(A) (B) (C) (D)

Solution: Set by the problem.

2. If real numbers A and B are satisfied, the range of A is ().

(A)a≤ (B)a≥4 (C)a≤ or a≥4 (D) ≤a≤4.

solution

Because b is a real number, the quadratic equation of one variable about b

The discriminant of is ≥0, and the solution is a≤ or a ≥ 4.

3. As shown in the figure, in quadrilateral ABCD, ∠ B = 135, ∠ C = 120, AB=, BC=, CD =, then the length of the AD side is ().

(A) (B)

(C) (D)

Solution: d

As shown in the figure, the intersection points A and D are AE, DF is perpendicular to the straight line BC, and the vertical feet are E and F respectively.

Known to be available

BE=AE=，CF=，DF=2，

So ef = 4+.

The intersection point a is AG⊥DF, and the vertical foot is g. In Rt△ADG, it is obtained according to Pythagorean theorem.

AD =。

4. In a column number, it is known that when k≥2,

(The integer symbol indicates the largest integer that does not exceed the real number, for example,) equals ().

1 (B) 2 (C) 3 (D) 4

Solution: b

Provide by sum

, , , ,

, , , ,

……

Because 20 10=4×502+2, it = 2.

5. As shown in the figure, in the plane rectangular coordinate system xOy, the vertex coordinates of the isosceles trapezoid ABCD are A( 1, 1), B(2,-1), C (-2,-1) and D (-/kloc-0) respectively. Point P 1 rotates around point b 180, point P2 rotates around point c 180, point P3 rotates around point d 180, …, and repeat the operation to get point P 1, P2, …, and then point P20655.

(A)(20 10，2) (B)(20 10，)

(C)(20 12)，(D)(0，2)

Solution: The coordinates of b and point sum are (2,0) and (2,0) respectively.

Remember, among them.

According to the symmetry relation, we can get:

, , , .

Order, you can also get that the coordinates of this point are (), that is, (),

Since 20 10=4 502+2, the coordinate of this point is (20 10,).

Second, fill in the blanks

6. Given a =- 1, the value of 2a3+7a2-2a- 12 is equal to.

Solution: 0

It is known that (A+ 1) 2 = 5, so A2+2A = 4, so

2 a3+7 a2-2a- 12 = 2 a3+4a 2+3 a2-2a- 12 = 3 a2+6a- 12 = 0。

7. A bus, a van and a car are driving in the same direction at a constant speed on a straight road. At a certain moment, the bus is in front, the car is behind, and the van is between the bus and the car. 10 minutes later, the car caught up with the van. After another 5 minutes, the car caught up with the bus; After another t minutes, the truck caught up with the bus, and then t =.

Solution: 15

At a certain moment, the distance between the truck and the bus and the car is S kilometers, and the speed of the car, the truck and the bus is (km/min) respectively. It is set that the truck catches up with the bus within X minutes.

, ①

, ② .③

From ① ②, you get, so, x = 30. So (points).

8. As shown in the figure, in the plane rectangular coordinate system xOy, the vertex coordinates of polygon OABCDE are O (0 0,0), A (0 0,6), B (4 4,6), C (4 4,4), D (6 6,4) and E (6 6,0) respectively. If the straight line l passes through the point M (2, 3)

Solution:

As shown in the figure, extend the X axis of BC intersection point to point f; Connect OB, AF CE, DF and intersect at point n.

It is known that point M (2 2,3) is OB and the midpoint of AF, that is, point M is the center of rectangular ABFO, so the straight line divides rectangular ABFO into two parts with equal areas. Because point N (5 5,2) is the center of the rectangular CDEF,

A straight line passing through point n (5 5,2) divides the rectangular CDEF into two parts with equal areas.

Then, the straight line is the straight line sought.

Let the function expression of a straight line be, then

Solution, so the functional expression of the straight line is.

