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The distance between the slide and the lens position changes between 15 ~ 30cm. In order to get an enlarged image on the screen, which of the following focal lengths should be selected () A. focal length 15cm B. focal length 30cm the correct answer to this question is a, but to get an enlarged image, the object distance should be between F < U < 2F. The farther the lens is, the larger the image will be. Why not choose B and a? Be clear,
Get an enlarged image. The object distance should be between F < U < 2F. Now the object distance changes between 15 ~ 30cm. This section is between F < U < 2F
So f = 30cm, so f = 15cm
If the focal length is 30cm and the object distance is between F < U < 2F, it changes between 30 ~ 60cm, not between 15 ~ 30cm
A car is traveling at a constant speed of 30m / s. when the driver suddenly finds an emergency ahead, he starts braking after 0.6s, taxis for 52m after 4.4S, and the car stops. The average speed from finding the situation to stopping is______ m/s．
The driving distance of 0.6s vehicle is s = VT = 30m / s × 0.6s=18m，
The average speed from discovery to stop is v = s total
Ttotal = 18m + 52m
0.6s+4.4s=70m
5s=14m/s，
So the answer is: 14
You must have eaten boiled eggs, which are delicious and nutritious, but you may not have boiled eggs yourself. You can try. What's the feeling of touching eggs when they are just cooked? When the water on the egg skin is gone, does it feel the same?
The eggs that have just been cooked are taken out of the pot and touched by hand. It doesn't feel very hot; The water on the egg skin is gone. If you touch it again, it feels very hot; Cause: water evaporates and absorbs heat
A cube object m with a side length of 0.1M and uniform mass is placed on the horizontal ground, and the pressure on the ground is 5.4 × one hundred and three Pa. as shown in the figure, the cross bar can rotate in the vertical plane around the fixed point O and is tied to the cross bar B The thin rope at the end is connected to the object m through the moving pulley. When the cross bar is lifted vertically at point a with force F, the cross bar is balanced in the horizontal position. At this time, the object M The pressure on the ground is 1.8 × one hundred and three PA, if the force F is still applied, lift the cross bar vertically at 0.1M from point a, so that the cross bar is still balanced in the horizontal position, and the object m The pressure on the ground is 1.0 × one hundred and three PA, it is known that the length of AB part on the cross bar is 0.2m, AB: OA = 1:3, G is 10N / kg, excluding the cross bar mass, moving pulley weight, rope mass and friction. Then the following options are correct （ ） A. The mass of object m is 54 kg B. The density of object m is 0.54 × 103kg/m3 C. The force F is 1n D. The force F is 24N
A. The bottom area of the cube is s = 0.01m2, and the gravity of the object is g = PS = 5.4 through the deformation of the pressure formula × 103Pa × 0.01m2 = 54n, the mass of the object calculated according to the gravity formula M = GG = 5.4kg, the option statement is wrong. B. from the formula ρ= mv=5.4kg0.001m3=5.4 × 103kg / m3, wrong option statement; Force F
Place a spring scale horizontally, fix the left end, and one person pulls the right end of the spring. When the indicated value of the spring scale is 20n, it indicates that the tension of people on the spring scale is______ N. If both ends of the spring are not fixed, Party A and Party B respectively pull the two ends of the spring with forces of equal size and opposite directions. The indicated value of the spring is 20n, then the tensile force of each person on the spring scale is______ N．
If one end of the spring dynamometer is fixed on the wall and the other end is pulled with a force of 20n, the indication of the spring dynamometer is equal to 20n
The spring dynamometer receives a pair of balance forces of 20n, and the indication of the spring dynamometer is equal to the magnitude of this force, equal to 20n;
So the answer is: 20, 20
The stiffness coefficient of a light spring is k = 1x103n / m. at both ends of the spring, Party A and Party B pull the spring with a force of 20n respectively, then the elongation of the spring is_______ m. The resultant force of the spring is ____
At both ends of the spring, a and B pull the spring with a force of 20n respectively
Equivalent to: hang a section of the spring on the wall, and one person pulls the spring with 20n tension
Therefore, elongation = 20 / 103 = 0.19M
Because the spring is always in a static state, the static state is a kind of equilibrium state
So the external force is 0
RELATED INFORMATIONS
- 1. As shown in the figure, use the spring scale to horizontally pull the object block weighing 10N, place the object block on the horizontal desktop, and gradually increase the tension. When the indication of the spring scale is 2.2n, the object block begins to slide. In order to maintain the uniform movement of the object block, the later tension is only 2.0N. Then the maximum static friction between the object block and the desktop is______ N. The dynamic friction coefficient between the block and the table is __
- 2. As shown in the figure, the parallel metal plates a and B placed along the horizontal direction are respectively connected with the positive and negative poles of the power supply. There is a small hole in the center of the two plates along the vertical direction. Now there is a positively charged droplet. The point P directly above the small hole falls freely from rest, and the speed after successively passing through the two small holes is V1. If the plate a does not move, if the key K is kept open or closed, B When the plate is translated upward or downward for a short distance, the same droplet still falls freely from rest at point P, and the velocity after passing through two small holes successively is v2. The following statement is correct without air resistance () A. If key K remains closed and plate B moves downward, V2 > v1 B. If key K is closed for a period of time and then disconnected, and plate B is moved downward, V2 > v1 C. If key K remains closed, V2 = V1 regardless of moving board B up or down D. If key K is closed for a period of time and then disconnected, no matter moving board B upward or downward, V2 < v1
- 3. Make up unit Zhang Wei's height is 16.6_____ 7.5km=______ m； 5.85mm=_____ m； 0.068m=_______ nm； 16.4m=______ um； 1.8h=____ min； 45min=_____ h
