Elastic Framework

n physcs, n lstc cllsn ccrs btwn tw physcl bjcts n whch th ttl kntc nrgy f th tw bds rmns th sm. n n dl, prfctly lstc cllsn, thr s n nt lss f kntc nrgy nt thr frms sch s ht, ns, r ptntl nrgy. Drng th cllsn f smll bjcts, kntc nrgy s frst cnvrtd t ptntl nrgy ssctd wth rplsv r ttrctv frc btwn th prtcls (whn th prtcls mv gnst ths frc, .. th ngl btwn th frc nd th rltv vlcty s bts), thn ths ptntl nrgy s cnvrtd bck t kntc nrgy (whn th prtcls mv wth ths frc, .. th ngl btwn th frc nd th rltv vlcty s ct). Cllsns f tms r lstc, fr xmpl Rthrfrd bckscttrng. sfl spcl cs f lstc cllsn s whn th tw bds hv ql mss, n whch cs thy wll smply xchng thr mmnt. Th mlcls—s dstnct frm tms—f gs r lqd rrly xprnc prfctly lstc cllsns bcs kntc nrgy s xchngd btwn th mlcls’ trnsltnl mtn nd thr ntrnl dgrs f frdm wth ch cllsn. t ny nstnt, hlf th cllsns r, t vryng xtnt, nlstc cllsns (th pr psssss lss kntc nrgy n thr trnsltnl mtns ftr th cllsn thn bfr), nd hlf cld b dscrbd s “spr-lstc” (pssssng mr kntc nrgy ftr th cllsn thn bfr). vrgd crss th ntr smpl, mlclr cllsns cn b rgrdd s ssntlly lstc s lng s Plnck's lw frbds nrgy frm bng crrd wy by blck-bdy phtns. n th cs f mcrscpc bds, prfctly lstc cllsns r n dl nvr flly rlzd, bt pprxmtd by th ntrctns f bjcts sch s bllrd blls. Whn cnsdrng nrgs, pssbl rttnl nrgy bfr nd/r ftr cllsn my ls ply rl.