the enameled wire is d=l n

As can be seen from the figure, there are 10 small squares between the 1cm of the scale, so the minimum graduation value of the scale is the indication of the scale.

Your drawings are very unfigurative, whether they come out from above or from below, but you look like the head and tail are the same. If you don't think about the graph, of course, it's divided by 20.

Read out the total length of 20 turns of enameled wire first, and then divide it by 20.

Reference selection:

Choose any and make it easy.

Generally, the ground is selected, or objects that do not move relative to the ground.

The reference frame is assumed to be immobile. (The frame of reference can be a moving object or a stationary object) 2Relatively stationary motion is based on air.

Assuming that the air is not moving, when there is no wind, it means that the car is also stationary, that is, the car and the air are relatively stationary;

When there is wind, the speed of the motorcycle and the air is different, assuming that one of them is still moving, the other is still moving, and the two are in relative motion.

1.Select c, d (if it is a single choice, it is d, because when the reference object is selected, any object can be used as a reference except for the object of study itself).

2.Relatively stationary motion.

There is no need to explain that any object can be a reference, the motion of the object is relative, if someone else runs with you and the speed is the same, then you see that person is stationary, and the person standing sees you as moving, which is caused by the difference in reference, which also shows the fault of ab.

Fill-in-the-blank questions about relative stationary movement.

Solution: Let the center of gravity be a meter away from end A, and the total length is L

1180×l=g×a（1）

2040×l=g×（l-a）（2）

1) Substitution (2).

2040l=gl-ga

2040l=gl-1180l

g=2040+1180

g=3220n

What is the length of the trunk?

Because it is a trunk of uneven thickness, its center of gravity is definitely not at the center point, so the distance between the center of gravity and point b is x, and the length of the trunk is l

fa*l=g*x

fb*l=g*（l-x)

1180=gl-2040

g=3220/l(kg)

Liquids are also pressurized. Think about it: the water in the container decreases and the pressure at point A is also smaller, so the pressure at point A is also smaller.

The correct answer to this question is ACD

1.Because of the decrease in water, the water level outside the glass tube drops (a correct).

2.Because the glass tube is fixed to the rubber stopper, the length of the insertion into the water remains the same. It can be seen from the absence of water in the tube at the initial equilibrium in the previous state (the lower nozzle of the glass tube is the object of study).

Atmospheric pressure = pressure of the gas inside the container + pressure of the tube immersed in the liquid.

In the process of water outflow, the height of the tube immersed in the liquid becomes smaller, then.

Atmospheric pressure = pressure of the gas inside the container + pressure of the tube immersed in the liquid.

When the liquid in the generator tube is pressed out or the air is immersed inside the container (the latter in this case), the pressure of the gas in the container becomes greater (d is correct).

3.Since there is no water in the tube at the beginning and the lower end of the glass tube is still in water after the valve is closed, the water surface in the glass tube remains unchanged (B error).

4.The pressure at point A on the inner surface of the bottom of the container is both at the beginning and when the valve is closed.

Pressure at point A = atmospheric pressure + liquid pressure from the lower end of the glass tube to the bottom of the container.

Since the length from the lower end of the glass tube to the bottom of the container does not change, the pressure at point A on the inner surface of the bottom of the container does not change (C is correct).

1 One understanding is that the first pure mathematical deduction is blocked.

You make an equation of terms a and b: g object (2 ) = g object + g motion ) 2 = f pull.

then g-object *(1-)g-motion is derived.

G-object = g-motion (1-).

That F-La=g-matter (2 )=G-motion (2(1-)2 2F=G-motion+G-matter.

G Thing G Movement + G Matter.

g motion = g object * (1- ) Here we use the g object key Jianhu 2f ) g motion = 2f * 1- )

Thus choose D

The force is balanced at a constant speed, and the rope tension is half of the gravity of the weight, a

b.=g thing to take the chong god h to eliminate the loss fs

f=g, hs=ghh=gnh=gn

Because n=2. Sentence Lu.

Choose C, the density of water is 1g cm 3, on this figure is the angular bisector of the coordinate axis, make a vertical straight line at any position in the figure, and a, b, water three straight lines have three intersection points, and the density is equal to the mass divided by the volume, in the case of the same volume, the mass of the material is dense, so a water b

m= v, i.e. m is a primary function with respect to v, which is the slope.

Water = , that is, the m and v function of water is a straight line with a slope of 1, and the angle between the straight line and the x-axis is 45° As can be seen from the figure, a water b chooses c

The answer is: c density = m v is actually the slope of the straight line on the graph.

That is, the greater the slope of the straight line in the graph, the greater the inclination of the straight line, the greater the density.

Therefore, the density of A is greater than the density of B, and the density of water is 1 (I will not hit the unit), the slope of the straight line in the image in the above figure is 1, so the inclination angle is 45 degrees, and the inclination angle of A can be judged from the figure to be greater than 45 degrees, and the inclination angle of B is less than 45 degrees.

So a> water" b

(First of all, it is not in the middle, otherwise there will be no time difference.) Also, the second echo we heard was only a difference, so the two echoes came from the cliffs on both sides. Now we divide the distance from the cliff into two sections, that is, with the seat we stand as the dividing line, assuming that the distance to our left is closer, then the response we hear after 1s is to first release the sound from our position to the cliff, and then reflect back to our ears from the cliff, and the second here contains two identical distances, that is, in this second we have walked twice the distance to the left, and of course the distance we hear is from the right, which is also twice the distance (note that the time taken on the right side is, Because the sound we make is moving to both sides at the same time, the right side is farther away, we will hear the echo from the left side first, that is, one second, and after a while, we will hear the echo from the right, the difference of seconds, so the movement time of the right side is) We calculate the distance between the left and right respectively, and pay attention to divide by 2.

The distance to the two sides is different, he is 170 meters from one side of the mountain, and 85 meters from the other.

So the distance between the two walls = 170 + 85 = 255m

When the slider P moves to the A terminal, only R1 is connected to the circuit.

Supply voltage u=i1·r1=

When the slide P is moved to the B-terminus, R1 and Rab are connected in series.

The total resistance in the circuit r total = u i = 6v

Rab = r total - r1 = 60 -30 = 30 The resistance Rab between the sliding rheostat AB is 30.

In the first case, Rab 0,R total R1 and U I1R total 6V

In the second case, R total R1 Rab U I2 60 ohms, Rab 60 30 30

Solution: When p is on the A side, u=i1*r1=

When p is at the b terminal, r total = u i2 = 6v

r2 = r total - r1 = 60 -30 = 30

First of all, it should be remembered: the condition for the induced current to arise is a change in the magnetic flux passing through the rectangular wire frame.

When the wireframe begins to enter the magnetic field, the magnetic flux through the wireframe increases, and an induced current is generated.

When the wireframe is all in the magnetic field, the magnetic flux does not change and there is no induced current.

When the magnetic field is generated, the magnetic flux through the wire frame decreases, and an induced current is generated.

So option c is correct.

The situation of moving from left to right in option C is that A cuts the magnetic inductance line to generate current, AB does not cut the magnetic inductance line and there is no current, and B cuts the magnetic inductance line to generate current. Our teacher just commented.