9. As shown in the figure, rays AM and BN are perpendicular to line AB, point E is a point above AM, perpendicular AC passing through point A intersects with BE respectively, BN is at points F and C, and perpendicular CD passing through point C is D. If CD = CF, then.

Solution:

See the picture, settings.

Because Rt△AFB∽Rt△ABC, so.

And because fc = DC = ab, that is,

Solve, or (give up).

Rt delta ∽ is rt delta again, so that's =

10. For i=2, 3, …, k, the remainder obtained by dividing the positive integer n by I is I- 1. If the minimum value of is satisfied, the minimum value of a positive integer is.

Solution: Because is a multiple of, the minimum value of is satisfied.

,

Where represents the least common multiple.

because

,

Therefore, the minimum value of the positive integer satisfied is.

III. Answer questions (***4 questions, 20 points for each question, 80 points for * * *)

1 1. As shown in the figure, △ABC is an isosceles triangle, AP is the height on the base BC, point D is the point on the line segment PC, and BE and CF are the circumscribed circle diameters of △ABD and △ACD respectively, connecting EF. Verification:.

Proof: Connect ED and FD as shown in the figure. Because BE and CF are both diameters, so

ED⊥BC，FD⊥BC，

Therefore, the three-point * * * line of D, E and F is .................. (5 points).

Connect Auto Exposure, Auto Focus and then

,

Therefore, △ ABC ∽△ AEF...........( 10)

Let AH⊥EF and vertical foot be h, then AH=PD. It can be obtained from △ABC∽△AEF.

,

Therefore,

So ........... (20 points)

12. As shown in the figure, parabola (a 0) and hyperbola intersect at point A and point B. It is known that the coordinate of point A is (1 4), point B is in the third quadrant, and the area of △AOB is 3(O is the coordinate origin).

(1) The values of real numbers A, B and K;

(2) Take the point A on the parabola as the straight line AC‖x axis, intersect with the parabola at another point C, and find the coordinates of all points E satisfying △EOC∽△AOB.

Solution: (1) Because point A (1, 4) is on a hyperbola,

Therefore, k=4. So the functional expression of hyperbola is.

If the function expression of the line where point B(t,) and AB are located is, then there is

Solve,.

Therefore, the coordinate of the intersection of straight line AB and Y axis is 0, so

, organized,

Solution, or t = (rounding). So the coordinates of point B are (,).

Because both point A and point B are on the parabola (a 0), the solution is .............. (10 point).

(2) As shown in the figure, because AC‖x axis is C (,4), so Co = 4. BO=2, so.

Let the parabola (a 0) intersect with the negative semi-axis of X axis at point D, and the coordinate of point D is (0).

Because ∠ COD = ∠ BOD =, ∠COB=.

(i) Rotate △ clockwise around point O to get △. At this time, point (2) is the midpoint of CO, and the coordinates of this point are (4,).

Extend to point so that =, then point (8,) is the qualified point.

(ii) making a symmetrical figure δ about the X axis to obtain a point (1,); Extend to point so that =, then point E2(2,) is the qualified point.

So, the coordinates of this point are (8,) or (2,). ...................................................................................................................................................

13. Find all prime numbers p and positive integers m that are satisfied.

Solution: set by the problem,

Therefore, since P is a prime number, or ... (5 points)

(1) If, let, k is a positive integer, then,

,

Therefore, therefore.

So the solution is ............ (10)

(2) If, let and k are positive integers.

When, when,

,

Therefore, therefore, it is still 2.

Because it's strange, so, therefore.

therefore

It is out of the question.

When,,; If,, there is no positive integer solution; When there is no positive integer solution.

To sum up, the prime number p=5 and the positive integer M = 9....................(20 points).

What is the maximum number of 20 10 positive integers of 14. 1 2, …, 20 10 that can be taken out, so that the sum of any three numbers can be divisible by 33?

Solution: First of all, the following figures are 61:1,,,,(i.e. 199 1) ......................... (5 points).

