变质量质点的动能定理与软绳问题

183　　　Vol.18No.31999　3COLLEGE　PHYSICSMar.1999　:1997-12-10;:1998-09-15石东平(,　402168)　,.　;;;　O313.61　1　1,L、M,F,v,,,F.1F=MLv2,,FL=12Mv2,F=M2Lv2.?.2　[1～4]mdvdt=F1+F2+(u-v)dmdt(1)udmmm,F1、F2mdm,F2+(u-v)dmdtdmm,,2,Υ=F2+(u-v)dmdt(2)mdvdt=F1+Υ(3).(a)(b)2(3)dr=vdtmdvdt·vdt=(F1+Υ)·dr12md(v2)=F1·dr+Υ·dr(4)(4).12md(v2)=12m(v+dv)2-12mv2,:.,,dm,dm.,[5～6],“”,.3　()mdm,ΥΥ′,:dm12dm(v+dv)2-12dmu2=(F2+Υ′)·dr′Υ′Υ′·dr′=12dm(v2-u2)-F2·dr′ΥΥ·dr=F2+(u-v)dmdtM2·dr=F2·dr+dm(u-v)·v　dA=Υ·dr+Υ′·dr′=F2·(dr-dr′)-12(u-v)2dm(5).(F1、F2)(Υ、Υ′),,.F2=0,Υ=(u-v)dmdt(6)dA=-12(u-v)2dm(7),,dm0,dA0,dEkdA,(1),;dm0,(7)dA0,,,dEkdA.4　4.1　1,x,(4)0=Fdx+0+(0-v)dmdtdxdmdt=ddtMLx=MLdxdt=MLvF=MLv2.x,A=Fx=MLxv2,ΔEk=12MLxv2,(7)A=∫x0-12(0-v)2MLdx=-12MLxv2,,F,,,,.4.2　2　λ,v(3),xF(x)?3x0=F(x)dx-G(x)dx+0+(0-v)dmdtTOdxG(x)=λxg,dm=λdx,dx=vdtF(x)=λv2+λgxx,()A=λxv2,ΔEk=λxv2/2,A,,.,.,,.(27)233　　　　　　　　　　　　:AMETHODTOCORRECTτ-VALUEOFTRANSIENTPROCESSINRLCSERIESCIRCUITCaiXuhong　　LiShaohui(DepartmentofPhysics,ShantouUniversity,Shantou,Guangdong,515063,China)Abstract　AtestmethodoftransientprocessinRLCseriescircuitisproposedandtheproblemofdeviationbetweenthemeasuredτ-valueandthecalculatatedoneisresolved.Keywords　transientstateprocess;relaxationtime;resonance(23)5　1　,.().:,1989.1652　..,1988,7(3):123　..,1989,8(11):184　..,1991,10(10):185　..:,1982.546　..,1995,(4):6ATHEOREMONKINETICENERGYFORVARIABLEMASS-POINTANDSOFT-ROPEQUERYShiDongping(DepartmentofPhysics,ChongqingTeacher'sCollege,Yongchuan,Chongqing,402168,China)Abstract　Atheoremonkineticenergyforvariablemass-pointandformulaofworkofrecoilin-ternalforcesaregivenandthereasonofsoft-ropequeryisdiscussed.Keywords　variablemass-point;theoremofkineticenergy;workofrecoilinternalforces;soft-ropequery(24)1　,..2.:,19852　СЭ.　.::,1958DISCUSSONEDDY-CURRENTINSILICONSTEELPLATEDuKaichu(DepartmentofPhysics,LongyanTeacher'sCollege,Longyan,Fujian,364000,China)Abstract　Itisanalysedquantitativelythatthedensityofeddy-currentlossareproportionattothesquareofthicknessofthesiliconsteelplate.Keywords　magneticflux;resistance;inductionelectromotiveforce;eddy-current;eddy-cur-rentloss273　　　　　　　　　　　　:RLCτ