英语周报八年级下册第37期答案

参考范文Then, clutching (5mff)the tin can, he headed for the shop. " Ihave the money. " he seriously told the owner. The man went to thewindow and fetched Reubens treasure, He wiped the dust of it andgently wrapped it in brown paper. Then he placed the parcel inReuhen's hands Racing home, Reuben burst through the front door.His mother was cleaning the kitchen stove. "Here, Mom! HereReuben exclaimed as he ran to her side. He placed a small box in herwork-roughened haHis mother unwrapped it carefully, to save the paper. A bluepaper box appeared, and she lifted the lid. In gold lettering on a pair ofred, lambskin gloves was the word Mother. And a card says,"It isMothers Day. We appreciate all the things that Mom does for usThank you! "His mother had never received such a gift; she had nogift except her wedding ring. Spccchlcss, she smiled sweetly andgathered her son into her arms, with tears running down her cheeks.Long live with this happy moment!

18.(1)h≥0.5m(2)45N(3)10【解析】本题考动量守恒定律和能量守恒定律的应用。(1)小球恰能完成一次完整的圆周运动时,设它到最高点的速度为vo,在最高点,仅有重力充当向心力,则有mg=m(1分)在小球从最低点运动到最高点的过程中,机械能守恒,以水平面为零势能面,并设小球在最低点的速度为n,则有m=mg·(1分解得t1=√5m/s(1分设滑块从ho高处运动到将与小球碰撞时速度为n2,对滑块由能星守恒定律有(1分因滑块与小球发生弹性碰撞,故碰潼后速度交换,则n2=√5m′s(1分解得h=0.5m(1分因此滑块B从斜面下滑的高度h≥0.5m(1分)(2)若滑块从h=5m处下滑到将要与小球碰撞时速度为u,同理有mgh′解得u=√95Im s(1分)滑块与小球碰后的瞬间,同理滑块静止,小球以u′=√95m/s的速度开始做圆周运动,因为a>v1,则小球能运动到圆周运动的最高点,根据小球从最低点运动到最高点的过程中,机械能守恒以水平面为零势能面,设小球在最高点的速度为v3,则有o mu=mg 2L+2 mu3(1分}设小球运动到最高点时绳的拉力大小为T,重力与绳的拉力的合力充当向心力,则有T十L(1分)T+mg=m(1分解得T=45N(1分)(3)滑块和小球最后一次碰撞时速度为t=5m/s滑块最后停在水平面上,设它通过的路程为s,同理有mgh=m+mgs(1分小球做完整圆运动的次数为n=(1分解得s′=19m,n=10

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