On the other hand, suppose it is a number that satisfies the conditions listed in 1, 2, …, 20 10. For any four of these n numbers, because

, ,

So ...

So the difference between any two numbers is a multiple of 33 (10 point).

Let, i= 1, 2, 3, …, n.

By, by,

Therefore, it is ≥11............ (15 points).

≤ ,

So ≤60. Therefore, n≤6 1.

To sum up, the maximum value of n is 61....................... (20 points).

Physics: First, multiple-choice questions (only one of the following questions meets the meaning of the question, with 4 points for each small question and 32 points for * * *).

1. When Xiao Ming was sitting in the front row listening to the lecture, he took the color image projected by the projector on the screen with his camera.

Let's go As the meeting was dark, he used a flash. The photo was taken like this: ()

(1) It is much clearer than without a flash.

(b) The effect is the same as that without a flash.

(c) The image projected on the screen is not clear.

(d) The color is "blinking" and only black characters and lines are photographed.

2. As shown in the figure, the included angle between the speed direction of the elastic ball before landing and the horizontal ground is α, and the elastic ball leaves after landing.

When the angle between the speed direction and the horizontal ground is β, the following statement is correct: ()

(a) whether the ground is smooth or not, there is always β = α.

(b) Whether the ground is smooth or not, there is always β < α.

(c) The greater the ground roughness, the smaller the β.

(d) The rougher the ground, the greater the β.

3. In order to reduce the risk of motorcycle rollover when crossing obstacles, it should be used when landing.

The measures taken are: ()

(a) Only front wheel braking (b) Only rear wheel braking

(c) Both front and rear wheels are braked. (d) The front and rear wheels are not braked.

On September 25th, 2008, at 2 1 p.m, Shenzhou VII spacecraft flew into the blue sky with three astronauts.

After completing missions such as extravehicular activities, he returned to Earth safely on 28th 17: 37. Known: "Shenzhou" flew on the 7th.

The spacecraft travels in a circular orbit 343 kilometers above the earth's surface at a speed of 7.76 kilometers per second. earth

The radius is 6.37× 103km. During the operation of Shenzhou VII spacecraft, the number of times the spacecraft circled the earth was as follows:

15 30 45 60

5. There is a fan-shaped homogeneous metal object, which has the property of expanding when heated and contracting when cooled, as shown in the figure. Room temperature state

The central angle formed by AB edge and CD edge is α. If the temperature of the object becomes uniform

When rising, the change of α angle is: ()

bigger and bigger

(b) unchanged

minify

nondeterminacy；uncertainty

6. As shown in the figure, place a cuboid block M in the container.

There is an iron block m on it, and the wood block rises to the surface.

The degree is h 1 (Figure A); Tie the iron block to the wood with a rope.

Below the block, the height of the surgical block exposed from the water is h2 (Figure

b)； After the string was cut (Figure C), the block of wood floated out.

The water height h3 is: ()

(A) h 1 +ρ iron (h2- h 1)/ρ water (B) h2 +ρ iron (h2- h 1)/ρ water.

(c) h 1 +ρ wood (h2- h 1)/ρ water (D) h2 +ρ iron (h2- h 1)/ρ wood.

7. The picture on the right shows the right rear wheel of a moving car from the back of the car on a horizontal highway.

As you can see: ()

(a) The car turns to the left, and the direction in which the right rear wheel acts on the ground is downward and left.

(b) The car turns to the left, and the direction in which the right rear wheel acts on the ground is downward to the right.

(c) The car is turning to the right, and the direction of the force exerted by the right rear wheel on the ground is downward and left.

(d) The car turns to the right, and the direction in which the right rear wheel acts on the ground is downward to the right.

