1, D 2, B 3, C 4, D 5, C 6, C 7, B 8, D 9, D 10, d 1, A 12, d1.
27. solution: train speed v= 1000km/ 14h time from Zhengzhou to Nanjing t = 700 km/v = 9.8 h.
28, solution, (1) As can be seen from the figure, get on at 8.48 and get off at 8.58, so the ride time = 10min (2) speed v = s/t = 4.2 km/10 min = 4200m/600s = 7m.
29, solution, (1) 30min = 0.5h (2) v = 50km/0.5h =100km/h (3) t = 70km/100km/h = 0.7h = 42min.
30. solution: t1= 60v1=1.5v2 = 4s2 = 50t2 =15v3 =13.
3 1, solution: correct 1.5
Chapter two, sound phenomenon.
1、C 2、A 3、D 4、A 5、B 6、A 7、C 8、C 9、D 10、B 1 1、ABD 12、ABC 13、BCD
14, vibration 340 15, big tone, high timbre 16, sound can propagate in water 17, B 18, wave sound energy, information 19, too small vibration frequency, vibration frequency.
22. Solution: Sound is absorbed by snow. It is caused by the structure of snowflakes.
Because the loose and porous structure of newly fallen snow can make sound waves reflect many times, thus reducing the sound.
When it snows, there will be many small gaps in the middle. Sound waves will reflect, reflect and reflect again after entering, and will never come out after entering. So the structure of snowflakes can absorb sound, just like activated carbon absorbs particles.
So it's quiet when it snows.
23. Yes. But listen carefully. When the water level in the thermos bottle is from low to high, the tone gradually rises and finally returns to its original state.
24. Solution: Because the speed of light is extremely high, the time for light to travel from the place where it thunders to the eyes can be ignored. It can be considered that lightning is the starting time of thunder, so the distance is s = vt = 2040 m.
25. solution: h=0.6s*340m/s divided by 2= 102m.
26. Vibration and sound can spread energy, and sound can spread through metal and air. Sound-producing objects cannot transmit sound in vibration, stillness and vacuum.
27.( 1) The lower the vibration amplitude of the ruler (initial) (2) The length amplitude of the steel ruler protruding from the edge of the table (3) Yes, the purpose is to exclude the influence of other variables and examine the role of a single variable.
28.( 1), DE (2), 3 124 (3) Control variable method (4) Change the vibration frequency of the string.
There may be mistakes. . .