21.BC解析:本题考査传送带问题。由υt图像得0~1s煤块的加速度大小a1=8m/s,方向沿传送带向下;1s~2s内,煤块的加速度大小a3-4m/s2,方向沿传送带向下;0~1s内,对煤块应用牛顿第二定律得nisinθ+ anglosθ-ma1;1s~2s,对煤块应用牛顿第二定律得 nisin0- anglosθ=mn2;解得θ=37°,0.25,Δ项错误、B项正确。σ-t图线与t坐标轴所围面积表示位移,所以煤块上滑的总位移x=10m,由运动学公式得下滑时间下=√/2x=5,所以煤块从冲上传送带到返回A端所用的时间为(2+5),C项正确。0~1s内煤块比传送带多运动4m,划痕长4m;1s~2s内传送带比煤块多运动2m,产生的热量为12J,D项错误。

Passage 2I felt sorry that i did nothing today. I wake up at 7: 00wokein the morning as usual. It was raining harder. so I decidedhardA to go to school. During the day times. I did nothinglimeonly sitting and watch You Tube videos. A1 5: 00 in theevening, I went to the market to buy everything to catsomethingAnd all of the sudden. I saw my professor sitting oppositemyself. I felt really embarrassing as I lied to him that Imeembarrasseddidnt go to school because of I wasn't feelingcomfortable. I promised that I wouldn t skip school again

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