8. The conductance (reciprocal of resistance) of tin oxide changes with the change of CO concentration in the surrounding environment. The straight line in Figure A reflects the relationship between its conductance and CO concentration. The sensor is made of tin oxide and consists of the circuit shown in Figure B, in which the reading of voltmeter can reflect the CO concentration in the surrounding environment of the sensor. Then in the following diagram showing the relationship between CO concentration and voltage representation number Uo, it is correct: ()

Second, fill in the blanks

9. There is a certain amount of iron filings on the desktop. Now, as shown in Figure A, the N pole of two identical bar magnets A and the S pole of magnet B are placed above the iron filings to absorb a certain amount of iron filings. If the poles with iron filings are put together, the iron filings adsorbed at the joint will be _ _ _ _ (select "increase", "unchanged" or "decrease"); As shown in Figure B, gently place a soft iron bar slightly longer than the distance between the two magnets.

On the magnet, the absorbed iron filings will be _ _ _ _ _ _ _ (optional: increase, unchanged or decrease).

IO。 As shown in the figure, it is a "drip irrigation" device for irrigating crops. In the figure, p is the total inlet width and q is the special emitter. The dripper has the same number of drops per minute and the same amount of drops each time. There are 100 emitters on a straight pipe with the same cross-sectional area. The water velocity in the water pipe between the 36th and 37th emitters is v 1. If the water velocity in the water pipe between the 64th and 65th drippers is v2, the relationship between the velocities is v1_ _ v2 (optional: ">", "<" or "=", and VL: v2 = _ _ _ _ _.

1 1. Put a solid cube with a side length of 10 cm into the bottom of a cylindrical thin-walled glass container with a bottom area of 200 cm2 and a height of 20 cm, and then gradually pour some liquid into the container. The figure on the right reflects the relationship between the pressure F on the block at the bottom of the container and the depth h (0 ~ 6 cm) of the liquid poured into the container.

It can be seen that the density of this liquid is _ _ _ _ _ _ _ kg/m3, and when the depth h of the poured liquid is l2 cm, the pressure F on the block at the bottom of the container is _ _ _ _ _ _ _ _ _.

12. As shown in the figure, there is a meadow along a long straight road, and the distance represented by the side length of each small square in the figure is 6 meters. The maximum speed of Xiao Zhang moving along the straight road on the edge of the grass is 6 meters per second, and the maximum speed of Xiao Zhang moving on the grass is 3 meters per second. Please mark the distance from the edge of the grass to the grass in 6 seconds. He started with A and chose the right path. The shortest time to reach P is _ _ _ seconds (accurate to 0. 1 second).

13. In the circuit shown in the figure, the current flowing through resistors R 1 and R2 can reach the specified values I 1 and I2 by adjusting the rheostat R and R', and the current value is indicated by the ammeter.

The correct operation steps are:

① Before connecting the circuit, the slider of rheostat R should be placed at the (optional: "A" or "B") terminal, and the slider of rheostat R' should be placed at the _ _ _ _ _ _ _ _ (optional: "A" or "B") terminal;

② After the circuit is connected, adjust the resistor _ _ _ _ _ _ _ (optional: "R" or "R'") to make the ratio of ammeter A 1 to ammeter A2 I 1/I2, and then adjust _ _ _ _ _ _ _ _ _ _ (optional: "R"

Three, the calculation problem (this question ***27 points)

14.(8 points) In the blender container.

There is water with a mass m of 0.5 kg, and the water is

After heating to 70℃, let it self-heat at room temperature.

Dynamic cooling. Its temperature changes with time.

As shown in the figure. Now start the electric agitator.

The cooling water is continuously stirred and driven by electricity.

The power of this machine is 900 watts, and the work it does.

80% is converted into internal energy of water. If you don't take the exam,

Considering the internal energy change of the container and the final temperature of water.

What is the temperature?

15.(9 points) The picture shows the working principle of a four-cylinder engine: the internal combustion engine connects the pistons of four cylinders to a crankshaft through connecting rods, so that the working processes of each cylinder are staggered. The crankshaft is connected with the flywheel. Every half revolution of the flywheel, one cylinder is doing work, and the other three cylinders are doing suction, compression and exhaust respectively. The main technical indicators of a four-cylinder engine are shown in the following table, in which the displacement is equal to the sum of the working volumes of four cylinders, and the working volume of the cylinders is equal to the product of the area of the piston and the distance of the piston moving up and down (that is, the stroke length), which is expressed by the rotational speed.

