Physics problems in the second grade of junior high school Zhang Hong wanted to measure the speed of sound propagation in the air. She carried out the following experiments: (1) with a leather tape measure, she measured the distance between her and the wall as 208m; (2) with a shout, she measured the time of hearing the echo as 1.2s with a stopwatch. Please help her find out the speed of sound propagation in the air

Physics problems in the second grade of junior high school Zhang Hong wanted to measure the speed of sound propagation in the air. She carried out the following experiments: (1) with a leather tape measure, she measured the distance between her and the wall as 208m; (2) with a shout, she measured the time of hearing the echo as 1.2s with a stopwatch. Please help her find out the speed of sound propagation in the air

Distance of sound transmission
s=208m×2=416m
sound velocity
v=s/t=416m÷1.2s=346.7m/s

In the second physics problem of junior high school, we use the formula to find the density: (ROU) = V / m
Measure the density of milk with balance, weight, enough water, beaker, etc

Steps:
(1) The mass of empty beaker measured by the adjusted balance is M1;
(2) Pour proper amount of water into the beaker and mark the water surface. The total mass of beaker and water is m2;
(3) Pour out the water, pour the milk into the beaker to the mark, and measure the total mass of milk and beaker is m3
(4) Mathematical expression: ρ milk = (m3-m1) ρ water / (m2-m1)

The calculation of physical density in grade two
A steel ball with a volume of 30cm and a mass of 89g
(1) This steel ball is hollow. What is the volume of the hollow part?
(2) If the hollow part is filled with water, what is the total mass? (density of copper = 8.9 * 10 ^ 3kg / M)

（1）
89g/8.9*10^3KG/M=10cm^3
The volume of hollow part is 30-10 = 20cm ^ 3
（2）
20*1=20g
89+20=109 g=0.109 Kg

It is known that the propagation velocity of infrasound wave in water is 1500m / s. if the distance from the center of a tsunami to the nearest land is 30000m, and the wave speed is 200m / s, how long does it take for the shore instruments to receive the infrasound wave from the earthquake to land the tsunami wave?

S=300000/200-300000/1500
=1500-200
=1300s

1. A car drives to a steep mountain at a speed of 10 m / s. The driver honks his horn and hears the echo after 4 s. how many meters is the car away from the mountain in front when he hears the echo?
When the bullet leaves the target, the distance between the bullet and the target is 340.5 meters per second?
To complete and standardize the process and answer,

If the sound speed is standard 340m / s, V1 = 10m / s, V sound = 340m / s, t = 4S, s 1 S2 S3 S4 solution, s = V sound * t = 340m / s * 4S = 1360ms1 = V1 * t = 10m / s * 4S = 40ms2 = S + S1 = 1360m + 40m = 1400ms3 = 1400m / 2 = 700ms4 = 700m-40m = 660m, the second question is that the sound speed is standard 340m / s, s = 340m

One student knocked on the straight iron pipe (full of water) conveying water, and the iron pipe made a clear sound. The other students put their ears close to the iron pipe at different positions to hear the sound. A said that they heard only one sound; B said that they heard two sounds; C said that they heard three sounds. It is known that the propagation speed of sound in the air is v gas = 340m / s, and water is v water = 1700m / s, Steel is v steel = 5100 M / s. please explain by calculation: what is the distance between the position of ABC three students and the knocking point? (if the two sounds reach the human ear, if the time interval between the two sounds reaches the ear is greater than or equal to 0.1 s, the human ear can distinguish the two sounds.)

(1) Suppose: the distance between student a and tapping point S1; the time of sound coming from air = S1 / 340; the time of sound coming from water = S1 / 1700; the time of sound coming from steel = S1 / 5100; student a only hears one sound, which means that the difference between the fastest time and the slowest time is less than 0.1 second, that is: (S1 / 340) - (S1 / 510

Xiaoming and Xiaotao go to the playground on Sunday to buy some helium balloons. The naughty Xiaoming unties one of them, inhales a big mouthful of helium, and then yells at Xiaotao. Xiaotao finds that Xiaoming's beautiful voice turns into an irritating "Donald Duck" sound. Xiaotao is puzzled,

The reason is that sound waves propagate faster in helium (short period and high frequency), so as long as there is enough helium concentration in the sound producing position (for example, vocal cord), the sound can be changed

Observation of flowers to write about 400 words of life sentiment composition, urgent

（1） Stimulate interest observation is the basis of perception and writing. Interest in observation is the premise of cultivating students' observation ability. Without interest, there is no cultivation of observation ability. Therefore, we should try every means to stimulate students' interest in observation

Write a sentiment on life through ants (composition)
More understanding of life

I found a team on the grass in the park. The team became a winding path. What's this? I fell down and saw that it was an ant team. They were busy! A group of ants moved soil from the cave, while another group of ants carried food to the cave, and a group of ants were shuttling back and forth, as if directing other ants to work
Out of curiosity, I took out a paper cup, poured some mineral water into the paper cup, and put some ants into it. Facing the sudden "flood", these ants panicked and struggled desperately in the water. Their tentacles kept swinging, as if they were sending out signals for help, Gradually, a few ants gathered together, as if they were discussing the way to escape the "flood". While discussing, the ants waved their antennae to send out signals, as if telling other partners how to escape. Finally, step by step, the ants swam to the edge of the paper cup and escaped the "flood", The ants scrambled out of the paper cup and joined the army in a neat line
After a while, I put a small biscuit not far away from the ants. Eh, the ants are so clever that they smell the smell of biscuits. One ant first climbed up to the biscuit, circled around the biscuit, and then climbed up. Then, four or five ants came up, and they also climbed up on the biscuit, touching the front ant, It seems to be muttering something. About a minute later, these ants climbed back to the army together. Why did these ants walk away without eating biscuits? Isn't the biscuit delicious? After a while, a huge ant team came to the biscuits and looked at the biscuits that were dozens of times bigger than their own body. They seemed to be worried about whether they could move them. After a long time, they were worried, These ants keep swinging all over their bodies, as if they are doing warm-up exercises. Then, the ants get under the biscuits one after another. Oh, the big biscuits are shouldered by the ants with concerted efforts. I am really surprised to see the ants carry the biscuits back to the cave step by step. It turns out that the collective strength is so strong. No wonder the saying goes, "people unite, Mount Tai moves"!

Writing training. Learn to write a poem with the title of "grass" to express the feelings of life. Write it yourself

Withered and withered every year
It's green all the time
Green onion does not change withered
It's not easy to be green when withered
The meaning is similar, but the charm is a little different
Withered and withered every year
It's green all the time
It's not going to change every year
Every year is not easy to be green