15.答案:(1)3.5(2)解析:(1)木块刚放在传送带上时,对木块由牛顿第二定律得:1mg=ma1,a1=2m/2加速时间:4=2=4s=2s(1分)加速的位移:x1=a14=×2×22m=4m(1分)木块与传送带共遠后做匀速直线运动;与速的时间21-x110=4s=1.55(1分)木块在传送带上运动的时间t=t1+12=3.5s(1分)(2)木块滑到平板车后,当滑到平板车最右端两者恰好共速时平板车长度最小对木块和小车系统由动量守恒定律得:mu=(M+m)v(1分)根据能量守恒定律H2mgLmin-2mu-2(M+m)uD),于(2分)解得:L==16(1分)

One possible versionSeveral minutes later, the ice broke suddenly and Harry's feet sank into the water. It was noep, but his legs got wet to the knees. Harry was so upset that the accident would delay hisschedule. He had to find some firewood to build a fire to dry his clothes. He pulled off his glovestook out his matches, and lighted the fire. Seeing the burning fire, he had thought he was safe. Butsuddenly, without warming, a heavy mass of snow dropped down. The flame of hope was put outUnluckily, his dog ran away and the fear of death came over Harry. He had no choice but torun to save himself. However, he did not feel his feet when they hit the ground. Having falleneveral times, he decided to rest a while. As he lay in the snow, he noticed he was not shaking. Hecould not feel his nose or fingers or feet. A feeling of horror came into his mind. It was when herealized he was going to die that he heard a bark and the noise his friends made

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