The revolutions per minute of the flywheel. Q:

(1) How much work does the engine do every half revolution of the flywheel?

(2) If the pressure of gas on the piston in the power stroke can be regarded as constant pressure, what is the pressure?

16( 10 minute) As shown in the figure, the device can measure the heat q required for each kilogram of water at 100℃ to evaporate into water vapor at 100℃ at atmospheric pressure. The measuring principle of the device is as follows: water is boiled by a heater, and vaporized water vapor is liquefied by a condenser and collected in a measuring cup; By measuring the heating power of the heater and the water quality collected by the measuring cup for a period of time, the value of Q can be obtained according to the energy conservation relationship. The heat dissipation power of this vaporization device is constant, but the value of heat dissipation power is unknown. When the heater power is measured to be 285.0 watts, the mass of liquid collected by condensation within 300 seconds is 28.0 grams; When the power of the heater is 100.0W, the mass of the liquid condensed and collected within 300 seconds is 4.0g g. According to the above data, the heat q needed to be absorbed by the evaporation of water at 100℃ per kilogram at atmospheric pressure is calculated.

Four, the experimental problem (this problem *** 14 points)

17, Xiao Wang needs to measure the resistance of an unknown resistor Rx, but there are only the following devices around him.

(a) a pow supply with a constant voltage and an unknown value;

(b) An ammeter with a total scale of 30 grids and an uncalibrated range;

(c) Resistance box R0(R0(0-9999 ohms);

(d) An electric key and enough wires.

(1)(4 points) Xiao Wang uses the above equipment to design as shown in the figure.

Circuit, measuring the resistance of unknown resistor. Please write the calculation relationship.

Point out the meaning of each quantity in the relationship.

(2)(4 points) In order to accurately measure the resistance of the unknown resistor, it is necessary to change the resistance of the resistor box for multiple measurements. When the key is closed for the first time, the value of the resistance box is _ _ _ _ _ _ _ (optional: "9999 Ohms max", "0 min" or "any value"). The first deflection of the ammeter is 1. Please calculate and analyze the acceptable range of the secondary measurement resistance box.

(3)(6 points) Considering the possible reading errors in the experiment, we can use images to find such errors and eliminate these data through repeated measurements. The following are six sets of data measured by Xiao Wang, one of which has an incorrect reading. Please circle the answer. The wrong reading is that the serial number is group _ _ _ _, and the resistance of the unknown resistor is _ _ _ ohm.

Serial number l 2 3 4 5 6

Resistance box reading/ohm 230 130 105 80 55 40

Current meter reading/(grid) 6.09.012.015.0 20.0 25.0

Verb (abbreviation of verb) judgment reasoning problem (47 points in this question * * *)

18.( 15 points)

(1) In ancient times, the ancient Greek philosophers represented by Aristotle had guessed that the earth was spherical through observation, and some phenomena in our daily life can also explain that the earth is spherical. Give an example to illustrate.

(2) When people observe objects outside the earth, they will be influenced by the earth's atmosphere. Assuming that there is no atmosphere on the earth's surface, the observed sunrise time is _ _ _ _ _ _ _ compared with the actual atmosphere: "it will be advanced", "it will be delayed" or "unchanged"). Simply put, the reasons for this phenomenon are: _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _.

(3) People observe the moon on the earth and find that the side of the moon facing the earth is always the same. This is due to: _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _.

