Physics papers within the scope of the second year of junior high school pay answers .

16、a

17、d18、a、b

19, B21, the ratio of useful work to total work; ＜j ；

Xiao Fei, the water continues to boil after reaching the boiling point, the temperature remains the same, and switching to low heat can keep the water in the pot still boiling, compared with Xiao Nan's practice, they produce the same amount of heat and absorb the same amount of heat, and Xiao Fei is more energy-efficient.

The simplest Galilean telescope consists of a convex lens and a concave lens, with the convex lens as the objective lens and the concave lens as the eyepiece. The second focal point of the objective coincides with the first focal point of the eyepiece. A beam of parallel light from a distant object should converge near the second focal point after entering the objective lens, because the concave lens is inserted in the optical path, and the distance from the second focal point of the objective lens is the focal length of the concave lens, so the light rays are imaged at infinity after passing through the concave lens, and an upright virtual image is observed through the eyepiece.

This seems to be from high school, you are a junior high school strongman class?

A real mirror.

Lessons learned. Look at yourself through the eyes of others.

With copper as a mirror, you can be dressed.

Concave and convex mirrors.

There are only two situations in which B can feel that there is no wind: it is the same speed and direction as the wind, or there is no wind.

And B senses a headwind. It would not be possible to be without the wind. He is faster than the wind; In contrast to him, A is slower than the wind.

So it should be in the same direction as them, the wind blowing eastward, but!! The wind that blows to the east is the west wind (meaning the wind blowing from the west).

Let the first time the air whistle is A, the point where the first time the air whistle echo is B, the point where the second air whistle is C, and the time when the second time the air whistle echo is D

The distance from point A to the cliff is s

then there is ab=10v, bc=32v, cd=6v

3400+10v)/2=s

2040+6v) 2=s-10v-32v, s=1785, v=17

A cunning criminal, in order to prove to the judge that he was not at the scene of the crime at 10 a.m., showed the judge a 10-hour ** that he was in the dance room, but the clever judge knew at a glance that he was committing perjury.

The reason is: Generally speaking, the dance room is surrounded by flat mirrors, and if you shine on the individual from one direction, the image in the plane mirror can also be illuminated. But the ** provided by the cunning criminal, but there is no image of him in the plane mirror, so it can be judged that the ** was grafted later, so it can be judged to be perjury.

32. (1) Why can I only see the presence of birds in room B, but not hear the birds' calls?

2) Because there is a vacuum between the double glazing in room B, and the vacuum cannot transmit sound.

33. Find the speed of the car first, pay attention to the length of the car when passing through the cave, and find the speed of the car to be 10m s. It takes 24s to cross a bridge at this speed, and the distance traveled by the car can be found to be 240m, and the length of the body should also be considered, so the bridge length is 200m.

34. First find the time taken by Xiaogang t=s v=2400m 2m s=1200s. Then find the time of Xiao Ming's movement on the bike, which is 1200s-600s=600s according to the title. Finally, the velocity of Xiao Ming can be found to be v=s, t=2400m, 600s=4ms.

This question can also compare the time taken by the two, Xiao Ming is half the time taken by Xiao Gang, and the distance is equal, so Xiao Ming's speed is twice the speed of Xiao Gang, so it is 4m s.

1. What is the difference between sound and light propagation?

2. Because the vacuum does not transmit sound.

1.When the ball is thrown upwards, the gravitational potential energy becomes larger due to the higher height, and the kinetic energy becomes smaller due to the gravitational force and air friction, and during the parachute descent, the gravitational potential energy becomes smaller due to the smaller height, and the kinetic energy formula: ek = one-half mv, the mass m remains the same, and the v does not change due to the uniform velocity decrease, so the kinetic energy remains the same.

2.Gravitational potential energy formula: ep=mgh, m unchanged, height h, apogee means that it is farther away from the earth, and h is also increased, g is the gravitational acceleration = meter seconds 2, so the gravitational potential energy becomes larger, through the law of conservation of mechanical energy we can know that energy will not disappear for no reason, will not appear for no reason, therefore, kinetic energy is converted into gravitational potential energy, so the kinetic energy becomes smaller, when the satellite moves from apogee to perigee, because the altitude keeps getting lower, so the gravitational potential energy decreases, and the mechanical energy is conserved, So the kinetic energy keeps increasing, so the gravitational potential energy is converted into mechanical energy.

1.In the process of rising the ball thrown upwards, the gravitational potential energy becomes larger and the kinetic energy becomes smaller. In the process of descending, the gravitational potential energy becomes smaller and the kinetic energy remains unchanged. Fill in "Bigger", "Smaller", "Unchanged").

2.Artificial satellites orbit the Earth in an elliptical orbit, and their position is sometimes closer to the Earth and sometimes farther from the center of the Earth. When a satellite is operating from perigee to apogee, the gravitational potential energy becomes larger and the kinetic energy becomes smaller, and when the satellite is operating from apogee to apogee, the gravitational potential energy is converted into kinetic energy.

1.The gravitational potential energy of the ball becomes greater and the kinetic energy becomes smaller as it gets higher and lower. The parachute potential energy becomes less and the kinetic energy becomes greater as it gets lower and lower and the velocity increases.

2.From perigee to apogee, the gravitational potential energy becomes smaller and the kinetic energy becomes greater as it gets farther and farther away from the earth, and from the point of transport to the perigee, the gravitational potential energy becomes greater and the kinetic energy becomes smaller.

Increase, decrease, decrease, and remain unchanged.

Increase decrease gravitational potential energy kinetic energy.

Increase, decrease, decrease, unchanged.

Increase, decrease, gravitational potential energy, kinetic energy.

Bigger, smaller, smaller, unchanged, bigger, smaller, gravitational potential.

, 3, the earth ground bicycle Sun Min himself.

4. The same is relatively stationary.

Relatively stationary. Cart.

Piece. m 8m

One, two, three

Three, the horizon road I me.

Fourth, the same stationary.

Five, six, seven, 293 cars (or drivers).

8. 9:8 8:9

IX, 7 1260

10. The reaction distance is 20m s when there is no drinking

Half an hour after drinking, the response distance is 8 m (20 m s) more than when no alcohol is consumed