题干

如图甲所示,物块A、B的质量分别是mA=4.0kg和mB=3.0kg.用轻弹簧栓接,放在光滑的水平地面上,物块B右侧与竖直墙相接触.另有一物块C从t=0时以一定速度向右运动,在t=4s时与物块A相碰,并立即与A粘在一起不再分开,物块C的v﹣t图象如图乙所示.求:

①物块C的质量mC

②墙壁对物块B的弹力在4s到12s的时间内对对B的冲量I的大小和方向;

③B离开墙后的过程中弹簧具有的最大弹性势能Ep

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解:①由图知,C与A碰前速度为v1=9m/s,碰后速度为v2=3m/s,

C与A碰撞过程动量守恒,以C的初速度反方向为正方向,

由动量守恒定律得:mCv1=(mA+mC)v2,解得:mC=2kg;

②由图知,12s末A和C的速度为v3=﹣3m/s,4s到12s,

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The Farmers' Fresh Market

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    ____It was a very small market,but a very interesting one. I don't know why farmers go there to sell vegetables or why people go there to buy them because there are many more products in large supermarkets.____

    Also ,all the things sold there were very novel. In addition to fresh food,there was homemade food and many other things such as woolen blankets,quilts ,and wooden products.

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G. I think the reason is that the ones sold in the Farmers' Market are fresher and cheaper.