(4) Astronomical observation shows that almost all distant stars (or galaxies) are moving away from us at their own speed, and the farther away the stars are from us, the greater the speed of moving away from us (called retrogression speed); That is to say, the retrogression speed V of different stars in the expansion of the universe is directly proportional to their distance R from us, that is, v=Hr, where H is a constant, which is called Bo constant. Therefore, scientists put forward the big bang theory: the universe was formed by a fireball of the big bang, which has been 65.438+037 billion years. After the Big Bang, all the stars moved outward at different speeds. What constant H = _ _ _ _ _ _ _ _ _ measured in the above astronomical observation? Recent measurements have found that the Hubble constant is increasing, which indicates that the universe is expanding at the speed of _ _ _ _ _ _ (optional: "acceleration", "uniform" or "deceleration").

19.(6 points) The center of gravity of the uniform triangular plate is at the intersection of the three midlines of the triangle, and the center of gravity of the uniform thin rod is at the midpoint of the rod. At present, there is an isosceles right-angled triangular plate and three uniform thin rods. The length of the three thin rods is equal to the side length of the triangular plate, and these three thin rods form a triangle as shown in the figure. Let the center of gravity of the triangle be p, and the center of gravity of the triangle composed of three thin rods be P'. P and P' are not drawn in the figure. The following are the views of three students: one student thinks that the positions of P and P' coincide; Student B thinks that the positions of P and P' are not coincident, and the distance from P to hypotenuse is greater than that from P' to hypotenuse, while student C thinks that the positions of P and P' are not coincident, and the distance from P to hypotenuse is less than that from P' to hypotenuse.

Please judge the views of the above three students through analysis.

20. (12min) There is a rectangular hole with the size of a×b at the horizontal bottom of the container. Cover the hole with a cylinder with radius a and length b (as shown in the figure, it is a side view). Now slowly inject the liquid with density ρ into the container, and try to analyze and explain how big the mass m of the cylinder should be so that it will not float on any liquid surface.

2 1.( 14 points)

(1)(8 points) Small light bulb (point light source)

Out of the light spread outward along the radius, in a single

The transmission of bit time is perpendicular to the propagation direction.

Light energy per unit area is called light.

Strong. Some students designed and made a picture.

The experiment shows: put a "6 volts, 8.0"

Waber's small light bulb is connected to the circuit to make it

Ordinary light, in the same bulb filament.

Put one on a horizontal surface, facing the light.

Light intensity sensor to determine the relationship between light source and light source.

When the distance is d, measure the corresponding light intensity value i.

Obtain a plurality of groups of data; Marking data in

On the I- 1/d2 coordinate diagram, a cross is obtained.

The straight line of the origin. Find out according to the chart.

This small light bulb converts electric energy into light energy.

Efficiency. (Calculation of surface area of known sphere

The formula is S=4πR2, and r is the radius of the sphere).

(2)(6 points) L 1 and L2 are two street lamps for lighting with rated electric power of 200 watts, and their luminous efficiency and light energy propagation law are the same as those in the above experiment. L 1, L2 is installed at a height of 5 meters from the ground, and P is a point on the road directly below the midpoint of the street lamp connection, so that the light intensity irradiated to point P is not less than 0.0 13W? M -2. What is the maximum distance between two adjacent street lamps?

Shanghai 23rd Junior High School Physics Competition (Datong Middle School Cup)

Reference answer

The total score of the topic is one two three four five.

14

Question 1 5

Topic 16

Topic 17

Topic 18

Topic 19

Question 20

Question 2l

title

score

cheque

score

grade papers

First, multiple-choice questions (4 points for each small question, 32 points for * * *)

Lu Hao 1 2 3 4 5 6 7 8

Answer c d b c b b b b d

score

grade papers

Fill in the blanks (6 points for each small question, 30 points for * * *)

9, reduce, reduce

10 、>; , 16 : 9

1 1、 1.25 x 103，7.5

12、

16.4

13、① B、a

②R’，R，A 1(A2)，I 1(I2)

Three, the calculation problem (this question ***27 points)

14, (8 points)

Solution: Agitator can increase the internal energy of water per second: E=Pt=900×l×0.8=720 angle ... (2 points)

Q=cm△t indicates that these energies can increase the temperature of water:

As long as it is found that the position of temperature change with time when water cooling is equal to 0.343℃/ s, corresponding to

The temperature represented by the point is the final temperature of water. Draw a straight line AB in the diagram and change the temperature along AB.

