牛顿第二定律以及变质量问题

2425920061()JournalofPhysicsTeachingVol124No1259(S)11200613711,2,3,400715200512(1),,2005(2),11:F_=kdp_dt,F_=ma_,,,()/(),F_=kdp_dt,:thechangeinmotionisproportionaltothemotiveforceimpressed;andismadeinthedirectionoftherightlineinwhichtheforceisimpressed,(I_),:I_(mv_),F=kd(mˆv)dt,1750F=mˆa,,,::,2,:tm,v_(vnc),mu_,tm,v_+v_mmF_,,(m+m)(v_+v_)-mv_-mu_=F_t,:ddt(mv_)-dmdtu_=F_,u_mmmdmdt451:2:345:,()Vol124No1259(S)1120061381JournalofPhysicsTeaching2425920061()(),F_,u_=0,F_=d(mv_)dt;u_=v_,F_=mdv_dt,,:,,S,()v,,dtvSdt,vSdtdt,v_=v_-0=v_,dp_=vSdtv_,dp_dt=vSv_,,dp_dt=vSv_=F_F_,ox,,,FX=Sv2,F_,Sv2,,,33:L0m0v,L,F?2L,Lt,(F-mg)t=mv,t,t,Fm,u_=0,dmdt=m0,dLdt=m0v_:ddt(mv_)=v_dmdt+mdv_dt=v_dmdt=m0v_2=F_,F_,v_GF_=F_+G_=m0Lg+m0v_24:,,,,,1:n(n),m0,(n-1),(n-1)m0g,,,,L,x(L,x),,v_=0,m=(L-x)dmdt=-dxdt,u_=dxdt=2g(L-x):F_=-dmdt=dxdtdxdt=(2g(L-x))2=2g(L-x)G_=g(L-x)F_=F_+G_=3g(L-x)=3G_,Lx,1,tm,m,1m:tmt:t=0.01smL,mt2gL:ŠF=m(g+2gL/t),:ŠF=m(g+2gL/t)=2gLt(g+2gL/t)=g2gLt+2gL2gLŠF1010m()