Physics in the second grade of junior high school Xiao Ming and Xiao Hong want to measure the length of a long enough steel pipe, but they only have two stopwatches and a hammer. At that time, the temperature was 15 degrees. Please help them figure out how to measure the length of the steel pipe

It's very simple. There are two ways to solve this problem. 1. The problem of acoustic distance measurement s = V * t. 2. It's solved by time difference

There is a vibrating mobile phone, which uses a micro motor to drive the blade on the shaft. When the blade rotates, the motor also vibrates, so as to drive the mobile phone to vibrate. So, what shape should the blade be
A: Wrench shape is the type of twist screw B: round C: Fan D: blade shape of fan

Because the mass distribution between the sector and the center of rotation is not uniform, it is easy to produce vibration when rotating

How do we hear sound
Du Lei designed a plan for astronauts to talk on the moon or in space. He said that when they connect their helmets with a stick, they use physics knowledge (), ()

Solid can transmit sound, but vacuum cannot

As shown in the picture, Xiaoming and Xiaogang made a "local telephone" by using the above string and two paper cups containing cola drinks. ① they can communicate between 10m with the "local telephone", which shows that: 1 (2) at the same distance, when the speaker speaks with the same loudness, if he is connected with a thin copper wire, he can hear a louder sound than when he is connected with a nylon string (3) if another student holds a part of the string with his hand while using the "native telephone", the other listener will not be able to hear it__ ．

(1) The sound can be transmitted through the strings of "local telephone", that is, it can be transmitted in solid state; (2) when the sound is transmitted through the thin copper wire, it is louder than that through the nylon rope, that is to say, the effect of metal wire is much better than that of nylon rope; (3) when the string is held by hand, it can't vibrate, so it can't make sound Secondly, the effect of metal wire is much better than nylon rope; thirdly, when the vibration of string stops, the sound can not continue to spread

Find a set of physics papers (sound phenomenon, light phenomenon and motion) in grade two of junior high school. Fill in 10 blanks, 5 experiment questions, 5 calculation questions,

Physics is a science based on. In the first physics class, the teacher put the flask which had just been taken away from the flame and the water had stopped boiling, turned it upside down and poured cold water on the bottom of the flask. 2. In the international system of units, the main unit of length is, the main unit of time

1. Xiaoxing used the sunlight to measure the focal length of the convex lens. With the convex lens facing the sunlight, he looked for the spot on the paper. But he did not carefully adjust the distance between the paper and the lens in the experiment. When the spot on the paper was not the smallest, he measured the distance L from the spot to the center of the convex lens. Then, the actual focal length of the convex lens ()
A. It must be less than L
B. Must be greater than L
C. It could be equal to L
D. It may be less than or greater than L
2. In 2008, the U.S. Mars probe Phoenix entered the Martian atmosphere and landed in the Martian arctic region. After opening the parachute, the vertical falling speed of Phoenix dropped from 1450km / h to 400km / h in 15s. The relationship between the atmospheric resistance F1 of Mars and the gravity F2 of Mars is ()
A. F1 is greater than F2
B. F1 equals F2
C. F1 is less than F2
D. Can't judge

Choose D for the first question, because the spot on the paper may be formed by the straight line propagation of light after focusing by convex lens
It may also be formed when the convex lens has not been focused, so the actual focal length size has two possibilities
Choose a for the second question, because it's a slow motion

6. As shown in the figure, the power supply voltage is u = 18V, and the rated voltage of the small bulb L is 9V. When the slide P of the sliding rheostat moves to the midpoint, the small bulb l will light normally; when the slide P moves to the rightmost end, the actual power of the small bulb L is 6W. Then the rated power of the small bulb and the maximum resistance of the sliding rheostat are ()
A．13.5W；6Ω B．6W；13.5Ω C．6W；12Ω D．13.5W；12Ω
7. There is a resistance wire, which is connected to both ends of a power supply, and its heating power is 100W. If it is folded and connected to the original circuit, its heating power will become ()
A．50W B．200W C．25W D．400W
I hope you can give a brief explanation, especially the first question
The first problem is a series circuit composed of switch, sliding rheostat and light bulb

Choose D
When p is at the middle point, the lamp lights normally, u = UL, then r = 2RL
When p is at the rightmost end, the resistance of sliding rheostat is the largest, and ∵ r sliding = 2RL
U slip / UL = R slip / RL ∵ u slip = 12V, UL = 6V. And ∵ RL = UL2 / P real,
Ψ RL = 6 Ω r slip = 12 Ω P forehead = u forehead 2 / RL = (9V) 2 / 6 Ω = 13.5w
Choose D
After folding in half, the length becomes half of the original, the cross-sectional area is twice of the original, and the R is 1 / 4 of the original
According to P = U2 / R, P is 4 times of the original

Multiple choice questions for the second year of junior high school
The following statements about melting point and freezing point are correct ()
A. All solids have a melting point. B. ice must melt when its temperature reaches the melting point
C. Water does not solidify until its temperature reaches the freezing point. D. the melting point of ice is different from that of water
Please tell me why the other three are wrong again. Thank you!

Let's analyze them one by one. A... crystal has melting point.. amorphous does not have.. b... the melting point of ice and the freezing point of water are both 0 ℃. At 0 ℃, it may be water, ice or ice water mixture... It may not melt... C... right... How can it solidify if it does not reach the freezing point

Two cranes lift two objects AB at a constant speed, GA = 2GB, the ratio of lifting height ha: HB = 1:3, and the ratio of time TA: TB = 2:1. The power ratio of the two cranes is calculated
A.1：3 B.4：3 C.1：12 D.3：1

The work is GH, so the power P = GH / T, and the ratio is 1:3, so a

Can a 220 V, 60 W lamp calculate its resistance

From P = UI = u ^ 2 / R
The resistance of the lamp in normal operation is
R = u ^ 2 / P = 220 * 220 / 60 = 806.7 Ω

Physics in the second grade of junior high school Xiao Ming and Xiao Hong want to measure the length of a long enough steel pipe, but they only have two stopwatches and a hammer. At that time, the temperature was 15 degrees. Please help them figure out how to measure the length of the steel pipe