The speed is equal to 0.343℃/ sec. Let the parallel line A'B' of AB be tangent to the drawing line, and read the vertical position of the tangent point P.

The final temperature of water is 30℃.

According to the drawing, the answer is given at 27℃ ~ 33℃. ............... (4 points)

15.(9 points)

Work done by the engine per second: w=P.t= 120 kJ.

The crankshaft completes 6000/60 = 100 revolutions per second.

(1) The work done by the engine on the flywheel every half cycle.

(2) In each power stroke, the work done by gas is W= p. △V= p×O.5L=600 joules.

P= 1.2× 106 Pa can be obtained.

Note: According to different solutions, the conclusion is given the same correct score.

16.( 10)

From the law of conservation of energy, we can know the power generated by heating system.

Where m is the mass of water collected within 300 seconds, and q is the water that needs to be absorbed for evaporation per kilogram 100 degrees Celsius.

Heat, P0 is the lost power. Two equations can be obtained by using the data in the table.

Four, the experimental problem (this problem *** 14 points)

17 (1) First measurement: the resistance box value is R0 1, and the current indication number is NI grid;

Change the resistance of the resistance box, and measure for the second time: the value of the resistance box is R02, and the current representation number is N2 grid.

This relationship can be obtained from the voltage across the battery:

(2) The maximum value is 9999 ohms ... (2 points)

Let the current represented by the ammeter deflection of 1 grid be I, and the power supply voltage is E=I(9999 ohms +Rx).

The maximum deflection grid number of the second ammeter pointer is 30 grids, corresponding to the R0 value of the resistance box, including:

30I(Ro+Rx)=I(9999 ohm +Rx) Ro=333.3 ohm -0.967Rx.

Since Rx is unknown, it can take any value, and it can be known that:

Acceptable value range of resistance box: the minimum value is 334 ohms, and the maximum value is 9999 ohms ... (2 points)

As can be seen from the figure:

The wrong reading is the serial number.

2 zu

The unknown resistance is 20.

Ohm.

Scoring standard:

Images (2 points)

2 (2 points)

20 (2 points)

Note: The resistance of the resistor is based on

The produced image gives points in the range of (15-25) ohms.

Four, judgment and reasoning questions (this question ***47 points)

18( 15 points)

(1) When a ship leaves port, the hull is not seen first, and then the mast is not seen at sea: solar eclipse.

(2) It will be postponed; Atmospheric refraction of solar light

(3) The rotation period of the moon is equal to the period of revolution of the moon around the earth.

(4) 2.19×10-5km/(sec? Light years) or 2.31×10-1 8 seconds-1acceleration.

19.(6 points)

Student B is right, but students A and C are wrong. (2 points)

For triangles: Let the height on the hypotenuse of a right triangle be h, then the center of gravity P of a uniform triangle is from the hypotenuse.

The distance is h/3 = o.333h.

For triangle: according to the length of three thin rods, the mass is set to

Obviously, the distance from the center of gravity of two right-angled sides with a total mass of 2m0 to the hypotenuse is h/2.

Considering the quality of the hypotenuse, we can know that the distance from the center of gravity to the bottom of the triangle is:

0.293h & lt； 0.333h, so the distance from p to the hypotenuse is greater than the distance from p' to the hypotenuse.

(2)(8 points)

Let the electric power of the street lamp be P0, and the included angle between the connecting line between point P and the street lamp on the ground and the ground is, then the ground

The light intensity perpendicular to the light propagation direction at point p is:

Considering that two adjacent lights are lit at the same time, and the ground is not perpendicular to the light propagation direction, the actual light intensity on the ground is:

According to the meaning of the question, this light intensity should be equal to 0.0 13W? Mi 2.