- 4. The derivation of the relationship between velocity and displacement in free fall motion
- 5. How to use the formula of uniform variable speed linear motion in senior high school physics compulsory 1 Is under what circumstances this formula is used
- 6. The time taken for an object to fall freely after the last 200 meters is 4 seconds, then what is the falling time and total height of the object
- 7. Particle is an idealized model. In the following examples, what can treat the research object as a particle is () A. Study the motion law of the earth around the sun B. Study the law of the alternation of day and night on the earth C. Study the time required for the train to run from Fuzhou to Shanghai D. Study the time required for a train to pass the Nanjing Yangtze River Bridge
- 8. A car traveling along a straight highway with uniform acceleration, passing two electric poles 50m apart by the roadside, shares 5S. When it passes the second electric pole, its speed is 15m / s, and when it passes the first electric pole, its speed is () A. 2m/s B. 10m/s C. 2.5m/s D. 5m/s
- 9. A physics problem about uniform variable speed linear motion in high school (about the proportional relationship in special laws) A train starts to move at a constant speed in a straight line from a standstill. One person stands in front of the first carriage to observe. The time for the first carriage to pass through him is 4 seconds, and the time for all trains to pass through him is 12 seconds. The distance between carriages is not recorded, and the length of each carriage is equal. It is required to (1) how many carriages in total; (2) How many cars passed this person in the last 4 seconds; How many seconds did the last three sections pass through this person? I hope it's best to use the proportional relationship in the special law, and attach the description and analysis, not just the formula and answer If the three questions are difficult to answer, you can answer the first one first
- 10. The object changes speed under the action of external force When an object changes speed under the action of external force, the kinetic energy must change The kinetic energy of an object remains unchanged, and the combined external force must be zero Why are these two sentences wrong?
- 11. 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
- 12. As shown in Figure 1, the current in a long straight wire changes according to the sinusoidal law as shown in Figure 2. It is specified that the current is positive from left to right. There is an open metal frame below the straight wire. Therefore, compared with point B, the moment when the potential at point a is the highest is () A. T1 time B. T2 time C. T3 time D. T4 time
- 13. An object with a mass of M = 2kg is stationary on the horizontal plane, and the sliding friction force between the object and the horizontal plane is equal to 0.25 times of the elastic force between them. Now, apply an oblique upward pull force with a size of F = 8N and an angle of 37 degrees with the horizontal direction to the object. It is known that sin37 = 0.6, cos37 = 0.8, g = 10, and calculate the displacement through the object in 5 seconds under the action of the pull force
- 14. If a car flies on rainy days at a speed of more than 150 kilometers per hour, will part of the rain rub shoulders with the body because of the strong airflow formed by too fast speed? (provided that there is no rain from the start to the acceleration above 150, and the rain is not very heavy, between heavy rain and light rain) I just want to know whether an object will form a strong airflow around the body because of its too fast speed, and whether this strong airflow will change the orbit of the object to be contacted?
- 15. Moving car and laser beam That's it. I can't understand this question. I want to know how the laser beam moves? Is it a line or a face? Why are there two situations in the car? Where is the entry point?
- 16. As shown in the figure, when a charged body is contacted with a positively charged electroscope, if (1) The opening angle of the metal foil becomes larger, indicating that the charged body carries () charge (2) The opening angle of the metal foil decreases to 0 °, indicating that the charged body carries () charge (3) After the metal foil is closed and opened, it indicates that the charged body carries () charge, and the amount of charge is greater than that of the electroscope ()
- 17. If the threshed grains (such as wheat, rice, etc.) are mixed with a small amount of stems and leaves, shell debris or small sand particles, the grains can be lifted into the air with a lifting machine to separate the grains from the sundries, so as to obtain clean grains. The main reason why grains can be separated from other sundries is that they () A. The speed of being thrown into the air by the hoister is different B. Different density C. The gravity is different D. The air resistance after throwing is different
- 18. A junior high school physics problem about energy, who can solve it? A car's fuel tank was filled with 35kg gasoline. Xiao Wang drove to a place to perform a task, with a distance of 300km. When driving at a constant speed on the highway, the resistance of the car was 920n Note: 30% of the internal energy generated by complete combustion of gasoline is converted into mechanical energy; The calorific value of gasoline is 4.6 * 10 to the seventh power of joules per kilogram Q: (1) the work done by the traction force when the car reaches the destination; (2) The calculation shows whether the car needs refueling during driving Thank you!
- 19. According to the redshift phenomenon of the star spectrum, scientists infer that the universe is expanding and the planet is getting farther and farther away from us. In fact, there is a similar phenomenon in the sound in daily life. At the moment when the train sped past us, although the frequency of the whistle emitted by the train is constant, the tone of the sound we hear is different______ (select "from high to low" or "from low to high")
- 20. On the same desktop, there are a wooden block and an iron block with the same smoothness and different quality on each surface. Explore whose contact surface is rough (there is a spring dynamometer)? (1) Design experiment: (2) Experimental phenomena: (3) Experimental results: Write (1), (2) and (3) respectively